Yeast-based production methods for culture media proteins

Abstract

The present disclosure relates to the field of methods for producing component proteins of culture media, such as transferrin, by yeast cells More particularly, this disclosure relates to methods for producing low purity extracts of such component proteins from yeast cells that may be used in cellular agriculture.

Description

[0001]Yeast-based production methods for culture media proteins Cross-reference to related applications The present application claims priority from Australian Provisional Patent Application No. 2023902613 filed on 17 August 2023, the contents of which are incorporated herein by reference in their entirety. Technical field The present disclosure relates to the field of methods for producing component proteins of culture media, such as transferrin, by yeast cells. More particularly, this disclosure relates to methods for producing low purity extracts of such component proteins from yeast cells that may be used in cellular agriculture. Background In vitro meat production is the process by which muscle or organ tissue from animals is grown in laboratories using cell culture techniques to manufacture meat and meat products. It is expected that these processes will use significantly less arable land and water and emit significantly less carbon that animal-based production. The culture media for such production processes requires components, such as amino acids, salts, vitamins and growth factors, necessary for cell proliferation. These components are generally provided by the addition of foetal bovine serum (FBS) to the media before use. As FBS is derived from unborn calves, an alternative source of these culture medium components is required to ensure that in vitro meat production is free from utilising animal products, particularly animal-derived serum. Accordingly, there remains a need for improved methods of producing component proteins, such as growth factors and serum proteins like transferrin, for use in an animal component-free culture medium for in vitro meat production. Summary The present disclosure is based on the surprising finding that a low purity extract of transferrin obtained from recombinant yeast cells can be utilised to supplement a cell culture medium for in vitro meat production. In a first aspect, the present disclosure provides a method of producing a component protein, said method including the steps of: (a) providing a yeast cell configured to express the component protein that has been cultured under suitable conditions to express the component protein and / or a culture medium in which the yeast cell has been cultured, wherein the component protein is of non-human origin; and (b) extracting a low purity extract of the component protein from the yeast cell and / or the culture medium. In a second aspect, the present disclosure provides a method of producing a component protein, said method including the steps of: (a) providing a yeast cell configured to express the component protein, wherein the component protein is of non-human origin; (b) culturing the yeast cell in a culture medium under suitable conditions to express the component protein; and (c) extracting a low purity extract of the component protein from the culture medium and / or the yeast cell. In particular examples, the component protein is present in an amount less than about 95% of a total protein content of the low purity extract. More particularly, the component protein may be present in an amount of less than about 80% of a total protein content of the low purity extract. Suitably, the extraction step of the present methods comprises a dialysis and / or filtration step. In certain examples, the dialysis and / or filtration step includes dialysing and / or filtering the culture medium with a membrane having a molecular weight cut off of at least about 2.5 kDa. More particularly, the membrane can have a molecular weight cut off of between about 5 kDa and about 15 kDa. In other examples, the membrane has a molecular weight cut off of about 10 kDa. In some examples, the membrane has a molecular weight cut off of about 30 kDa. Suitably, the present methods further include the step of lyophilising the low purity extract of the component protein. According to certain examples, the yeast cell is or comprises a Pichia pastoris cell. For various examples, the component protein is of an origin selected from the group consisting of a non-human mammalian species, an avian species and a teleost species, such as a bovine, a chicken or a salmon. Suitably, the component protein is selected from the group consisting of transferrin, albumin, insulin, insulin-like growth factor 1 (IGF-1), insulin-like growth factor 2 (IGF-2), fibroblast growth factor 2 (FGF2) activin, transforming growth factor β (TGF-β), neuregulin 1 (NRG1) and any combination thereof. In some examples, the component protein is transferrin. In a third aspect, the present disclosure provides an isolated component protein produced according to the methods of the first or second aspect. In a fourth aspect, the present disclosure relates to a low purity extract of a component protein, wherein the component protein has been produced by a yeast cell configured to express the component protein and wherein the component protein is of non-human origin. In a fifth aspect, the present disclosure provides a kit for producing a culture medium for culturing non-human animal cells, said kit comprising a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to the non-human animal cells. In particular examples, the kit further comprises a base medium. In a sixth aspect, the present disclosure relates to a culture medium for culturing non- human animal cells comprising: (a) a base medium; and (b) a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to the non-human animal cells. Suitably, the culture medium is free or substantially free of serum. In a seventh aspect, the present disclosure provides a method of producing a culture medium for culturing non-human animal cells, said method including the steps of: (a) providing a base medium; (b) adding a low purity extract of a component protein to the base medium, wherein the component protein has been produced by a yeast and is of a same or similar species to the non- human animal cells. In an eighth aspect, the present disclosure relates to a method of culturing non-human animal cells, said method including the step of culturing the non-human animal cells in a culture medium comprising a base medium and a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to that of the non-human animal cells. In a ninth aspect, the present disclosure provides a method of in vitro meat production, said method including the steps of: (a) providing a plurality of non-human animal cells; and (b) culturing the non-human animal cells in a culture medium comprising a base medium and a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to that of the non-human animal cells. Suitably, the methods of the eighth and ninth aspects further include the earlier step of isolating the plurality of non-human cells from an animal source. For the methods of the above two aspects, the component protein is suitably of an origin selected from the group consisting of a non-human mammal, an avian or a teleost. Referring to the methods of the eighth and ninth aspects, these may further include the step of harvesting a meat product produced from culturing the non-human animal cells. For the kit of the fifth aspect, the culture medium of the sixth aspect or the method of the seventh to ninth aspects, the low purity extract of the component protein suitably has been produced by the method of the first or second aspects. Referring to the above aspects, the component protein is suitably present in the low purity extract in an amount less than about 95% of a total protein content of the low purity extract. More particularly, the component protein can be present in the low purity extract in an amount less than about 80% of a total protein content of the low purity extract. In a tenth aspect, the present disclosure provides a meat product produced according to the method of the eighth or ninth aspects. Brief description of the drawings The following figures form part of the present specification and are included to further demonstrate certain aspects of the present disclosure. The disclosure may be better understood by reference to one or more of these figures in combination with the detailed description of specific embodiments presented herein. It will be appreciated by persons skilled in the art that numerous variations and / or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. Figure 1. Schematic of the vector construct used for production of recombinant transferrin production strains. Figure 2. The sum of the areas under the peak for the 3 most abundant MS / MS transitions of transferrin specific peptides for different colonies of the transformed P. pastoris strain. Figure 3. SDS-PAGE analysis and tryptic digest gel slice proteomics confirmation of transferrin protein production and secretion into the supernatant by strain Tfn 1-8 Pichia pastoris BG16. Figure 4. Toxicity assays for Pichia culture extracts. Right hand panel: C2C12F12 original cells are cultured in a DMEM F12 media supplemented with 10% FBS. B9 original cells are cultured in B9 media with no FBS and no transferrin. Left hand panel: The remaining 4 blocks are C2C12 cells cultured in B9 supplemented with 20 µg / mL store bought bovine recombinant transferrin. From left to right these blocks represent raw Pichia extract, Pichia extract supernatant after centrifugation, Pichia extract after centrifugation and passing through a 10 kDa cut off filter (keeping the retentate) and Pichia extract after centrifugation and passing through a 30 kDa cut off filter (keeping the retentate). The concentrations of Pichia extract (dw / v) used for each block were (from left to right) 200, 100, 80, 50, 20, 10, and 0 µg / ml. Figure 5. Functionality assay using C2C12 cells. Recombinant bovine transferrin (Tfn) partially purified from Pichia supernatant was added at various concentrations (1-1000 ug / ml media) to the B9 cell culture media and compared with cell growth in transferrin-negative B9 media and standard DMEM media supplemented with 10% FBS (denoted GM media). Cells were grown for 7 days and cell numbers measured every day from day 3 to day 7. Tube B: TFN 1.8 T1 ferment supernatant filtered and dialysed; Tube C: TFN 1.8 T1 ferment supernatant filtered, dialysed and 30 kDa; Tube F: TFN 1.8 T1 ferment supernatant freeze dried #5 filtered and dialysed; Flask A: TFN 1.8 flask supernatant filtered, dialysed; Flask B: TFN 1.8 flasks supernatant filtered, dialysed, free-dried and resuspended in PBS. Figure 6. Functionality assay using C2C12 cells (simplified data set). Recombinant bovine transferrin (Tfn) partially purified from Pichia supernatant was added at various concentrations (1- 1000 ug / ml media) to the B9 cell culture media and compared with cell growth in transferrin- negative B9 media and standard DMEM media supplemented with 10% FBS (denoted GM media). Cells were grown for 7 days and cell numbers measured every day from day 3 to day 7. Key to the Sequence Listing SEQ ID NO: 1 full length amino acid sequence of a bovine transferrin protein g t c a Ķ acaccatgactttattagcctgtctatcctggcccccctggcgaggttcatgtttgttta tttccgaatgcaacaagctccgcattacacccgaacatcactccagatgagggctttctg t g t t t t t t a t a a a c a g c c c a t t c g t c g t t a c g a a a c t g t a ķ tgaaagatattatggttatactggtgcttttagatgtttggtggaaaaaggggacgttgc tttcgttaaggatcagactgttattcagaatactgatggcaataataacgaggcttgggc t a c t g a a t a a g g c t c t t t t c a g g g t a t a c t a a c t g c c c c t aacaggattagcagagcgaggtatgtaggcggtgctacagagttcttgaagtggtggcct aactacggctacactagaaggacagtatttggtatctgcgctctgctgaagccagttacc t g c of g t c a a g t g t t t t t t a t a a a c a g c c c a t t c g t c g t tggtaaagataaacctgataattttcaattgtttcaatctccacacggtaaagatttgtt gttcaaagattctgcagatggcttcttgaaaattccttctaaaatggatttcgaattgta c g a a a c t g t a c c t a c t g a a t a a g g c g t c a a g t g t t t t t atataaacagaaggaagctgccctgtcttaaacctttttttttatcatcattattagctt actttcataattgcgactggttccaattgacaagcttttgattttaacgacttttaacga t t t a g t t g c t Detailed description General Techniques and Definitions Unless specifically defined otherwise, all technical and scientific terms used herein shall be taken to have the same meaning as commonly understood by one of ordinary skill in the art (e.g., in genomics, immunology, molecular biology, immunohistochemistry, biochemistry, oncology, and pharmacology). The present disclosure is performed without undue experimentation using, unless otherwise indicated, conventional techniques of molecular biology, microbiology, recombinant DNA technology and immunology. Such procedures are described, for example in Sambrook, Fritsch & Maniatis, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratories, New York, Fourth Edition (2012), whole of Vols I, II, and III; DNA Cloning: A Practical Approach, Vols. I and II (D. N. Glover, Second Edition., 1995), IRL Press, Oxford, whole of text; Oligonucleotide Synthesis: A Practical Approach (M. J. Gait, ed, 1984) IRL Press, Oxford, whole of text, and particularly the papers therein by Gait, ppl-22; Atkinson et al, pp35-81; Sproat et al, pp 83-115; and Wu et al, pp 135-151; 4. Nucleic Acid Hybridization: A Practical Approach (B. D. Hames & S. J. Higgins, eds., 1985) IRL Press, Oxford, whole of text; Immobilized Cells and Enzymes: A Practical Approach (1986) IRL Press, Oxford, whole of text; Perbal, B., A Practical Guide to Molecular Cloning (1984) and Methods In Enzymology (S. Colowick and N. Kaplan, eds., Academic Press, Inc.), whole of series. Those skilled in the art will appreciate that the present disclosure is susceptible to variations and modifications other than those specifically described. It is to be understood that the disclosure includes all such variations and modifications. The disclosure also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features. The present disclosure is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the disclosure, as described herein. Each feature of any particular aspect or embodiment or embodiment of the present disclosure may be applied mutatis mutandis to any other aspect or embodiment or embodiment of the present disclosure. Throughout this specification, unless specifically stated otherwise or the context requires otherwise, reference to a single step, composition of matter, group of steps or group of compositions of matter shall be taken to encompass one and a plurality (i.e., one or more) of those steps, compositions of matter, groups of steps or group of compositions of matter. As used herein, the singular forms of “a”, “and” and “the” include plural forms of these words, unless the context clearly dictates otherwise. For example, a reference to “a bacterium” includes a plurality of such bacteria, and a reference to “an allergen” is a reference to one or more allergens. The term “and / or”, e.g., “X and / or Y” shall be understood to mean either “X and Y” or “X or Y” and shall be taken to provide explicit support for both meanings or for either meaning. Throughout this specification, the word “comprise’ or variations such as “comprises” or “comprising” will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. By “consisting essentially of”, in the context of an amino acid sequence, is meant the recited amino acid sequence together with an additional one, two or three amino acids at the N- and / or C-terminus. By “consisting essentially of”, in the context of a nucleic acid sequence, is meant the recited nucleic acid sequence together with an additional one, two or three nucleic acids at the 5’ end and / or 3’ end. The term “substantially” does not exclude “completely” (e.g., a composition which is “substantially free” from Y may be completely free from Y). Throughout the present specification, various aspects and components of the disclosure can be presented in a range format. The range format is included for convenience and should not be interpreted as an inflexible limitation on the scope of the present disclosure. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub- ranges as well as individual numerical values within that range, unless specifically indicated. For example, description of a range such as from 1 to 5 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 5, from 3 to 5 etc., as well as individual and partial numbers within the recited range, for example, 1, 2, 3, 4, 5, 5.5 and 6, unless where integers are required or implicit from context. This applies regardless of the breadth of the disclosed range. Where specific values are required, these will be indicated in the specification. The term “about” in relation to a numerical value x is optional and means, for example, any number within 1, 5 or 10% of the referenced number. In certain embodiments, the term “about” encompasses the exact number recited. All computer programs, algorithms, patent and scientific literature referred to herein is incorporated herein by reference. For the present disclosure, the database accession number or unique identifier provided herein for a gene, protein or yeast strain, as well as the gene and / or protein sequence or sequences associated therewith, are incorporated by reference herein. Methods of producing component proteins by yeast cells The inventors have surprisingly shown that low purity extracts of transferrin protein of a non-human origin (e.g., bovine transferrin) produced by recombinant Pichia pastoris cells can be successfully used in culturing non-human animal cells obtained from the same or similar species as that of the transferrin protein. Advantageously, this demonstrated functionality of low purity extracts of non-human component proteins of cell culture media may lead to improved economics for in vitro meat production, whilst also obviating the need for animal-derived components like FBS in the culture process. In one broad form, the present disclosure provides a method of producing a component protein, said method including the steps of: (a) providing a yeast cell configured to express the component protein that has been cultured under suitable conditions to express the component protein and / or a culture medium in which the yeast cell has been cultured, wherein the component protein is of non-human origin; and (b) extracting a low purity extract of the component protein from the yeast cell and / or the culture medium. For such examples, the method may further include the earlier step of culturing the yeast cell in a culture medium under suitable conditions to express the component protein. In some examples, step (b) may include extracting the low purity extract of the component protein from the yeast cell (e.g., not from the culture medium). In other examples, step (b) may include extracting the low purity extract of the component protein from the culture medium (e.g., not from the yeast cell). For various examples, step (b) may include extracting the low purity extract of the component protein from the yeast cell and the culture medium. Accordingly, in one form, the present disclosure provides a method of producing a component protein for a culture media, said method including the steps of: (a) providing a yeast cell configured to express the component protein, wherein the component protein is of non-human origin; (b) culturing the yeast cell in a culture medium under suitable conditions to express the component protein; and (c) extracting a low purity extract of the component protein from the culture medium and / or the yeast cell. In a related form, the present disclosure provides a low purity extract of a component protein, wherein the component protein has been produced by a yeast cell configured to express the component protein and wherein the component protein is of non-human origin. In another form, the present disclosure relates to a component protein produced according to the method described herein. The term “component protein” refers to any protein, whether of chemical or biological origin, that can be used in cell culture media to maintain or promote the growth or proliferation of cells. The terms “component,” “nutrient” and ingredient” can be used interchangeably herein and are all meant to refer to such compounds. Typical ingredient proteins that are used in cell culture media include serum proteins (e.g., albumin), growth factors, carrier proteins (e.g., fetuin), extracellular matrix proteins (e.g., fibronectin) and iron transport proteins. Other ingredients that promote or maintain cultivation of cells ex vivo can be selected by those of skill in the art, in accordance with the particular need. Suitably, the component protein is selected from the group consisting of transferrin, albumin, insulin, insulin-like growth factor 1 (IGF-1), insulin-like growth factor 2 (IGF-2), fibroblast growth factor 2 (FGF2), activin, transforming growth factor β (TGF-β), neuregulin 1 (NRG1) and any combination thereof. In particular examples, the component protein is a transferrin protein. It is contemplated that the yeast cell can be of any species or strain known in the art, such as those described herein. Suitably, the yeast cell is capable of being modified to recombinantly express (e.g., transiently express, stably express, constitutively express and / or conditionally express) the component protein. Accordingly, the present method may include the earlier or initial step of modifying, or more particularly genetically modifying, the yeast cell to express the component protein, such as by those methods provided herein. In particular examples, the yeast cell is a Pichia pastoris yeast cell. The yeast cells of the present disclosure can also be cultured using conventional methods and conventional and established culture media known in the art. Suitably, a culture medium used should contain all nutrients essential for the growth and survival of a yeast cell. In particular examples, the culture medium contains a carbon source, a nitrogen source, an inorganic salt and optionally additional components which a yeast cell, such as that of the genus Pichia, can utilize, and is capable of efficiently culturing a yeast cell, such as that of the genus Pichia. The carbon source may, for example, be a sugar, such as glucose, fructose or sucrose. The nitrogen source may, for example, be an inorganic acid or an ammonium salt of an inorganic acid, such as ammonia, ammonium chloride or ammonium acetate, peptone or casamino acid. The inorganic salt may, for example, be magnesium phosphate, magnesium sulphate or sodium chloride. The culture medium may be a natural medium or a synthetic medium. The culture medium may, for example, be MMA medium (Minimal medium with agar), MB medium (Okazaki et al., Nucleic Acids Res., 1990, 18, 6485-6489), SDC medium (Synthetic dextrose complete medium), TES medium (0.5% Bacto-yeast extract, 3% glucose, supplemented with uracil, leucine, histidine, lysine, adenine), YES medium (0.5% Bacto-yeast extract, 3% glucose, supplemented with uracil, leucine, histidine, lysine, adenine), or YPD medium (1% Bacto-yeast extract, 2% Bactopeptone, 2% glucose). According to particular examples, the culture medium for the yeast cells is or comprises YPD medium (Rose et al., “Methods In Yeast Genetics”, Cold Spring Harbor Laboratory Press (1990)). In order to produce the recombinant component protein, the yeast cells are suitably cultivated or cultured under conditions that allow for expression of said component protein. This means that substances or agents may be added or removed from the culture medium (e.g., the addition or removal of substances or agents may occur by changing the culture medium) in order to activate recombinant expression of the component protein (e.g., activate a promoter operably linked to a nucleic acid encoding the component protein). In other examples, the component protein is constitutively expressed, rather than conditionally or inducibly expressed, by the yeast cells. In view of the foregoing, the yeast cells may be cultured in the presence of an agent that is capable of modulating, and more particularly inducing, expression of the component protein by the yeast cells. In those examples relating to: (a) methylotrophic yeast cells, such as Pichia pastoris; and / or (b) yeast cells comprising a nucleotide sequence encoding the component protein that is operably linked to a methanol-inducible promoter; the culturing step suitably includes culturing the yeast cells in the presence of methanol. Methods of culturing methylotrophic yeast cells are known in the art (see, e.g., Pichia Protocols, Methods in Molecular Biology, 389, Cregg, Ed., 2007, 2nd Ed., Humana Press, Inc). To this end, methanol may be introduced or added to the culture media in an appropriate amount and as required so as to promote expression of the component protein by the yeast cells. Suitably, the yeast cell is cultured for a period of time, such as after transfection thereof, that is sufficient to allow for expression and / or secretion of the component protein. In particular examples, the yeast cell is cultured for at least about 1 day (e.g., at least about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100 days or any range therein). For some examples, the yeast cell is cultured for about 1 day to about 10 days, about 2 days to about 8 days, about 3 days to about 7 days or about 4 days to about 6 days. The cells of the present invention can be cultured with any known cultivation method such as batch culture, continuous culture and fed-batch culture. Culture may also be carried out by publicly known cultivation methods for yeasts, such as a shaking cultivation, a stirring cultivation, a semi-continuous fermentation (see, e.g., Wang et al., Front. Bioeng. Biotechnol., 30 November 2021), a bioreactor or a high cell density fermentation (see, e.g., Liu et al., Front. Microbiol., 21 August 2020). Culture conditions suitable for selected yeast strains may be easily adjusted by those skilled in the art. Suitably, the yeast cells are cultured at a temperature from about 23°C to about 37°C (e.g., about 23°C, 24°C, 25°C, 26°C, 27°C, 28°C, 29°C, 30°C, 31°C, 32°C, 33°C, 34°C, 35°C, 36°C, 37°C or any range therein), more particularly at a temperature between about 25°C to about 35°C or even more particularly at a temperature between about 28°C to about 30°C. In various examples, extraction of the low purity extract of the component protein can be performed following harvesting, isolation, purification or separation of the culture medium or a supernatant thereof from the respective yeast cells, particularly in those examples in which the component protein is secreted from the yeast cells into the culture medium. To do this, for example, the yeast cells or cell residues or debris may be separated from the culture media by methods known to the person skilled in the art, such as centrifugation, separators or filters. The component protein present in the culture media (or a supernatant thereof) may then be extracted, isolated, concentrated or harvested by methods known to the person skilled in the art, such as those described herein (e.g., dialysis, filtration, precipitation, gradient centrifugation and the like). Accordingly, the present method can also comprise the steps of: (i) isolating the culture medium or a supernatant thereof from the yeast cells; and / or (ii) isolating the component protein from the supernatant of the culture medium if the component protein is secreted from the cell. According to alternative examples, particularly in those examples in which the component protein is not or only minimally secreted by the yeast cells, extraction of the low purity extract of the component protein can be performed in respect of the yeast cells and optionally together with the culture medium or a supernatant thereof. For such examples, the yeast cells are suitably lysed before recovering or isolating the low purity extract of the component protein therefrom. The yeast cells may be lysed biologically, such as by contacting them with an enzyme able to digest their cell walls (e.g., zymolyase). Alternatively, the yeast cells may be lysed mechanically, by breaking their cell walls, such as by using a vortex with glass beads, or lysed chemically, such as by osmotic shock or alkali treatment. Once lysed, the component protein present in the yeast cells and optionally the culture medium can be extracted, such as by those method provided herein. During extraction of the low purity extract in step (c), the yeast cells and / or lysed debris thereof may be separated, isolated or purified from the culture medium by standard methods like centrifugation, separation, filtration or ultrafiltration. The component proteins produced therefrom can then be concentrated and / or purified according to methods known to those skilled in the art, such as dialysis, gradient ultracentrifugation (GUC), filtration, precipitation (e.g., “salting out”), chromatography, column purification, non-chromatographic purification tags (e.g., an elastin- based or cellulose binding domain containing tag, optionally with a self-cleaving option, such as an intein-based tag), column-based purification (e.g., column comprising Fe2+to bind transferrin), magnetic particle purification (e.g., purification of Fe2+-bound transferrin), and any combination thereof. Suitably, the culture medium and / or the yeast cell can be subjected to a sterilisation step, such as before and / or after extracting the low purity extract of the component protein therefrom. Such a sterilisation step may be performed by any means known in the art, including, but not limited to, passage of the culture medium and / or the yeast cell through a sterilisation filter of sufficient size (e.g., about 0.2, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.3 μm or any range therein) to remove one or more microbes therefrom. According to particular examples, the low purity extract of the component protein is isolated or extracted from the culture media and / or the yeast cells by dialysis and / or filtration. More particularly, the component protein can be isolated or extracted from the culture media and / or the yeast cells by dialysis. In other examples, the component protein is isolated or extracted from the culture media and / or the yeast cells by filtration. For particular examples, the component protein is isolated or extracted from the culture media and / or the yeast cells by dialysis and filtration. In such examples, a centrifugation step may be performed before or after dialysis and / or filtration or between filtration and dialysis irrespective of their order. As used herein, the term “filtration” refers to a process of separating particulate matter from a fluid, such as a culture medium, by passing the fluid through a medium or membrane that will not pass the particulates. As used herein, the term “dialysis” refers to a form of filtration, or a process of selective diffusion through a membrane (e.g., a dialysis membrane). Dialysis is typically used to separate low-molecular weight solutes that diffuse through the membrane from high- molecular weight solutes which do not. For example, a feed of fluid can be passed over a semipermeable membrane or filter, and a feed of dialysate is passed over the other side of that membrane or filter, and then there is diffusive and selective transport of dissolved solutes, typically below a particular molecular weight, between the fluids. The composition of one fluid, the dialysate, may be used to deplete the composition of the other fluid, the feed fluid, of some molecule or molecules. By extension, the dialysate may be used to concentrate particular molecules, particularly those of a high molecular weight within the feed fluid (e.g., a component protein like transferrin). Suitable membranes for dialysis and filtration methods are commercially available and include, for example, cellulose acetate, regenerated cellulose, polyester and cellulose ester. Commercially available dialysis kits are also known in the art (see, e.g., Slide-A-Lyzer Dialysis Flasks from ThermoFisher Scientific). It is also envisaged that the dialysis time may vary depending on the dialysis membrane used, the solvent used, and the target compound. As such, optimal dialysis or filtration conditions for step (c) can be selected or determined by measuring dialysis times and dialysis rates in advance. In particular examples, however, the filtration or dialysis step is conducted for a period of time of about 1 hour to about 48 hours (e.g., about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48 hours or any range therein). In particular examples, the filtration or dialysis step is conducted for a period of time of about 12 hours to about 48 hours, more particularly about 18 hours to about 42 hours or even more particularly about 24 hours to about 36 hours. The separation characteristic determined by the pore size-range of a dialysis membrane or a filtration membrane is most often referred to as the molecular weight-cut off (MWCO) of the membrane. Traditionally, a membrane’s MWCO refers to the smallest average molecular mass of a standard molecule that will not effectively diffuse across the membrane. Typically, the smallest size globular macromolecule (in Daltons) that is retained by greater than 90% upon extended dialysis (e.g., greater than 8-12 hours) defines the nominal MWCO. Thus, a dialysis membrane or a filtration membrane with a 10 kDa MWCO will generally retain proteins having a molecular mass of at least 10 kDa. According to certain examples, the dialysis and / or filtration step includes dialysing and / or filtering the culture medium with a membrane having a MWCO of at least about 2 kDa (e.g., at least about 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70 kDa or any range therein). More particularly, the membrane can have a MWCO of at least about 2.5 kDa, even more particularly, at least about 5 kDa or still even more particularly, at least about 10 kDa. For particular examples, the membrane has a MWCO of between about 2 kDa and about 40 kDa. In some examples, the membrane has a MWCO of between about 2 kDa and about 20 kDa. In other examples, the membrane has a MWCO of between about 5 kDa and about 15 kDa. For various examples, the membrane has a MWCO of between about 8 kDa and about 12 kDa. According to particular examples, the membrane has a MWCO of about 10 kDa. In some examples, the membrane has a MWCO of between about 20 kDa and about 40 kDa. In other examples, the membrane has a MWCO of between about 25 kDa and about 35 kDa. For various examples, the membrane has a MWCO of between about 28 kDa and about 32 kDa. According to particular examples, the membrane has a MWCO of about 30 kDa. Referring to some examples, the dialysis and / or filtration step includes: (i) dialysing the culture medium with a membrane having a MWCO of between about 5 kDa and about 15 kDa, or more particularly about 10 kDa, to produce a first extract of the component protein; and (ii) filtering the first extract with a membrane having a MWCO of between about 5 kDa and about 15 kDa, or more particularly about 10 kDa, to produce the low purity extract of the component protein. In other examples, the dialysis and / or filtration step includes: (i) dialysing the culture medium with a membrane having a MWCO of between about 5 kDa and about 15 kDa, or more particularly about 10 kDa, to produce a first extract of the component protein; and (ii) filtering the first extract with a membrane having a MWCO of between about 25 kDa and about 35 kDa, or more particularly about 30 kDa, to produce the low purity extract of the component protein. For the above examples, the dialysis and / or filtration step may further include a centrifugation step before, between and / or after, or more particularly between, steps (i) and (ii). Moreover, for such examples, the dialysis and / or filtration step may further include a lyophilisation step before, between and / or after, or more particularly between, steps (i) and (ii). By way of example, the culture medium may be lyophilised prior to step (i), the first extract may be lyophilised prior to step (ii) and / or the low purity extract may be lyophilised after step (ii). Further, the culture medium may be centrifuged prior to step (i), the first extract may be centrifuged prior to step (ii) and / or the low purity extract may be centrifuged after step (ii). According to certain examples, the extraction step (i.e., the step of extracting the low purity extract) described herein includes a buffer exchange step, such as to remove one or more contaminating proteins or molecules from the low purity extract. In this regard, the culture medium and / or the yeast (or further extracts thereof) may be contacted with a buffer exchange membrane or column. For such examples, the buffer exchange membrane or column has a MWCO of between about 2 kDa and about 20 kDa (e.g., about 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 kDa, or any range therein), more particularly between about 2 kDa and about 15 kDa, even more particularly between about 3 kDa and about 10 kDa, yet even more particularly between about 3 kDa and about 7 kDa or still even more particularly about 5 kDa. Such a buffer exchange step may be performed in addition to the dialysis and / or filtration steps disclosed herein or may be performed as an alternative means or method of purifying the culture medium and / or yeast expressing the component protein to generate a low purity extract thereof. The terms “lowly purified” and “low purity” are intended to mean an extract, isolate or product, such as a protein extract (e.g., a low purity extract or a low purity protein extract), in which a component protein, such as a transferrin protein, is present in an amount less than 100% (e.g., less than about 99%, 98%, 97%, 96%, 95%, 94%, 93%, 92%, 91%, 90%, 89%, 88%, 87%, 86%, 85%, 84%, 83%, 82%, 81%, 80%, 79%, 78%, 77%, 76%, 75%, 74%, 73%, 72%, 71%, 70%, 69%, 68%, 67%, 66%, 65%, 64%, 63%, 62%, 61%, 60%, 59%, 58%, 57%, 56%, 55%, 54%, 53%, 52%, 51%, 50%, 49%, 48%, 47%, 46%, 45%, 44%, 43%, 42%, 41%, 40%, 39%, 38%, 37%, 36%, 35%, 34%, 33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%, 22%, 21%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, 10% or any range therein), more particularly less than about 95%, even more particularly less than about 90%, yet even more particularly less than about 85%, still even more particularly less than about 80%, further still even more particularly less than about 75%, yet still even more particularly less than about 70%, even more particularly less than about 65%, still yet even more particularly less than about 60%, further even more particularly less than about 55% or yet still more particularly less than about 50% of a total protein content, such as a total soluble protein content, thereof. Suitably, the component protein is present in an amount of about 5% to about 95% (e.g., about 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or any range therein) of a total protein content of the low purity extract. In particular examples, the component protein is present in an amount of about 10% to about 95% of a total protein content of the low purity extract. For certain examples, the component protein is present in an amount of about 30% to about 80% of a total protein content of the low purity extract. In other examples, the component protein is present in an amount of about 65% to about 90% of a total protein content of the low purity extract. According to various examples, the component protein is present in an amount of about 70% to about 80% of a total protein content of the low purity extract. For certain examples, the component protein is present in an amount of about 20% to about 60% of a total protein content of the low purity extract. In other examples, the component protein is present in an amount of about 30% to about 50% of a total protein content of the low purity extract. In particular examples, the component protein is present in an amount of about 30% to about 70% of a total protein content of the low purity extract. Referring to other examples, the component protein is present in an amount of about 60% to about 80% of a total protein content of the low purity extract. According to some examples, the component protein is present in an amount of about 35% to about 45% of a total protein content of the low purity extract. According to some examples, the component protein is present in an amount of about 1% to about 95% (e.g., about 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or any range therein) by weight of the low purity extract, such as a lyophilised form or a dry form thereof. For certain examples, the component protein is present in an amount of about 10% to about 80% by weight of the low purity extract. In other examples, the component protein is present in an amount of about 65% to about 90% by weight of the low purity extract. According to various examples, the component protein is present in an amount of about 70% to about 80% by weight of the low purity extract. For certain examples, the component protein is present in an amount of about 20% to about 60% by weight of the low purity extract. In other examples, the component protein is present in an amount of about 30% to about 50% by weight of the low purity extract. According to some examples, the component protein is present in an amount of about 35% to about 45% by weight of the low purity extract. In various examples, the component protein is present in an amount of about 8% to about 40% by weight of the low purity extract. According to particular examples, the component protein is present in an amount of about 5% to about 50% by weight of the low purity extract. For some examples, the component protein is present in an amount of about 1% to about 30%, more particularly about 2% to about 25%, even more particularly about 3% to about 20%, yet even more particularly about 4% to about 15% or still even more particularly about 5% to about 10% by weight of the low purity extract. With respect to assessing yield and / or purity of the component protein (e.g., a transferrin protein), this may be performed by any method or means known in the art. Yield of the component protein, such as in a culture supernatant or the low purity extract, may be measured by any means of protein quantification known in the art, such as by gel electrophoresis (e.g., Western Blot); by ELISA, or by chromatographic methods, such as HPLC (high performance liquid chromatograph), HPLC-UV or mass spectrometry methods, such as LC-MS (Liquid Chromatography-Mass Spectrometry) and LC-MRM-MS (Liquid Chromatography-Multiple Reaction Monitoring-Mass Spectrometry). In particular examples, yield of the component protein in the low purity extract is measured by LC-MS or LC-MRM-MS. In view of the foregoing and in certain examples, a MRM assay is used which employs specific peptides (such as shown in Figure 2; e.g., SEQ ID NOs: 21-24 of a transferrin protein) and their fragments (transitions) as discriminators of the component protein and / or indicators of an expression level thereof. In principle, any unique peptide derived from a component protein can be used to determine the amount or expression level of said component protein in a sample or extract using an MRM assay, but not every peptide lends itself well for LC-MRM_MS quantification. This is dependent on various factors such as the matrix present, the peptide hydrophobicity and ionisation efficiency. Thus, peptide choice was optimised for the LC-MRM- MS assay that was developed herein. Further to the above, a level of purity of the component protein may be determined for the present method. To this end, an extract or isolate containing the component protein will typically include impurities or contaminating proteins (e.g., non-component proteins, or more particularly non-transferrin proteins), such as host cell proteins derived from the yeast cells. Purity of the component protein in the low purity extract may be measured by any means known in the art. Exemplary methods may include calculating a ratio of a level, content or concentration of the component protein, such as determined by those methods described above, in the low purity extract to a total protein content thereof. The total protein content can be assessed, for example, by a bichinonic acid (BCA) assay, gel electrophoresis, such as by Bradford assay, SDS-PAGE and Western Blot, or Coomassie Stain; by ELISA; by chromatographic methods, such as HPLC; or by mass spectrometry methods. In particular examples, the purity of the low purity extract is determined by the ratio of a level, concentration or content of a component protein therein to a total protein content therein. For such examples, the level of the component protein is suitably measured by LC-MS or LC-MRM-MS and the total protein content is suitably measured by a Bradford assay. In other examples, the level of the component protein and total protein is suitably measured by LC-MS or LC-MRM-MS. The level of purity of the component protein in the low purity extract may then be measured by a matrix matched calibration curve. The level of purity the component protein can also be expressed as a wt% or by weight of the total protein content, such as hereinbefore described. As used herein, the terms “by weight”, “weight %” and “wt%” refer to the weight of a substance (for example, a transferrin protein) as a percentage of the weight of the composition comprising the substance or a component in the composition (e.g., the crude extract, the first extract or the low purity extract). For example, the weight of a particular protein, such as a transferrin protein, may be determined as a percentage of the weight of the total protein content or total soluble protein content of the low purity extract. Once isolated, the low purity extract of the component protein may be lyophilized, so as to assist with storage and stability thereof. Methods of lyophilising protein extracts are known in the art. In particular examples, a lyoprotectant, such as hyaluronic acid, γ-cyclodextrin and / or trehalose, is used. Accordingly, the low purity extract may further comprise a lyoprotectant. In such examples, a lyophilised form of the low purity extract of the component protein can be resuspended in any suitable buffer or carrier prior to use. The buffer or carrier may be any suitable buffer or carrier known in the art. For example, the buffer or carrier may be a TRIS, acetate, glutamate, lactate, maleate, tartrate, phosphate, citrate, carbonate, glycinate, histidine, glycine, succinate, triethanolamine, or phosphate buffer. In particular examples, the lyophilised low purity extract is resuspended in PBS prior to use. Once resuspended or reconstituted in a suitable buffer or carrier, the low purity extract can be further subjected to one or more steps of centrifugation, dialysis and / or filtration as described herein, such as prior to addition to a culture medium for culturing non-human animal cells. According to various examples, the method described herein includes: (i) dialysing the culture medium and / or yeast cells, such as with a membrane having a MWCO of between about 5 kDa and about 15 kDa, or more particularly about 10 kDa, to produce a low purity extract of the component protein; and (ii) lyophilising the low purity extract. For such examples, the low purity extract may be reconstituted in an appropriate buffer or carrier, such as those described herein, prior to addition to the culture medium. Referring to some examples, the method described herein includes: (i) dialysing the culture medium and / or yeast cells, such as with a membrane having a MWCO of between about 5 kDa and about 15 kDa, or more particularly about 10 kDa, to produce a first extract of the component protein; (ii) lyophilising the first extract; (iii) optionally reconstituting the lyophilised form of the first extract in a carrier or buffer; (iv) optionally centrifuging the reconstituted form of the first extract; and (v) optionally, filtering the first extract of step (iii) or (iv), such as with a membrane having a MWCO of between about 5 kDa and about 15 kDa, or more particularly about 10 kDa, to produce the low purity extract of the component protein. In other examples, the method described herein includes: (i) dialysing the culture medium and / or yeast cells, such as with a membrane having a MWCO of between about 5 kDa and about 15 kDa, or more particularly about 10 kDa, to produce a first extract of the component protein; (ii) lyophilising the first extract; (iii) optionally reconstituting the lyophilised form of the first extract in a carrier or buffer; (iv) optionally centrifuging the reconstituted form of the first extract; and (v) optionally, filtering the first extract of step (iii) or (iv), such as with a membrane having a MWCO of between about 25 kDa and about 35 kDa, or more particularly about 30 kDa, to produce the low purity extract of the component protein. In some examples, the method described herein includes: (i) dialysing the culture medium and / or yeast cells with a membrane having a MWCO of between about 5 kDa and about 15 kDa, or more particularly about 10 kDa, to produce the low purity extract of the component protein. For various examples, the method described herein includes: (i) dialysing the culture medium and / or yeast cells with a membrane having a MWCO of between about 5 kDa and about 15 kDa, or more particularly about 10 kDa, to produce the low purity extract of the component protein; and (ii) lyophilising the low purity extract. Referring to further examples, the method described herein includes: (i) dialysing the culture medium and / or yeast cells with a membrane having a MWCO of between about 5 kDa and about 15 kDa, or more particularly about 10 kDa, to produce a first extract of the component protein; (ii) lyophilising the first extract; and (iii) reconstituting the lyophilised form of the first extract in a carrier or buffer; (iv) optionally centrifuging the reconstituted form of the first extract; and (v) filtering the first extract of step (iii) or (iv) with a membrane having a MWCO of between about 25 kDa and about 35 kDa, or more particularly about 30 kDa, to produce the low purity extract of the component protein. Non-human transferrin According to certain examples, the component protein described herein is a transferrin protein. Thus, in one form, the present disclosure provides a method of producing a transferrin protein, said method including the steps of: (a) providing a yeast cell configured to express the transferrin protein, wherein the transferrin protein is of non-human origin; (b) culturing the yeast cell in a culture medium under suitable conditions to express the transferrin protein; and (c) extracting a low purity extract of the transferrin protein from the culture medium and / or the yeast cell. Transferrin is an approximately 80 kDa glycoprotein which binds to Fe3+ions and is intimately involved in iron metabolism. In this regard, serum transferrin belongs to a family of iron-binding proteins that is involved in iron transport / metabolism in the body and can be used as an additive in media for animal cell culture. Transferrin contains two domains, namely the N-lobe and the C-lobe. It is typically synthesized as an immature protein having a secretion signal peptide at its N-terminal end and is then secreted out of the cell as a mature glycoprotein having the secretion signal peptide cleaved. The transferrin protein of the present disclosure may be any natural transferrin protein, or a modified or variant version thereof (e.g., transferrin-like proteins, transferrin analogues etc as are known in the art), derived from any non-human animal species. It is envisaged, however, that such modified or variant versions of the transferrin protein suitably do not comprise or consist of a naturally occurring, native or wildtype human transferrin protein. Suitably, the transferrin protein is derived from a non-primate animal species. Exemplary transferrin proteins include blood serotransferrin (also referred to as siderophilin), lactotransferrin (lactoferrin), milk transferrin, egg white ovotransferrin (conalbumin) and membrane-associated melanotransferrin. For some examples, the transferrin protein is derived from a livestock species, such as domestic cattle, pigs, sheep, goats, camels, water buffalo, rabbits and the like. Alternatively, the transferrin protein can be derived from poultry, such as domestic chicken, turkeys, ducks, geese, pigeons and the like. According to certain examples, the transferrin protein is derived from a game species, such as wild deer, gallinaceous fowl, waterfowl, hare, rabbit and the like. In other examples, the transferrin protein (inclusive of transferrin-like proteins, variants and analogues thereof) is derived from an aquatic species or semi-aquatic species, such as fish, crustaceans, molluscs, cephalopods, cetaceans, crocodilians, turtles, frogs and the like. Suitably, the transferrin protein, inclusive of functional variants, derivatives and fragments thereof, is derived from a mammal, and more particularly a mammalian livestock species, such as cattle, pigs, sheep, goats, horse, chickens, deer and rabbit. In certain examples, the transferrin protein is derived from a bovine animal. In other examples, the transferrin protein is derived from a porcine animal. As used herein, “bovines” are members of the mammalian sub-family Bovinae and include cattle, buffalo, bison and yaks. Cattle include all breeds and sub-species of the genus Bos, including Bos indicus and Bos taurus and hybrids thereof. The nucleotide and protein sequences of bovine transferrin are publicly available (e.g., Q29443 and NM_177484.3). An exemplary amino acid sequence is set forth in SEQ ID NOs: 1 and 2. Thus, the transferrin amino acid sequence may be a protein which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 1 or SEQ ID NO: 2 or a fragment or derivative thereof. An exemplary encoding nucleotide sequence (i.e., mRNA transcript) is set forth in SEQ ID NO: 3. Thus, the encoding nucleotide sequence of transferrin may be a nucleic acid which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 3 or a fragment or derivative thereof. Full length Bovine transferrin protein sequence (SEQ ID NO: 1) MRPAVRALLACAVLGLCLADPERTVRWCTISTHEANKCASFRENVLRILESGPFVSCVKK TSHMDCIKAISNNEADAVTLDGGLVYEAGLKPNNLKPVVAEFHGTKDNPQTHYYAVAVVK KDTDFKLNELRGKKSCHTGLGRSAGWNIPMAKLYKELPDPQESIQRAAANFFSASCVPCA DQSSFPKLCQLCAGKGTDKCACSNHEPYFGYSGAFKCLMEGAGDVAFVKHSTVFDNLPNP EDRKNYELLCGDNTRKSVDDYQECYLAMVPSHAVVARTVGGKEDVIWELLNHAQEHFGKD KPDNFQLFQSPHGKDLLFKDSADGFLKIPSKMDFELYLGYEYVTALQNLRESKPPDSSKD ECMVKWCAIGHQERTKCDRWSGFSGGAIECETAENTEECIAKIMKGEADAMSLDGGYLYI AGKCGLVPVLAENYKTEGESCKNTPEKGYLAVAVVKTSDANINWNNLKDKKSCHTAVDRT AGWNIPMGLLYSKINNCKFDEFFSAGCAPGSPRNSSLCALCIGSEKGTGKECVPNSNERY YGYTGAFRCLVEKGDVAFVKDQTVIQNTDGNNNEAWAKNLKKENFEVLCKDGTRKPVTDA ENCHLARGPNHAVVSRKDKATCVEKILNKQQDDFGKSVTDCTSNFCLFQSNSKDLLFRDD TKCLASIAKKTYDSYLGDDYVRAMTNLRQCSTSKLLEACTFHKP Mature Bovine transferrin protein sequence (SEQ ID NO: 2) DPERTVRWCTISTHEANKCASFRENVLRILESGPFVSCVKK TSHMDCIKAISNNEADAVTLDGGLVYEAGLKPNNLKPVVAEFHGTKDNPQTHYYAVAVVK KDTDFKLNELRGKKSCHTGLGRSAGWNIPMAKLYKELPDPQESIQRAAANFFSASCVPCA DQSSFPKLCQLCAGKGTDKCACSNHEPYFGYSGAFKCLMEGAGDVAFVKHSTVFDNLPNP EDRKNYELLCGDNTRKSVDDYQECYLAMVPSHAVVARTVGGKEDVIWELLNHAQEHFGKD KPDNFQLFQSPHGKDLLFKDSADGFLKIPSKMDFELYLGYEYVTALQNLRESKPPDSSKD ECMVKWCAIGHQERTKCDRWSGFSGGAIECETAENTEECIAKIMKGEADAMSLDGGYLYI AGKCGLVPVLAENYKTEGESCKNTPEKGYLAVAVVKTSDANINWNNLKDKKSCHTAVDRT AGWNIPMGLLYSKINNCKFDEFFSAGCAPGSPRNSSLCALCIGSEKGTGKECVPNSNERY YGYTGAFRCLVEKGDVAFVKDQTVIQNTDGNNNEAWAKNLKKENFEVLCKDGTRKPVTDA ENCHLARGPNHAVVSRKDKATCVEKILNKQQDDFGKSVTDCTSNFCLFQSNSKDLLFRDD TKCLASIAKKTYDSYLGDDYVRAMTNLRQCSTSKLLEACTFHKP Bovine transferrin nucleotide sequence (SEQ ID NO: 3) GCTCGGCCAGCCCCGCGGGAGATATAGGGGCGCGGGGAGCAGGGAGGCTACGCAGAAGCCGGTCGGTCTG TGCTCTGCACTCCACATTACAGACCGGGGAGGATGAGGCCCGCTGTCCGCGCTCTGTTAGCCTGCGCGGT TCTGGGGCTGTGTCTGGCGGACCCTGAGAGAACTGTGAGATGGTGCACCATTTCAACTCATGAGGCCAAT AAGTGTGCCAGTTTTCGTGAAAATGTGCTTCGTATCTTGGAGAGTGGTCCTTTTGTCTCCTGTGTGAAGA AAACCTCACACATGGATTGCATCAAGGCTATCTCGAATAACGAAGCAGATGCTGTGACGTTGGATGGAGG TTTGGTGTATGAGGCAGGCCTGAAGCCCAACAACCTGAAGCCTGTGGTGGCAGAGTTCCATGGGACAAAA GACAACCCACAAACTCACTATTATGCTGTGGCGGTGGTGAAGAAGGACACTGACTTCAAGCTGAATGAGC TCAGAGGCAAGAAGTCCTGCCACACAGGCCTCGGCAGGTCCGCTGGGTGGAACATCCCCATGGGCAAACT TTATAAGGAATTGCCTGATCCACAGGAATCAATTCAGAGAGCTGCGGCCAACTTCTTCTCTGCCAGCTGT GTCCCCTGTGCGGATCAATCATCATTTCCCAAACTCTGTCAACTGTGTGCGGGGAAAGGGACAGACAAGT GTGCCTGCTCCAACCACGAACCATACTTCGGCTACTCAGGGGCCTTTAAATGTCTGATGGAGGGCGCTGG GGATGTGGCCTTTGTCAAGCACTCAACGGTATTCGACAACCTGCCTAACCCAGAAGACAGGAAAAACTAT GAGCTGCTCTGCGGGGACAACACCCGGAAGTCTGTAGATGATTACCAAGAATGCTACTTGGCAATGGTTC CTTCCCATGCGGTTGTGGCTCGAACTGTGGGCGGCAAGGAGGATGTGATCTGGGAACTTCTCAATCACGC CCAGGAACATTTTGGCAAAGATAAACCAGACAATTTCCAGCTTTTCCAATCCCCTCATGGGAAGGACCTG CTGTTTAAGGACTCTGCTGATGGGTTTTTAAAGATTCCTTCTAAGATGGATTTTGAGCTGTACTTGGGAT ATGAATATGTCACTGCTCTTCAGAATCTAAGAGAAAGTAAACCCCCGGACTCCTCAAAGGATGAGTGCAT GGTGAAGTGGTGTGCAATTGGTCACCAGGAGAGGACAAAGTGTGATCGGTGGAGTGGGTTCAGCGGCGGG GCAATAGAGTGTGAAACAGCAGAGAACACTGAAGAGTGCATCGCCAAGATCATGAAAGGAGAAGCTGATG CCATGAGCTTGGACGGAGGCTACCTCTACATAGCAGGCAAATGTGGCCTGGTGCCTGTCCTGGCAGAGAA CTACAAAACTGAGGGTGAAAGTTGCAAGAACACACCAGAAAAAGGGTATCTTGCTGTAGCTGTGGTTAAA ACATCAGATGCTAATATCAACTGGAACAATCTGAAAGACAAGAAGTCCTGCCACACTGCAGTAGACAGAA CCGCTGGCTGGAACATCCCCATGGGTTTGCTCTACAGCAAGATTAATAACTGTAAATTTGATGAATTTTT CAGCGCAGGTTGTGCACCTGGATCTCCGCGAAATTCCAGTCTCTGTGCTCTGTGCATTGGCTCAGAGAAG GGTACAGGAAAGGAGTGTGTTCCCAACAGCAATGAAAGATACTATGGCTATACAGGGGCTTTCAGGTGTC TGGTGGAGAAGGGAGACGTGGCCTTTGTGAAGGACCAGACTGTCATACAGAACACTGACGGAAATAATAA TGAAGCATGGGCAAAAAATCTGAAGAAGGAAAATTTTGAAGTACTATGCAAAGATGGCACCAGGAAACCT GTGACAGATGCTGAGAACTGCCACCTGGCCCGAGGCCCGAATCATGCTGTGGTCTCACGGAAAGATAAGG CAACTTGTGTGGAGAAAATATTAAACAAACAGCAGGATGATTTTGGAAAATCTGTAACCGACTGCACGAG CAATTTTTGTTTATTCCAATCAAATTCCAAGGACCTTCTGTTCAGGGATGACACTAAATGTTTGGCTTCA ATTGCGAAAAAAACATATGACTCCTACTTAGGGGATGACTACGTCAGAGCTATGACCAACCTGAGACAAT GCTCAACCTCAAAACTCCTGGAAGCATGCACTTTCCACAAACCTTAAAATCCAAGAGTGGAGCCAACACC TGATGGAGATGGGAGCTCATGGGACCCATAAGCTTCATCTGGTTTCGCTGGTCTGAGTGATTTGGTTGCC CTCACAATTTGGTGGTGGCGCCTCTGCAGGACAAAATAAAAATAAACATTATTATTGGTTTTATCTGTTA AAAAAACTTCATTTTCCCCTCTTAATGCTTGGTCTGCAACTAGCCCTTCCTTCAGAGCTCAAGATTCATT TGCTCTTTCCCACAGCCACAGTTCCTGGGTACAGTCCAAGTAGGAATTCTTTCTAAAAGGGGTCTGCATG GTCATTTTAATAAAATCAAGTGTACAAGGAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA As used herein, the term “porcine” refers to animals of the Suidae family, which include pigs, boars and warthogs. The term also refers to any pig species or strain, such as Large White, Landrace, Meishan and Minipig. The nucleotide and protein sequences of porcine transferrin are publicly available (e.g., CAQ34904.1 and NM_001244653.1). An exemplary amino acid sequence is set forth in SEQ ID NOs: 4 and 5. Thus, the transferrin amino acid sequence may be a protein which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO:4 or SEQ ID NO: 5 or a fragment or derivative thereof. An exemplary encoding nucleotide sequence which has been codon optimised for expression in yeast cells is set forth in SEQ ID NO: 6. Thus, the encoding nucleotide sequence of transferrin may be a nucleic acid which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO:6 or a fragment or derivative thereof. Full length Porcine transferrin protein sequence (SEQ ID NO: 4) MRLAVRALLACAVLGLSLAVAQKTVRWCTISNQEANKCSSFRENMSKAVKNGPLVSCVKKSSYLDCIKAI RDKEADAVTLDAGLVFEAGLAPYNLKPVVAEFYGQKDNPQTHYYAVAVVKKGSNFQWNQLQGKRSCHTGL GRSAGWIIPMGLLYDQLPEPRKPIEKAVASFFSSSCVPCADPVNFPKLCQQCAGKGAEKCACSNHEPYFG YAGAFNCLKEDAGDVAFVKHSTVLENLPDKADRDQYELLCRDNTRRPVDDYENCYLAQVPSHAVVARSVD GQEDSIWELLNQAQEHFGRDKSPDFQLFSSSHGKDLLFKDSANGFLRIPSKMDSSLYLGYQYVTALRNLR EEISPDSSKNECKKVRWCAIGHEETQKCDAWSINSGGKIECVSAENTEDCIAKIVKGEADAMSLDGGYIY IAGKCGLVPVLAENYKTEGENCVNTPEKGYLAVAVVKKSSGPDLNWNNLKGKKSCHTAVDRTAGWNIPMG LLYNKINSCKFDQFFGEGCAPGSQRNSSLCALCIGSERAPGRECLANNHERYYGYTGAFRCLVEKGDVAF VKDQVVQQNTDGKNKDDWAKDLKQMDFELLCQNGAREPVDNAENCHLARAPNHAVVARDDKVTCVAEELL KQQAQFGRHVTDCSSSFCMFKSNTKDLLFRDDTQCLARVGKTTYESYLGADYITAVANLRKCSTSKLLEA CTFHSAKNPRVETTT Mature Porcine transferrin protein sequence (SEQ ID NO: 5) QKTVRWCTISNQEANKCSSFRENMSKAVKNGPLVSCVKKSSYLDCIKAI RDKEADAVTLDAGLVFEAGLAPYNLKPVVAEFYGQKDNPQTHYYAVAVVKKGSNFQWNQLQGKRSCHTGL GRSAGWIIPMGLLYDQLPEPRKPIEKAVASFFSSSCVPCADPVNFPKLCQQCAGKGAEKCACSNHEPYFG YAGAFNCLKEDAGDVAFVKHSTVLENLPDKADRDQYELLCRDNTRRPVDDYENCYLAQVPSHAVVARSVD GQEDSIWELLNQAQEHFGRDKSPDFQLFSSSHGKDLLFKDSANGFLRIPSKMDSSLYLGYQYVTALRNLR EEISPDSSKNECKKVRWCAIGHEETQKCDAWSINSGGKIECVSAENTEDCIAKIVKGEADAMSLDGGYIY IAGKCGLVPVLAENYKTEGENCVNTPEKGYLAVAVVKKSSGPDLNWNNLKGKKSCHTAVDRTAGWNIPMG LLYNKINSCKFDQFFGEGCAPGSQRNSSLCALCIGSERAPGRECLANNHERYYGYTGAFRCLVEKGDVAF VKDQVVQQNTDGKNKDDWAKDLKQMDFELLCQNGAREPVDNAENCHLARAPNHAVVARDDKVTCVAEELL KQQAQFGRHVTDCSSSFCMFKSNTKDLLFRDDTQCLARVGKTTYESYLGADYITAVANLRKCSTSKLLEA CTFHSAKNPRVETTT Porcine transferrin nucleotide sequence (SEQ ID NO: 6) ATGAGATTGGCCGTTAGAGCTTTGTTGGCCTGTGCTGTTTTGGGTTTGTCTTTGGCTGTTGCCCAAAAGACCGTTAG ATGGTGTACCATCTCCAATCAAGAGGCTAACAAGTGCTCCTCCTTCCGTGAAAACATGTCCAAGGCTGTTAAGAACG GTCCATTGGTGTCCTGTGTTAAGAAGTCCTCCTACCTGGACTGTATCAAGGCCATCAGAGACAAAGAAGCTGACGCT GTTACTTTGGACGCCGGTTTGGTTTTTGAAGCTGGTTTGGCTCCATACAACCTGAAGCCAGTTGTTGCTGAGTTCTA CGGTCAAAAGGACAACCCACAGACTCACTACTACGCTGTTGCTGTTGTCAAGAAGGGTTCCAACTTCCAGTGGAACC AGTTGCAAGGTAAGAGATCCTGTCACACCGGTTTGGGTAGATCTGCTGGTTGGATTATCCCAATGGGTTTGTTGTAC GACCAGCTGCCAGAACCTAGAAAGCCAATTGAAAAGGCTGTGGCCTCCTTCTTCTCCTCTTCTTGTGTTCCATGTGC TGACCCAGTCAACTTCCCAAAGTTGTGTCAACAGTGTGCTGGTAAGGGTGCTGAGAAGTGTGCTTGTTCTAACCACG AACCATACTTCGGTTACGCTGGTGCCTTCAACTGTTTGAAAGAAGATGCTGGCGACGTTGCCTTCGTTAAGCACTCT ACTGTTTTGGAGAACCTGCCAGACAAGGCTGACAGAGATCAATACGAGTTGCTGTGCAGAGACAACACTAGAAGGCC AGTTGACGACTACGAGAACTGTTACTTGGCTCAGGTTCCATCTCACGCTGTTGTTGCTAGATCTGTTGATGGTCAAG AGGACTCCATCTGGGAGTTGTTGAATCAAGCTCAAGAGCACTTCGGTAGAGACAAGTCTCCAGACTTTCAGCTGTTC TCTTCATCCCACGGTAAGGACTTGTTGTTCAAGGACTCTGCCAACGGTTTCCTGAGAATCCCTTCTAAGATGGACTC CTCCTTGTACCTGGGTTACCAGTACGTTACTGCCCTGAGAAACCTGAGAGAAGAGATTTCCCCAGACTCCTCCAAGA ACGAGTGCAAGAAAGTCAGATGGTGTGCTATCGGTCACGAAGAGACTCAAAAGTGTGACGCTTGGTCCATTAACTCC GGTGGTAAGATTGAGTGTGTTTCCGCTGAGAACACCGAGGACTGTATTGCCAAGATCGTTAAGGGTGAAGCCGACGC TATGTCTTTGGATGGTGGTTACATCTACATTGCCGGTAAGTGTGGTTTGGTTCCAGTTTTGGCTGAGAACTACAAGA CCGAGGGTGAGAACTGTGTGAACACTCCAGAAAAGGGTTACCTGGCAGTTGCCGTTGTGAAGAAATCTTCTGGTCCA GACTTGAACTGGAACAACTTGAAGGGTAAGAAGTCTTGCCACACCGCTGTTGATAGAACTGCCGGTTGGAACATTCC TATGGGTCTGCTGTACAACAAGATCAACTCCTGCAAGTTCGACCAGTTCTTCGGTGAAGGTTGTGCTCCAGGTTCTC AAAGAAACTCTTCCTTGTGCGCCTTGTGTATTGGTTCTGAACGTGCTCCTGGTAGAGAGTGTTTGGCTAACAACCAC GAAAGGTACTACGGTTACACCGGTGCTTTCAGATGCTTGGTTGAAAAGGGTGACGTCGCTTTCGTCAAGGACCAGGT TGTTCAACAAAACACCGACGGTAAGAACAAGGACGACTGGGCTAAAGACTTGAAGCAGATGGACTTCGAGCTGTTGT GTCAGAACGGTGCTAGAGAGCCAGTTGATAACGCTGAAAACTGTCATTTGGCTAGAGCCCCAAACCATGCCGTTGTT GCAAGAGATGACAAGGTTACTTGTGTCGCCGAAGAGTTGCTGAAGCAACAAGCTCAATTCGGTAGACACGTTACCGA CTGTTCCTCATCCTTCTGCATGTTCAAGTCCAACACCAAGGACCTGCTGTTCAGAGATGATACTCAGTGCTTGGCCA GAGTTGGTAAGACTACTTACGAGTCCTACTTGGGTGCTGACTACATCACTGCTGTTGCCAACTTGAGAAAGTGCTCC ACCTCTAAGTTGTTGGAGGCTTGTACTTTCCACTCCGCTAAGAACCCAAGAGTTGAGACTACTACTTAA Alternatively, the transferrin protein, inclusive of functional variants, derivatives and fragments thereof, can be derived from a fish species. Exemplary fish species include, but are not limited to, salmon, tuna, pollack, catfish, cod, haddock, sea bass, tilapia, and carp. In particular examples, the transferrin protein is derived from a teleost species, such as perch, salmon, tuna, trout, bass, swordfish etc. The nucleotide and protein sequences of teleost transferrin are publicly available (e.g., P79815 and D89084.1 of Coho salmon). An exemplary amino acid sequence is set forth in SEQ ID NOs: 7 and 8. Thus, the transferrin amino acid sequence may be a protein which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 7 or SEQ ID NO: 8 or a fragment or derivative thereof. An exemplary encoding nucleotide sequence which has been codon optimised for expression in yeast cells is set forth in SEQ ID NO: 9. Thus, the encoding nucleotide sequence of transferrin may be a nucleic acid which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 9 or a fragment or derivative thereof. Full length Teleost transferrin protein sequence (SEQ ID NO: 7) MKLLLVSALLGCFATVYAAPAEGMVRWCVKSEKELKKCHDLAANVAGFSCVRRDDSLECI QAIKREEADAITLDGGDIYIAGLHNYNLQPIIAEDYGEDSDTCYYAVAVAKKGTEFGFLD LRGKKSCHTGLGKSAGWNIPIGTLVTVGQIQWAGIEDRPVESAVSDFFNASCAPGANRDS QLCQLCMGDCSRSHNEPYYDYSGAFQCLKDGAGEVAFIKHLTVPAAEKASYELLCKDNTR APIDSYKTCHLARVPAHAVVSRKDPRLANLIYSKLMAVTNFNLFSSDGYAAKNLMFKDST QNLVQLPMTTDSFLYLGAEYMSTIRSLTKAQATGVTSRAIKWCAVGHKEKVKCDAWTINS FTDGDSRIECQDAPTVDECIKKIMRKEADAIAVDGGEVFTAGKCGLVPVMVEQYDEVRCS APGEASSYFAVAVAKRGSGLTWTTLKGKRSCHTGLGRTAGWNIPMGLIHRRTMNCDFTTY FSKGCAPGFEVDSPFCAQCKGSGKSVGGDGSKCKASSEEQYYGYNGAFRCLVEDAGDVAF IKHTIVPEMTDGSGPVWAQNLMSSDFELLCQDGTTKPVTHFRECHLAKVPAHAVITRPES RGEVVSILLEQQARFGSSGSDSSFNMFKPDFGKNLLFKDSTKCLQEIPSGTKFQGFLGEE YMIAMQSLRECSNSTSDLEKACTFHSC Mature Teleost transferrin protein sequence (SEQ ID NO: 8) APAEGMVRWCVKSEKELKKCHDLAANVAGFSCVRRDDSLECI QAIKREEADAITLDGGDIYIAGLHNYNLQPIIAEDYGEDSDTCYYAVAVAKKGTEFGFLD LRGKKSCHTGLGKSAGWNIPIGTLVTVGQIQWAGIEDRPVESAVSDFFNASCAPGANRDS QLCQLCMGDCSRSHNEPYYDYSGAFQCLKDGAGEVAFIKHLTVPAAEKASYELLCKDNTR APIDSYKTCHLARVPAHAVVSRKDPRLANLIYSKLMAVTNFNLFSSDGYAAKNLMFKDST QNLVQLPMTTDSFLYLGAEYMSTIRSLTKAQATGVTSRAIKWCAVGHKEKVKCDAWTINS FTDGDSRIECQDAPTVDECIKKIMRKEADAIAVDGGEVFTAGKCGLVPVMVEQYDEVRCS APGEASSYFAVAVAKRGSGLTWTTLKGKRSCHTGLGRTAGWNIPMGLIHRRTMNCDFTTY FSKGCAPGFEVDSPFCAQCKGSGKSVGGDGSKCKASSEEQYYGYNGAFRCLVEDAGDVAF IKHTIVPEMTDGSGPVWAQNLMSSDFELLCQDGTTKPVTHFRECHLAKVPAHAVITRPES RGEVVSILLEQQARFGSSGSDSSFNMFKPDFGKNLLFKDSTKCLQEIPSGTKFQGFLGEE YMIAMQSLRECSNSTSDLEKACTFHSC Teleost transferrin nucleotide sequence (SEQ ID NO: 9) ATGAAGTTGTTGCTGGTTTCCGCCTTGTTGGGTTGTTTCGCTACTGTTTATGCTGCTCCAGCCGAAGGTATGGTTAG ATGGTGTGTTAAGTCCGAGAAAGAACTGAAGAAGTGCCACGACTTGGCTGCTAACGTTGCCGGTTTTTCCTGTGTCA GAAGAGATGACTCCTTGGAGTGTATCCAGGCTATCAAGAGAGAAGAGGCTGACGCTATTACTTTGGACGGTGGTGAC ATCTACATTGCCGGATTGCACAACTACAACCTGCAGCCAATTATCGCTGAGGATTACGGTGAAGATTCCGACACTTG TTACTACGCTGTTGCCGTTGCTAAGAAGGGTACTGAGTTCGGTTTCTTGGACCTGAGAGGTAAGAAGTCCTGTCACA CTGGTTTGGGAAAGTCTGCTGGTTGGAACATCCCAATCGGTACTTTGGTTACCGTTGGTCAGATTCAGTGGGCTGGT ATTGAGGACAGACCAGTTGAATCTGCTGTCTCCGATTTCTTCAACGCTTCTTGTGCTCCAGGTGCTAACAGAGACTC TCAATTGTGTCAGTTGTGCATGGGTGACTGCTCTAGATCTCACAACGAACCTTACTACGACTACTCCGGTGCTTTCC AGTGTTTGAAGGATGGTGCTGGTGAGGTCGCTTTCATCAAGCACTTGACTGTTCCAGCTGCTGAGAAGGCTTCTTAC GAGTTGTTGTGTAAGGACAACACTAGAGCCCCAATCGACTCCTACAAGACTTGTCACTTGGCTAGAGTTCCAGCTCA CGCTGTTGTTTCCAGAAAGGATCCAAGATTGGCCAACCTGATCTACTCCAAGTTGATGGCCGTTACCAACTTCAACT TGTTCTCCTCTGATGGTTACGCCGCCAAGAACTTGATGTTCAAGGACTCCACTCAGAACTTGGTCCAGTTGCCAATG ACTACCGACTCCTTCTTGTACTTGGGTGCTGAGTACATGTCCACCATCAGATCCTTGACTAAGGCTCAGGCTACTGG TGTTACTTCCAGAGCCATTAAGTGGTGTGCTGTCGGTCACAAAGAAAAGGTTAAGTGTGACGCCTGGACTATCAACT CCTTCACTGATGGTGACTCCAGAATCGAGTGTCAAGACGCTCCAACTGTTGACGAGTGCATCAAGAAAATCATGAGG AAAGAGGCCGACGCCATTGCTGTTGATGGTGGTGAAGTTTTCACTGCCGGTAAGTGTGGTTTGGTTCCAGTTATGGT TGAGCAGTACGACGAGGTTAGATGTTCTGCTCCTGGTGAAGCTTCTTCCTACTTTGCTGTTGCTGTCGCCAAGAGAG GTTCTGGTTTGACTTGGACTACCCTGAAGGGTAAGAGATCTTGTCATACCGGTCTGGGTAGAACTGCCGGATGGAAC ATTCCTATGGGTTTGATCCATAGAAGGACCATGAACTGCGACTTCACTACCTACTTCTCCAAGGGTTGTGCCCCAGG TTTCGAAGTTGATTCTCCATTCTGTGCCCAGTGCAAAGGTTCCGGTAAATCTGTTGGTGGTGACGGTTCTAAGTGTA AGGCTTCCTCTGAGGAACAGTACTACGGTTACAACGGTGCCTTCAGATGTTTGGTTGAAGATGCTGGTGACGTGGCC TTTATTAAGCACACTATCGTTCCAGAGATGACCGACGGTTCTGGTCCTGTTTGGGCTCAAAATTTGATGTCCTCCGA CTTCGAGCTGTTGTGCCAAGATGGTACTACCAAGCCAGTCACTCACTTCAGAGAGTGTCATTTGGCCAAGGTTCCAG CACATGCCGTTATTACTAGACCAGAATCCAGAGGTGAGGTGGTTTCCATTTTGTTGGAGCAACAGGCTAGATTCGGT TCTTCTGGTTCCGACTCCAGCTTCAATATGTTCAAGCCAGACTTCGGTAAGAACCTGCTGTTTAAGGATTCTACCAA GTGCCTGCAAGAGATCCCATCCGGTACTAAGTTCCAAGGTTTTCTGGGTGAAGAGTACATGATCGCCATGCAATCTT TGAGAGAGTGCTCCAACTCTACTTCCGACTTGGAAAAGGCTTGTACCTTCCACTCCTGTTAA According to other examples, the transferrin protein, inclusive of functional variants, derivatives and fragments thereof, can be derived from a bird or avian species and more particularly a poultry species. The nucleotide and protein sequences of poultry transferrin (i.e., ovotransferrin) are publicly available (e.g., P02789 and NM_205304.2 of Gallus gallus (chicken)). An exemplary amino acid sequence is set forth in SEQ ID NO: 10 and 11. Thus, the transferrin amino acid sequence may be a protein which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 10 or SEQ ID NO: 11 or a fragment or derivative thereof. An exemplary encoding nucleotide sequence which has been codon optimised for expression in yeast cells is set forth in SEQ ID NO: 12. Thus, the encoding nucleotide sequence of transferrin may be a nucleic acid which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 12 or a fragment or derivative thereof. Full length Chicken transferrin protein sequence (SEQ ID NO: 10) MKLILCTVLSLGIAAVCFAAPPKSVIRWCTISSPEEKKCNNLRDLTQQERISLTCVQKAT YLDCIKAIANNEADAISLDGGQAFEAGLAPYKLKPIAAEVYEHTEGSTTSYYAVAVVKKG TEFTVNDLQGKTSCHTGLGRSAGWNIPIGTLLHRGAIEWEGIESGSVEQAVAKFFSASCV PGATIEQKLCRQCKGDPKTKCARNAPYSGYSGAFHCLKDGKGDVAFVKHTTVNENAPDQK DEYELLCLDGSRQPVDNYKTCNWARVAAHAVVARDDNKVEDIWSFLSKAQSDFGVDTKSD FHLFGPPGKKDPVLKDLLFKDSAIMLKRVPSLMDSQLYLGFEYYSAIQSMRKDQLTPSPR ENRIQWCAVGKDEKSKCDRWSVVSNGDVECTVVDETKDCIIKIMKGEADAVALDGGLVYT AGVCGLVPVMAERYDDESQCSKTDERPASYFAVAVARKDSNVNWNNLKGKKSCHTAVGRT AGWVIPMGLIHNRTGTCNFDEYFSEGCAPGSPPNSRLCQLCQGSGGIPPEKCVASSHEKY FGYTGALRCLVEKGDVAFIQHSTVEENTGGKNKADWAKNLQMDDFELLCTDGRRANVMDY RECNLAEVPTHAVVVRPEKANKIRDLLERQEKRFGVNGSEKSKFMMFESQNKDLLFKDLT KCLFKVREGTTYKEFLGDKFYTVISSLKTCNPSDILQMCSFLEGK Mature Chicken transferrin protein sequence (SEQ ID NO: 11) APPKSVIRWCTISSPEEKKCNNLRDLTQQERISLTCVQKAT YLDCIKAIANNEADAISLDGGQAFEAGLAPYKLKPIAAEVYEHTEGSTTSYYAVAVVKKG TEFTVNDLQGKTSCHTGLGRSAGWNIPIGTLLHRGAIEWEGIESGSVEQAVAKFFSASCV PGATIEQKLCRQCKGDPKTKCARNAPYSGYSGAFHCLKDGKGDVAFVKHTTVNENAPDQK DEYELLCLDGSRQPVDNYKTCNWARVAAHAVVARDDNKVEDIWSFLSKAQSDFGVDTKSD FHLFGPPGKKDPVLKDLLFKDSAIMLKRVPSLMDSQLYLGFEYYSAIQSMRKDQLTPSPR ENRIQWCAVGKDEKSKCDRWSVVSNGDVECTVVDETKDCIIKIMKGEADAVALDGGLVYT AGVCGLVPVMAERYDDESQCSKTDERPASYFAVAVARKDSNVNWNNLKGKKSCHTAVGRT AGWVIPMGLIHNRTGTCNFDEYFSEGCAPGSPPNSRLCQLCQGSGGIPPEKCVASSHEKY FGYTGALRCLVEKGDVAFIQHSTVEENTGGKNKADWAKNLQMDDFELLCTDGRRANVMDY RECNLAEVPTHAVVVRPEKANKIRDLLERQEKRFGVNGSEKSKFMMFESQNKDLLFKDLT KCLFKVREGTTYKEFLGDKFYTVISSLKTCNPSDILQMCSFLEGK Chicken transferrin nucleotide sequence (SEQ ID NO: 12) ATGAAGTTGATCTTGTGCACCGTCTTGTCCTTGGGTATCGCTGCTGTTTGTTTTGCTGCTCCACCAAAGTCCGTTAT CAGGTGGTGTACTATCTCCTCACCTGAAGAGAAGAAGTGCAACAACCTGAGAGACTTGACCCAGCAAGAGAGAATCT CCTTGACCTGTGTTCAGAAGGCTACCTACTTGGACTGCATCAAGGCTATCGCTAACAACGAGGCTGACGCTATTTCT TTGGATGGTGGTCAAGCTTTCGAGGCTGGTTTGGCTCCATACAAGTTGAAGCCAATTGCTGCCGAAGTTTACGAGCA CACTGAAGGTTCTACTACCTCCTACTACGCTGTTGCCGTTGTCAAGAAGGGTACTGAGTTCACCGTTAACGACTTGC AGGGTAAGACTTCCTGTCACACTGGTTTGGGTAGATCTGCTGGTTGGAACATCCCAATCGGTACTTTGTTGCACAGA GGTGCTATTGAGTGGGAGGGTATTGAATCTGGTTCCGTTGAACAGGCTGTCGCCAAGTTTTTCTCTGCTTCTTGTGT TCCAGGTGCCACCATCGAGCAAAAGTTGTGTAGACAATGTAAGGGTGACCCCAAGACCAAGTGTGCTAGAAACGCTC CATACTCTGGTTACTCCGGTGCTTTCCACTGTTTGAAGGATGGAAAGGGTGACGTCGCTTTCGTTAAGCACACTACT GTTAACGAGAACGCCCCAGACCAAAAGGACGAATACGAGTTGTTGTGTCTGGACGGTTCCAGACAACCAGTCGACAA CTACAAGACTTGTAACTGGGCTAGAGTTGCTGCTCACGCTGTTGTTGCTAGAGATGACAACAAGGTTGAGGACATCT GGTCCTTTTTGTCCAAGGCTCAATCCGACTTCGGTGTTGACACTAAGTCTGACTTCCACTTGTTTGGTCCACCAGGT AAGAAGGACCCAGTCTTGAAGGACTTGTTGTTCAAGGACTCCGCCATCATGTTGAAGAGGGTTCCATCTTTGATGGA CTCCCAGTTGTACCTGGGTTTCGAGTACTACTCCGCTATCCAGTCTATGAGGAAGGACCAGTTGACTCCATCTCCAC GTGAGAACAGAATTCAGTGGTGTGCTGTTGGTAAGGACGAGAAGTCTAAGTGCGACAGATGGTCCGTTGTTTCCAAC GGTGACGTTGAGTGTACTGTTGTCGACGAAACTAAGGACTGTATCATCAAGATCATGAAGGGTGAAGCTGACGCCGT TGCTCTTGATGGTGGTTTGGTTTACACTGCCGGTGTTTGTGGATTGGTTCCAGTTATGGCTGAGAGATACGACGACG AATCCCAGTGTTCTAAGACCGACGAAAGACCAGCTTCTTACTTCGCAGTTGCTGTTGCCAGAAAGGACTCCAACGTC AACTGGAACAACTTGAAGGGAAAGAAGTCTTGTCACACCGCCGTTGGAAGAACTGCTGGATGGGTTATTCCAATGGG TCTGATCCACAACAGAACCGGTACTTGCAACTTCGACGAATACTTCTCTGAGGGTTGTGCTCCAGGTTCTCCACCTA ACTCTAGATTGTGTCAGTTGTGCCAAGGTTCCGGTGGTATTCCACCAGAAAAGTGTGTTGCTTCCAGCCACGAAAAG TACTTCGGTTACACTGGTGCCTTGAGATGCTTGGTTGAGAAGGGTGATGTTGCCTTCATTCAGCACTCTACCGTTGA AGAAAACACCGGTGGTAAGAACAAGGCTGACTGGGCTAAGAACTTGCAGATGGATGACTTCGAGCTGTTGTGTACCG ATGGTAGAAGGGCTAACGTCATGGACTACAGAGAGTGTAACTTGGCCGAAGTTCCAACTCATGCCGTTGTTGTTAGA CCAGAGAAGGCCAACAAGATCAGAGACTTGTTGGAGAGACAAGAGAAGAGATTCGGTGTCAACGGTTCCGAAAAGTC CAAGTTCATGATGTTCGAGTCCCAGAACAAGGATCTGCTGTTTAAGGACTTGACCAAGTGCCTTTTCAAGGTCAGAG AGGGCACCACCTACAAAGAGTTCTTGGGAGACAAGTTCTACACCGTCATCTCTTCCCTGAAAACTTGCAACCCATCC GACATCCTGCAGATGTGTTCATTCTTGGAGGGAAAGTAA By “protein” is meant an amino acid polymer. The amino acids may be natural or non- natural amino acids, D- or L-amino acids as are well understood in the art. The term “protein” includes and encompasses “peptide”, which is typically used to describe a protein having no more than fifty (50) amino acids (e.g., no more than 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49 or 50 amino acids and any range therein) and “polypeptide”, which is typically used to describe a protein having more than fifty (50) amino acids. Suitably, the transferrin protein produced by the methods herein is a mature transferrin protein (i.e., a transferrin protein which does not include a wildtype signal or leader sequence). Thus, the transferrin amino acid sequence may be a protein which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to any one of SEQ ID NOs: 1, 4, 7 or 10 or a fragment or derivative thereof. In some examples, however, the transferrin protein includes a wildtype signal sequence and one or more heterologous signal sequences. Accordingly, the transferrin amino acid sequence may be a protein which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to any one of SEQ ID NOs: 2, 5, 8 or 11 or a fragment or derivative thereof. Transferrin protein variants are contemplated for the present disclosure. As used herein, a protein, polypeptide or peptide “variant” shares a definable amino acid sequence relationship with a reference amino acid sequence. In particular examples, the reference amino acid sequence is that of a transferrin protein. As such, the reference amino acid sequence may be the amino acid sequence of any one of SEQ ID NOs: 1, 2, 4, 5, 7, 8, 10 or 11. The “variant” protein, polypeptide or peptide may have one or a plurality of amino acids of the reference amino acid sequence deleted, inserted / added or substituted by different amino acids. It is well understood in the art that some amino acids may be substituted, inserted / added or deleted without changing the activity of the transferrin protein (i.e., conservative substitutions). Accordingly, one or more of the residues of a transferrin protein (e.g., those defined by SEQ ID NOs: 1, 2, 4, 5, 7, 8, 10 or 11) may be conservatively modified (e.g., by amino acid substitution or deletion) without altering the biological activity, function, or other desired property of the transferrin protein, such as its affinity or its specificity for Fe3+molecules and / or a transferrin receptor or a transferrin binding protein on the cell surface to be incorporated into the cell. It is further contemplated that one or more amino acid residues of a transferrin protein may be modified to, for example, improve the stability of the transferrin protein in the plant (e.g., improved resistance to degradation by plant proteases and degradation pathways) and / or improve the binding affinity of the transferrin protein for a transferrin receptor, such as a transferrin receptor of a different species. Suitably, the transferrin protein, inclusive of variants thereof, does not comprise or consist of a wild-type transferrin protein derived from a human or a primate species, as are known in the art. Typically seen as conservative substitutions are the replacements, one for another, among the aliphatic amino acids Ala, Val, Leu, and Ile; interchange of the hydroxyl residues Ser and Thr; exchange of the acidic residues Asp and Glu; substitution between the amide residues Asn and Gln; exchange of the basic residues Lys and Arg; and replacements among the aromatic residues Phe and Tyr. Guidance concerning which amino acid changes are likely to be phenotypically silent can be found in, for example, Bowie et al., Science 247:1306-1310 (1990). In some examples, a variant of the transferrin protein provided herein substantially retains the Fe3+and / or transferrin receptor binding ability of the unmodified, wild-type or reference transferrin protein (e.g., a functional variant). Thus, one or more amino acid residues within the transferrin protein of the present disclosure can be deleted or replaced with other amino acid residues, and the variant transferrin protein can be tested for retained function (e.g., the ability to specifically bind an Fe3+molecule and / or a transferrin receptor at high affinity) using functional assays described in the art. The terms “specifically binds” or “selectively binds” can be used interchangeably herein and shall be taken to mean that the binding interaction between a transferrin protein disclosed herein and a target molecule described herein (e.g., a Fe3+molecule or a transferrin receptor) is dependent on detection of the target molecule by the transferrin protein. Accordingly, the transferrin protein, inclusive of fragments, variants or derivatives thereof, suitably preferentially binds or recognizes the target molecule even when present in a mixture of other molecules. The formation of a complex with a target molecule that is relatively stable under physiologic conditions can also be a characteristic of such selective or specific binding by a transferrin protein. The terms “high affinity” and “relatively high affinity” are used interchangeably herein and generally refer to a binding affinity between a transferrin protein, inclusive of fragments, variants or derivatives thereof, and the target molecule of interest with a KDof at least about 10-6M (e.g., at least about 10-6, 10-7, 10-8, 10-9, 10-10, 10-11, 10-12, 10-13, 10-14, 10-15, 10-16, 10-17, 10-18, 10-19, 10-20, 10-21, 10-22M or any range therein), more particularly at least about 10-7M, even more particularly at least about 10-8M and still even more particularly at least about 10-9M. Again, the determination of such affinity may be conducted under standard competitive binding immunoassay procedures, such as equilibrium dialysis, surface plasmon resonance (SPR; e.g., Biacore assays), fluorescent-activated cell sorting (FACS) binding assays and the like. In particular examples, a binding affinity of the transferrin protein, inclusive of fragments, variants or derivatives thereof, for Fe3+is defined by a KDof about 10-18M to about 10-22M. Suitably, protein, polypeptide or peptide variants provided herein share at least 70% or 75%, more particularly at least 80% or 85% or even more particularly at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% sequence identity with a reference amino acid sequence, such as those set forth in SEQ ID NOs: 1, 2, 4, 5, 7, 8, 10 or 11. The present disclosure also provides fragments of the transferrin proteins. As used herein, a “fragment” is a segment, domain, portion or region of a protein or peptide (such as those set forth in SEQ ID NOs: 1, 2, 4, 5, 7, 8, 10 or 11) which constitutes less than 100% of the amino acid sequence of the protein or peptide. In general, fragments may comprise, consist essentially of or consist of up to 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475, 500, 525, 550, 575, 600, 625, 650, 655, 660, 665, 670, 675, 676, 677, 678, 679, 680, 681, 682, 683, 684, 685, 686, 687, 688, 689, 690, 691, 692, 693, 694, 695, 696, 697, 698, 699, 700, 701, 702, 703, 704, 705, 706, 707, 708, 709, 710, 711, 712, 713, or 714 contiguous amino acids of a transferrin protein (such as of SEQ ID NOs: 1, 2, 4, 5, 7, 8, 10 or 11). In a particular example, the protein fragment is or comprises a conserved region or one or more conserved amino acids of a transferrin protein. In various examples, the fragment comprises, or is contained within, a transferrin protein, such as those set forth in SEQ ID NOs: 1, 2, 4, 5, 7, 8, 10 or 11. The present disclosure also contemplates derivatives of the transferrin proteins described herein. As used herein, “derivatives” are molecules such as proteins, fragments or variants thereof that have been altered, for example, by conjugation or complexing with other chemical moieties, by post-translational modification (e.g., phosphorylation, acetylation and the like), modification of glycosylation (e.g., adding, removing or altering glycosylation), lipidation and / or inclusion of additional amino acid sequences as would be understood in the art. Additional amino acid sequences may include fusion partner amino acid sequences which create a fusion protein. By way of example, fusion partner amino acid sequences may assist in detection and / or purification of the isolated fusion protein. Non-limiting examples include metal- binding (e.g., polyhistidine) fusion partners, maltose binding protein (MBP), Protein A, glutathione S-transferase (GST), fluorescent protein sequences (e.g., GFP), polylysine, epitope tags, such as myc, FLAG and haemagglutinin tags or an affinity tag (e.g., a streptavidin tag). Such derivatives, variants and fragments for use according to the present disclosure are suitably functional derivatives, variants and fragments of the transferrin protein. By “functional transferrin protein” is meant a transferrin protein able to reversibly bind to iron (e.g., one or two Fe3+ions) and / or a transferrin receptor with a high affinity, and more particularly the same or similar affinity as an unmodified or wild-type transferrin protein. Determining whether a transferrin protein derivative, variant or analogue is functional may be assessed by any method or means known in the art. Yeast It is contemplated that any species or strain of yeast may be utilised for the methods of the present disclosure. Exemplary yeast cells include those of a Pichia species, such as Pichia pastoris, a Candida species, such as Candida boidinii, a Hansenula species, such as Hansenula polymorpha, a Saccharomyces species, a Schizosaccharomyces species, a Yarrowia species, such as Yarrowia lipolytica, a Kluyveromyces species and an Aspergillus species. In particular examples, however, the yeast cell is not of the species Schizosaccharomyces pombe. Accordingly, in one form, the present disclosure provides an isolated yeast cell configured to express a component protein, such as a transferrin protein, of a non-human origin. According to various examples, the yeast cell is a methylotrophic yeast cell. Methylotrophic yeast cells, like those from the Pichia genera, are able to grow on methanol as the sole carbon source. Methanol utilization is initiated by the conversion of methanol to formaldehyde by the action of alcohol oxidase. For example, the methylotrophic yeast, Pichia pastoris, contains two genes for alcohol oxidases, AOX1 and AOX2. Strains with reduced alcohol oxidase activity (“methanol utilization slow” or MutS strains) often produce more of a recombinant protein expressed from the AOX1 promoter than strains that do not have reduced alcohol oxidase activity. Strains mutated in both AOX genes and completely lacking alcohol oxidase activity cannot metabolize methanol, but can still be induced for expression from the AOX1 promoter by methanol. These strains retain the ability to use other carbon sources for growth, but still express heterologous proteins from the AOX1 promoter upon the addition of methanol. Because these strains do not metabolize methanol (“methanol utilization minus” or Mut- strains), much less methanol is required for induction of protein expression, and strains carrying these mutations avoid issues related to methanol feeding in large-scale fermentations (see e.g., Chiruvolu et al., 1997, Enzyme Microb Technol, 21:277-83). Thus, a methylotrophic yeast having a mutation in one or both of the AOX1 and AOX2 genes can be used in the methods described herein. Representative methylotrophic yeast can be of the genus Candida, Hansenula, Pichia or Toruplosis. In particular examples, the methylotrophic yeast is Pichia pastoris. According to certain examples, the yeast cells are protease deficient. To this end, the yeast cells may be genetically modified so as to not express or only minimally express one or more proteases, as are known in the art. Such yeast cells may be suitable for improved recombinant expression of the component protein, as endogenous protease activity has been previously reported to cause proteolysis of heterologous proteins. In particular examples, the yeast cells do not or only minimally express PEP4 (i.e., the vacuolar protease proteinase A) and / or PRB1 (i.e., the vacuolar protease proteinase B). Referring to some examples, the yeast cells are genetically modified so as to promote or facilitate enhanced recombinant protein production and / or secretion thereof. For example, the yeast cells can be modified to express altered (e.g., increased or decreased) levels of one or more genetic helper factors, such as a chaperone protein (e.g., Kar2p, Sec63, Ydj1p, Ssa1p, BiP, Pdi) and a secretion pathway protein. Nucleic acid constructs are provided herein that allow for genetically engineering a methylotrophic yeast cell. While the methods are exemplified herein using a Pichia species (i.e., P. pastoris), it is envisaged that other species of the Pichia genus or other species from any of the Candida, Hansenula, Pichia and Torulopsis genera can be used in a similar manner to recombinantly express the component protein provided herein. The component proteins, low purity extracts, nucleic acids, genetic constructs and yeast cells described herein may be considered to be isolated. For the purposes of the present disclosure, by “isolated” is meant material that has been removed from its natural state or otherwise been subjected to human manipulation. Isolated material may be substantially or essentially free from components that normally accompany it in its natural state, or may be manipulated so as to be in an artificial state together with components that normally accompany it in its natural state. Isolated material may be in native, chemical synthetic or recombinant form. Recombinant expression of component proteins Methods for producing recombinant proteins or polypeptides are well known in the art. The yeast cell according to the present disclosure can be genetically modified to comprise a nucleotide sequence or nucleic acid molecule encoding a component protein of non-human origin. In some examples, the nucleic acid molecule encoding the component protein is stably integrated into the genome of the yeast cell. Alternatively, the nucleic acid molecule encoding the component protein is extra-chromosomally expressed from, for example, a replication-competent genetic construct or plasmid. For the present disclosure, it is envisaged that the nucleotide sequence encoding the component protein described herein may be part of a genetic construct (e.g., a vector, such as an expression vector or cassette). An exemplary genetic construct for the expression of a component protein is provided in Figure 1 and SEQ ID NO: 13. The nucleotide sequence encoding the component protein can be part of a genetic construct or integrated into the genome (e.g., transiently and / or stably expressed) by using respective means and methods which are well known to the person skilled in the art, such as those described in “Pichia Protocols”, Cregg JM, Humana Press; 2nd edition (August 8, 2007). By way of example, the transformation of the nucleotide sequence described herein into the yeast cells can be conducted by electroporation, protoplast fusion, calcium phosphate precipitation and calcium chloride precipitation, agitation with silicon carbide fiber, PEG-mediated transformation, dextran sulfate-mediated transformation and lipofectamine- mediated transformation. For the purposes of host cell expression, the genetic construct is suitably an expression construct. For some examples, the expression construct comprises the nucleotide sequence encoding the component protein operably linked to one or more additional sequences in an expression vector. An exemplary expression cassette encoding a transferrin protein for inclusion within such a genetic construct is provided in SEQ ID NO: 14. In one form, the present disclosure provides an isolated nucleic acid comprising, consisting of or consisting essentially of the nucleotide sequence set forth in SEQ ID NOs: 14 or a fragment, variant or derivative thereof. In another form, the present disclosure provides an isolated genetic construct or vector comprising, consisting of or consisting essentially of the nucleotide sequence set forth in SEQ ID NOs: 13 or a fragment, variant or derivative thereof. The terms “polynucleotide” and “nucleic acid” are used interchangeably herein. They refer to a polymeric form of nucleotides of any length, either deoxyribonucleotides or ribonucleotides. A polynucleotide defined herein may be of genomic, cDNA, semisynthetic, or synthetic origin, double-stranded or single-stranded and by virtue of its origin or manipulation: (1) is not associated with all or a portion of a polynucleotide with which it is associated in nature, (2) is linked to a polynucleotide other than that to which it is linked in nature, or (3) does not occur in nature. Preferred polynucleotides of the invention encode a component protein which are capable of being transcriptionally and translationally expressed in yeast cells. Also contemplated are variants of the nucleic acids described herein. Variants may comprise a nucleotide sequence at least 70%, at least 75%, preferably at least 80%, at least 85%, more preferably at least 90%, 91%, 93%, 94%, 95%, 96%, 97%, 98% or 99% nucleotide sequence identity with any nucleotide sequence encoding the component protein of the present disclosure (e.g., SEQ ID NOs: 3, 6, 9, 12-20). The present disclosure also provides nucleic acids that have been modified such as by taking advantage of codon sequence redundancy. In a more particular example, codon usage may be modified to optimize expression of a nucleic acid in a particular organism or cell type. In some examples, the nucleic acids provided herein (e.g., SEQ ID NOs: 3, 6, 9 and 12) are codon optimized for expression in a yeast. The nucleic acids, expression cassettes, genetic constructs, vectors and the like disclosed herein can be conveniently prepared using standard protocols such as those described in Chapter 2 and Chapter 3 of CURRENT PROTOCOLS IN MOLECULAR BIOLOGY (Eds. Ausubel et al. John Wiley & Sons NY, 1995-2008). As used herein, the terms “vector” and “expression vector” are understood to mean any nucleic acid molecule including a nucleotide sequence competent to be incorporated into a host cell and integrated into the host cell genome, or to replicate autonomously as an episomal DNA. Such vectors include linear nucleic acids, plasmids, phagemids, cosmids, RNA vectors, viral vectors and the like. Suitable expression vectors may comprise regulatory nucleotide sequences, such as promoter sequences, leader or signal sequences, ribosomal binding sites, polyadenylation sequences, transcriptional start and termination sequences, translational start and termination sequences, and enhancer or activator sequences. The expression vector may also include an additional nucleotide sequence encoding a selection marker or selection cassette, such as ampR, neoR, zeoRor kanR, although without limitation thereto. In particular examples, the expression vector includes a Zeocin™ (phleomycin D1) resistance selection cassette or marker. By “operably linked” is meant that said additional nucleotide sequence(s) is / are positioned relative to the nucleic acid of the present disclosure suitably to initiate, regulate or otherwise control transcription. Suitably, the nucleotide sequence encoding the component protein is operably linked to a promoter, such as an inducible or conditional promoter or a constitutive promoter. The use of any kind of promoter is envisaged, provided that the promoter is able to control the transcription of the nucleotide sequence encoding the component protein operably linked thereto in a yeast cell. However, in particular examples, the promoter is derived from a yeast cell, such as an inducible yeast promoter. According to particular examples, the promoter is an inducible promoter, inclusive of modified and unmodified versions thereof. To this end, an inducible promoter can be used to advantageously control the expression rate of the nucleotide sequence encoding the component protein within the yeast cell. It may therefore be possible to control the expression rate of the component protein in various stages of cultivation of the yeast cells and consequently to determine the point in time when the level of the component protein within the culture medium and / or the yeast cell reaches a predetermined or threshold amount or concentration. Alternatively, the present disclosure further contemplates the use of promoters which act constitutively in the yeast cell, such as a constitutive yeast promoter. Constitutive promoters for yeast cells are well known in the art. Moreover, it is contemplated that a combination of inducible (e.g., methanol-inducible) and constitutive promoters (or promoter elements therefrom) can be combined to further increase the expression of any of the nucleotide sequences operably linked thereto. According to particular examples, the promoter to be operably linked to the nucleotide sequence encoding the component protein is a yeast promoter, or a functional portion or element thereof. Exemplary yeast promoters include an AOX1 promoter, a GAL1 promoter, a PGK promoter, an FDH promoter, an FLD1 promoter, an ADH promoter, a PEX8 promoter, a DAS / DHAS promoter, a MOX promoter, AOD1 promoter, a MOD1 promoter and a HIS4 promoter, inclusive of functional fragments and modified versions or variants thereof. There are a number of inducible promoters that can be used when genetically engineering yeast, and more particularly methylotrophic yeast. For example, a methanol-inducible promoter, or a promoter element therefrom, can be used. Thus, in some examples, the promoter is or comprises a methanol-inducible promoter, such as AOX1 promoter from Pichia pastoris, an AOD1 promoter from Candida boidinii (see, e.g., GenBank Accession No. YSAAOD1A), a MOX promoter from Hansenula polymorpha (see, e.g., GenBank Accession No. X02425), a MOD1 promoter or a MOD2 promoter from Pichia methanolica (see, e.g., Raymond et al., 1998, Yeast, 14: 11- 23; and Nakagawa et al., 1999, Yeast, 15: 1223-30), a dihydroxyacetone synthase (DAS or DHAS) promoter from Pichia pastoris (see, e.g., GenBank Accession No. FJ752551), a FLD1 promoter from Pichia pastoris (see, e.g., GenBank Accession No. AF066054), or a PEX8 promoter from Pichia pastoris (see, e.g., Kranthi et al., 2010, Yeast, 27:705-11). In certain examples the yeast promoter is or comprises an alcohol oxidase 1 (AOX1) promoter, or a fragment, variant or derivative thereof. A non-limiting example of an AOX1 promoter is provided in SEQ ID NO: 15. Thus, the AOX1 promoter nucleotide sequence may be a nucleic acid which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 15 or a fragment or derivative thereof. The nucleic acid molecule described herein may also comprise a nucleic acid sequence which encodes a signal sequence which promotes secretion of the component protein into the supernatant of the culture medium in which the yeast cells are cultivated. The term “signal sequence” as used herein refers to a segment which directs the secretion of the biologically active molecule. The signal sequence used in the present invention may be an amino acid sequence initiating transport of a component protein, and more particularly, a transferrin protein, across the membrane of the endoplasmic reticulum (ER). Non-limiting examples of signal sequences include an MFα (mating factor α) signal sequence, a K1 killer toxin signal sequence, an invertase secretion signal sequence, a killer toxin of Kluyveromyces lactis signal sequence, a killer toxin of Pichia acaciae signal sequence, a killer toxin of Hanseniaspora uvarum signal sequence, a killer toxin of Pichia (Hansenula) anomala signal sequence, a SCW10 signal sequence (see encoding nucleotide sequence of SEQ ID NO: 18) and a C4QXW2 signal sequence (see encoding nucleotide sequence of SEQ ID NO: 19). The signal sequence may also include a native or wildtype signal sequence of the component protein (e.g., see encoding nucleotide sequence of a signal sequence of bovine transferrin of SEQ ID NO: 19). In particular examples, the signal sequence is an MFα signal sequence. An exemplary nucleotide sequence encoding an MFα signal sequence is provided in SEQ ID NO: 16, but functional fragments, variants and derivatives thereof are envisaged. Thus, the MFα signal sequence may be encoded by a nucleic acid which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 16 or a fragment or derivative thereof. In other examples, the signal sequence is a SCW10 signal sequence. An exemplary nucleotide sequence encoding a SCW10 signal sequence is provided in SEQ ID NO: 18, but functional fragments, variants and derivatives thereof are envisaged. Thus, the SCW10 signal sequence may be encoded by a nucleic acid which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 18 or a fragment or derivative thereof. In certain examples, the signal sequence is a C4QXW2 signal sequence. An exemplary nucleotide sequence encoding a C4QXW2 signal sequence is provided in SEQ ID NO: 19, but functional fragments, variants and derivatives thereof are envisaged. Thus, the C4QXW2 signal sequence may be encoded by a nucleic acid which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 19 or a fragment or derivative thereof. In various examples, the signal sequence is a native bovine signal sequence. An exemplary nucleotide sequence encoding a native bovine signal sequence is provided in SEQ ID NO: 20, but functional fragments, variants and derivatives thereof are envisaged. Thus, the native bovine signal sequence may be encoded by a nucleic acid which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 20 or a fragment or derivative thereof. The nucleotide sequence encoding the component protein may also be operably linked to a terminator sequence which blocks or prevents transcription by a polymerase. The term “terminator sequence” refers to DNA sequences located downstream or 3’ of a coding sequence, including polyadenylation recognition sequences and other sequences encoding regulatory signals, capable of affecting mRNA processing or gene expression. Typically, a promoter will be associated to a corresponding or operably linked terminator sequence. Accordingly, exemplary yeast terminator sequences include an AOX1 terminator, a GAL1 terminator, a PGK terminator, an FDH terminator, an FLD terminator, an ADH terminator and a HIS4 terminator, inclusive of modified versions or variants thereof. According to some examples, the yeast terminator is or comprises an AOX1 terminator, or a fragment, variant or derivative thereof. A non-limiting example of an AOX1 terminator is provided in SEQ ID NO: 17 below. Thus, the AOX1 terminator nucleotide sequence may be a nucleic acid which is at least 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5% or 100% identical to SEQ ID NO: 17 or a fragment or derivative thereof. Terms used generally herein to describe sequence relationships between respective proteins and nucleic acids include "comparison window", "sequence identity", "percentage of sequence identity" and "substantial identity". Because respective nucleic acids / proteins may each comprise (1) only one or more portions of a complete nucleic acid / protein sequence that are shared by the nucleic acids / proteins, and (2) one or more portions which are divergent between the nucleic acids / proteins, sequence comparisons are typically performed by comparing sequences over a "comparison window" to identify and compare local regions of sequence similarity. A "comparison window" refers to a conceptual segment of, for example, 6, 9, 12 or 20 contiguous residues that is compared to a reference sequence. The comparison window may comprise additions or deletions (i.e., gaps) of about 20% or less as compared to the reference sequence for optimal alignment of the respective sequences. Optimal alignment of sequences for aligning a comparison window may be conducted by computerised implementations of algorithms (Geneworks program by Intelligenetics; GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package Release 7.0, Genetics Computer Group, 575 Science Drive Madison, WI, USA, incorporated herein by reference) or by inspection and the best alignment (i.e. resulting in the highest percentage homology over the comparison window) generated by any of the various methods selected. Reference also may be made to the BLAST family of programs as for example disclosed by Altschul et al., 1991, Nucl. Acids Res. 25 3389, which is incorporated herein by reference. A detailed discussion of sequence analysis can be found in Unit 19.3 of CURRENT PROTOCOLS IN MOLECULAR BIOLOGY Eds. Ausubel et al. (John Wiley & Sons Inc NY, 1995-1999). The term “sequence identity” is used herein in its broadest sense to include the number of exact nucleotide or amino acid matches having regard to an appropriate alignment using a standard algorithm, having regard to the extent that sequences are identical over a window of comparison. Thus, a "percentage of sequence identity" is calculated by comparing two optimally aligned sequences over the window of comparison, determining the number of positions at which the identical nucleic acid base (e.g., A, T, C, G, U) or amino acid residue occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the window of comparison (i.e., the window size), and multiplying the result by 100 to yield the percentage of sequence identity. For example, "sequence identity" may be understood to mean the "match percentage" calculated by the DNASIS computer program (Version 2.5 for windows; available from Hitachi Software engineering Co., Ltd., South San Francisco, California, USA). Genetic constructs of the present disclosure may comprise companion sequences upstream and downstream to the expression cassette comprising the nucleotide sequence encoding the component protein. The companion sequences may be of plasmid or viral origin and provide necessary characteristics to the genetic construct to permit, for example, amplification thereof in a secondary host to the plant host, such as, sequences containing an origin of replication and a selectable marker. Typical secondary hosts include bacteria and yeast. The expression construct may also include an additional nucleotide sequence encoding a fusion partner (typically provided by the expression vector) so that the recombinant protein is expressed as a fusion protein. In this regard, the component protein (e.g., transferrin) may be expressed as a fusion protein with one or more further growth factors or cell culture components (e.g., albumin) for use in a serum-free media, such as a serum-free media for use in in vitro meat production (e.g., cellular agriculture or cellular aquaculture). Such fusion proteins may be configured to be cleavable, such as by way of enzymatic digestion, into their individual protein sequences. Culturing non-human animal cells It is contemplated that the non-human component protein (e.g., transferrin protein) described herein is suitable for use in culturing non-human animal cells in the process of in vitro meat production. Accordingly, in one form, the present disclosure provides a composition for producing a culture medium for culturing non-human animal cells, said composition comprising a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to the non-human animal cells and optionally one or more carriers, diluents or excipients acceptable for use in cell culture, as are known in the art. Suitably, the component protein is a transferrin protein. In a related form, the present disclosure relates to a kit for producing a culture medium for culturing non-human animal cells, said kit comprising a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to the non-human animal cells. Suitably, the component protein is a transferrin protein. To this end, it is envisaged that the component protein described herein can be formulated as discrete doses, such as in the form of a kit. Such a kit may further comprise a package insert comprising printed instructions for producing a culture medium for culturing non-human animal cells. Accordingly, the aforementioned kits are suitably for use in methods of culturing non-human animal cells and / or in vitro meat production, as described herein. The kit may further include a base medium. The terms “base medium” and “basal medium” generally refer to a solution of, for example, amino acids, vitamins, salts, and nutrients that is effective to support the growth of cells in culture, although normally these compounds will not support cell growth unless supplemented with additional compounds or components. Non-limiting examples of base media include Leibovitz's L-15 medium, Eagle's Minimum Essential Media (MEM), Medium 199, Dulbecco's Modified Eagle Medium (DMEM), Ham's F12 Nutrient Mix, Ham's F10 Nutrient Mix, MacCoy's 5A Medium, Glasgow Modified Eagle Medium (GMEM), Iscove's Modified Dulbecco's Medium, and RPMI 1640. In another form, the present disclosure provides a culture medium for culturing non-human animal cells comprising: (a) a base medium; and (b) a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to the non-human animal cells. According to certain examples, the culture medium or the kit includes one or more further additives. By way of example, the culture medium or the kit may include one or more growth factors or cytokines. These one or more further additives may include one or more of insulin growth factor 1 (IGF-1), insulin, an interleukin (e.g., IL-6, IL-11), interleukin 6 receptor (IL-6R), fibroblast growth factor (FGF; e.g., FGF-1 and FGF-2), epidermal growth factor (EGF), platelet derived growth factor (PDGF), connective tissue growth factor (CTGF), transforming growth factor-beta (TGF-beta), a bone morphogenetic protein (BMP), hepatocyte growth factor (HGF), vascular endothelial growth factor (VEGF), activin, WNT and / or albumin, inclusive of homologs, paralogs, orthologs, variants, precursors, isoforms, and combinations thereof. Suitably, the one or more growth factors or cytokines are recombinant growth factors or cytokines of the same or similar species to that of the non-human cells. In some examples, the growth factor is a chicken growth factor, a duck growth factor, a bovine growth factor, a sheep growth factor, a fish growth factor, a porcine growth factor, a mammalian growth factor, an avian growth factor or a teleost growth factor. Bacterial, yeast, insect, mammalian, or any other appropriate protein expression systems may be used to produce such recombinant growth factors or cytokines. In a further related form, the present disclosure provides a method of producing a culture medium for culturing non-human animal cells, said method including the steps of: (a) providing a base medium; (b) adding a low purity extract of a component protein to the base medium, wherein the component protein has been produced by a yeast and is of a same or similar species to the non- human animal cells. Suitably, the component protein is a transferrin protein. The step of adding one or more further additives, such as those described herein, to the base medium is further contemplated for the present method. In one particular form, the present disclosure also relates to a method of culturing non- human animal cells, said method including the step of culturing the non-human animal cells in a culture medium comprising a base medium and a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to that of the non-human animal cells. Suitably, the component protein is a transferrin protein. In another broad form, the present disclosure provides a method of in vitro meat production, said method including the steps of: (a) providing a plurality of non-human animal cells; and (b) culturing the non-human animal cells in a culture medium comprising a base medium and a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to that of the non-human animal cells. Suitably, the component protein is a transferrin protein. In still another related form, the present disclosure provides a meat product produced by the aforementioned methods. Cells for the production of in vitro meat may be cells (e.g., muscle cells, somatic cells, stem cells, lipid cells etc.) taken from an animal source, which may then be grown separately from the animal in a culture media in a bioreactor or other suitable type of sterile environment. Accordingly, the present method may further include the earlier step of isolating the plurality of non-human cells from an animal source, such as an animal biopsy. In some examples, the non-human animal cells are from poultry, seafood (e.g., teleost), game or livestock (e.g., bovine, porcine, ovine, caprine), such as one of those species described herein for the source of the component protein. The cells may grow into a semi-solid or solid form mimicking an animal organ by attaching to an edible or food-grade three-dimensional scaffold that is placed in the bioreactor. Alternatively, the cells can be grown at least partly in a suspension culture. Such suspension cells may then be subsequently isolated to be used directly as a final food product or a component ingredient of a food product (e.g., a hybrid food product including one or more further cell types and / or plant material). The starter cells may be primary cells directly obtained from an animal’s tissues, or continuous cell lines. In particular examples, the cells can be stem cells (e.g., embryonic stem cells, iPSCs, non-embryonic or somatic stem cells). If grown under the right conditions in appropriate culture media, primary cells will grow and proliferate, but only a finite number of times that is related to the telomere length at the end of the cell's DNA. Continuous cell lines, on the other hand, can be cultured in vitro over an extended period. After an appropriate period, the present method can further include the step of harvesting a meat product produced from culturing the non-human animal cells described herein. Suitably, the non-human animal cells are derived from any non-human animal species intended for human or non-human dietary consumption (e.g. cells of teleost, avian, and non-human mammalian origin such as ovine, caprine, porcine, and bovine, cells of livestock, poultry, avian, game, or aquatic species). For some examples, the non-human animal cells are derived from livestock, such as domestic cattle, pigs, sheep, goats, camels, water buffalo, rabbits and the like. In other examples, the non-human animal cells are derived from poultry, such as domestic chicken, turkeys, ducks, geese, pigeons and the like. According to certain examples, the non-human animal cells are derived from game species such as wild deer, gallinaceous fowl, waterfowl, hare, rabbit and the like. In various examples, the non-human animal cells are from aquatic species or semi- aquatic species harvested commercially from wild fisheries or aquaculture operations, or for sport, including certain fish (e.g., a teleost species), crustaceans, molluscs, cephalopods, cetaceans, crocodilians, turtles, frogs and the like. In particular examples, the non-human animal cells are derived from a bovine animal or, more particularly, cattle. For other examples, the non-human animal cells are derived from a porcine animal or, more particularly, a domestic pig or wild boar (Sus scrofa). According to various examples, the non-human animal cells are derived from a poultry animal or, more particularly, a chicken or a turkey. In some examples, the non-human animal cells are derived from a teleost animal, such as a salmon or tuna species. The culture media for culturing the non-human animal cells described herein suitably contains components necessary for cell proliferation, such as amino acids, salts, vitamins, growth factors, and buffering systems to control pH. Current methods typically add foetal bovine serum (FBS) or the like to the culture media prior to use as it provides vital macromolecules, growth factors, and immune molecules. It is contemplated, however, that the culture medium is suitably free or substantially free (e.g., less than about 1%, 0.5%, 0.1% or 0.05% by weight of the culture medium) of animal-derived components, such as animal-derived proteins. Moreover, it is envisaged, that the culture medium is suitably free or substantially free (e.g., less than about 1%, 0.5%, 0.1% or 0.05% by weight of the culture medium) of serum. Similarly, the kit described herein suitably does not include serum. For the purposes of the present invention, the term “serum- free” means the absence of any measurable amounts of blood serum of any species including, but not limited to, the absence of foetal bovine serum, calf bovine serum, human serum, or the like, or combinations thereof. The term “substantially free of serum” means the culture medium is essentially free of serum except as otherwise incidentally incorporated as a minor component (e.g., serum is present in a concentration less than about 1%, 0.5%, 0.1% or 0.05% by weight of the culture media). The culture medium may be a buffered solution containing components such as, but not limited to, inorganic salts (e.g., calcium chloride (CaCl2), potassium chloride (KCl), sodium chloride (NaCl), sodium bicarbonate (NaHCO3), sodium dihydrogen phosphate (NaH2PO4), magnesium sulfate (MgSO4), etc.), amino acids, vitamins (e.g., thiamine, riboflavin, folic acid, etc.), and other components, such as glucose, β-mercaptoethanol, ethylenediaminetetraacetic acid (EDTA) and sodium pyruvate. Suitably, the component protein, such as the transferrin protein, is of the same or similar species, such as from the same genus, to the non-human animal cells. A “similar species” may be one which comprises a component protein that demonstrates similarities with respect to the sequence, structure and / or function of the component protein in question. In particular examples, an amino acid sequence of a component protein of a first species that is similar to a second species demonstrates sequence similarity with an amino acid sequence of a component protein of the second species of at least about 50% (e.g., at least about 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or any range therein), more particularly at least about 55%, more particularly at least about 60%, more particularly at least about 65%, more particularly at least about 70%, more particularly at least about 75%, more particularly at least about 80%, more particularly at least about 85%, even more particularly at least about 90%, still even more particularly at least about 95% or yet even more particularly at least about 98% similarity therewith. According to particular examples, the component protein is of the same family to the non- human animal cells. For some examples, the component protein is of the same genus to the non- human animal cells. In certain examples, however, the component protein is of the same species to the non-human animal cells (e.g., both the component protein and the non-human animal cells are derived from a bovine species, a teleost species or a poultry species). It is expected that the component protein should then exert a higher bioactivity in relation to the non-human animal cells in culture compared to the use of a component protein, such as that derived from animal serum, of a genetically distant or distinct species to that of the non-human animal cells. Due to the enhanced compatibility, a reduced amount of growth factors, including the component protein, may be needed in the culture medium and improvements in overall cell growth may be observed. Suitably, the low purity extract of the component protein (e.g., the transferrin protein) of the above aspects is that hereinbefore described (e.g., produced by those methods described herein). So that preferred embodiments of the present disclosure may be fully understood and put into practical effect, reference is made to the following non-limiting examples. Examples Example 1. The aim of the present Example was to generate a P. pastoris yeast strain that expresses and secretes recombinant bovine transferrin (Uniprot Q29443, Bos taurus) under standard fermentation or culture conditions. Materials / Methods and Results Protein production in P. pastoris The P. pastoris host strain utilised for the present examples was the protease deficient BG16 strain. The bovine transferrin protein (Uniprot Q29443, Bos taurus) sequence was used to design the gene fragments for vector construction using SnapGene (Dotmatics, USA) or Geneious Prime 2021 v2.2 (Dotmatics, USA). These fragments were codon-optimised using Geneart codon optimisation tool (Invitrogen, Thermofisher) for P. pastoris. Next, the gene fragments were synthesised as priority gene plasmid inserts (Genewhiz, Azenta, USA) and cloned into a plasmid backbone — by restriction enzyme digestion and T4 ligase— that was fitted with a selection cassette for containing an EM7 promotor (for E. coli), TEF promotor (for yeast), a Zeocin™ resistance gene and CYC1 terminator. Also included was a recombinant protein production cassette for P. pastoris containing an AOX1 promotor upstream from the inserted transferrin CDS with an N-terminal alpha-mating factor secretion signal followed by an AOX1 terminator. Figure 1 shows the developed plasmid. Next the developed ligates were transformed into chemically competent Escherichia coli (NEB 5 Alpha) and selected on Luria agar plates containing the appropriate antibiotic selection reagent (Zeocin™, Invitrogen, Thermo, USA). The sequence of the plasmid constructs was then confirmed by DNA Sanger sequencing (Macrogen, S. Korea). Next P. pastoris cells were prepared to be electrocompetent and transformed with linearized plasmid via electroporation. After 2-4 hr outgrowth at 30°C with shaking, transformation reactions were diluted appropriately, plated onto suitable selection media (e.g. YPD plus Zeocin™) and incubated at 28°C for 48-72 hrs. The concentration of Zeocin was increased above standard selection concentrations in order to enhance selection for high copy number recombinant cassette genomic insertions for base recombinant expression strains, using e.g. 500- 1000 µg / mL Zeocin™. After incubation, colonies were patched onto similar YPD selection media for working stocks and short-term storage. The copy number of the insertions was determined, and only clones having high copy numbers of the expression construct were kept. These transformed cells were then cultured in flasks and the AOX promotor was induced to drive plasmid expression and enable the production of transferrin. Protein extraction from P. pastoris supernatant Protein extraction was simplified to obtain low-purity (<80%) protein extracts that can be used directly as a supplement for cell culture media. To this end, supernatant was simply dialysed using a 10 kDa molecule weight cut off membrane, lyophilised, resuspended and passed through a 0.22 uM filter for sterilisation then used directly in a culture medium. Alternatively, after sterilisation the sample was centrifuged and then filtered with a 10kDa cut off spin filter (Amicon). The retentate of the filter was kept and resuspended to be used in cell culture assays. Transferrin characterisation and quantification A targeted multiple reaction monitoring approach (LC-MRM-MS) was developed to enable the quantification of transferrin in the supernatant from culturing the yeast cells. Briefly, proteins were extracted from the supernatant using 8M urea and were then reduced with DTT and alkylated with iodoacetamide. The linearized alkylated and reduced proteins were then tryptically digested and eluted using a 10 kDa cut off filter. The peptides were lyophilised and resuspended in 0,1% formic acid and then analysed via LC-MRM-MS using an optimised protocol. Figure 2 shows the sum of the areas under the peak for the 3 most abundant MS / MS transitions of transferrin specific peptides for different colonies of the transformed strain. Colony Tfn 1-8 produced the most recombinant transferrin and was kept for larger scale production. Figure 3 shows an SDS- PAGE analysis of Pichia expression supernatants and identified a dominant protein band of ~ 80kDa molecular weight, similar to a commercial Sigma transferrin standard, as well as several other bands containing transferrin. Example 2 This Example was designed to determine levels of purity of transferrin in yeast supernatant extracts that may be suitable for use in cell culture (e.g., demonstrate functionality with little or no toxicity in cultured cells). Estimate the toxicity of Pichia extracts Pichia supernatant samples were directly dialysed and lyophilised, as described above. These powders were then sterilised (passed through a 0.22 uM filter) and tested for toxicity directly, or after additional centrifugation and passing through a 10 kDa Amicon cut-off filter (retentate kept) or additional centrifugation and passing through a 30 kDa Amicon cut-off filter (retentate kept). Tests were performed in traditional media (DMEM with FBS) and beefy-9 with or without added 20 ug / mL store-bought TF. In this base media, 20 μg / mL of commercially sourced transferrin protein and decreasing amounts of Pichia supernatant samples (200, 100, 80, 50, 40, 30, 20, 10 and 0 μg / mL of base media in order from left to right) that were either not cleaned up (first panel of Figure 4), centrifuged (second panel of Figure 4), centrifuged and passed through a 10 kDa Amicon cut-off filter (third panel of Figure 4) or centrifuged and passed through a 30 kDa Amicon cut-off filter (fourth panel of Figure 4) prior to use were added. The retentate fraction from the filtration step was then re- solubilised. Two positive controls (i.e., B9 media with 20 μg / mL transferrin and no Pichia culture supernatant added to each block of experiments shown in the figure and a positive control featuring C2C12 cells grown in DMEM F12 with 10% FBS) are included in Figure 4. There is also a negative control (B9 media only) included in Figure 4. Use of the direct (raw) lyophilised transferrin product generally resulted in minimal or no toxicity in the C2C12 cells, particularly for the lower concentration extracts (i.e., 10 to 50 μg of Pichia culture supernatant per mL of media). Centrifugation and passing through a molecular cut- off filter improved growth performance of the media containing the higher concentrations of Pichia culture supernatant. Fig. 4 shows the cell viability assay for Pichia extracts when used raw, after centrifugation, after centrifugation and 10 kDa or 30 kDa filtering. Little or no toxicity is observed when the extracts are centrifuged and passed through a 10 or 30 kDa cut off filter, with concentrations up to 200 μg / mL not affecting cell viability. These results show that low purity extracts of transferrin from yeast culture supernatant are a viable option for media supplement use. Example 3 This Example was designed to determine the functionality of recombinant bovine transferrin produced using Pichia yeast cells for supplementation of depleted cell culture media to enhance the proliferation of muscle / myoblast cells in cell culture. Functionality of partially purified recombinant bovine transferrin derived from Pichia cells Pichia supernatant samples containing recombinant bovine transferrin were directly dialysed and lyophilised, as described above. These powders were then sterilised (passed through a 0.22 μM filter) and tested for the ability to enhance proliferation of myoblast C2C12 cells, either directly or after additional centrifugation and passing through a 10 kDa Amicon cut-off filter (retentate kept). Tests were performed in traditional media (DMEM with FBS) and Beefy-9 media with or without added varying amounts of recombinant bovine TF. The treatment groups tested and their respective treatments are outlined in Table 1 below. In this base media, 20 μg / mL of commercially sourced transferrin protein or decreasing amounts of Pichia recombinant transferrin supernatant samples (1000, 100, 10, 1 ug / mL were added to base media in order from left to right). Two positive controls (i.e., B9 media with 20 μg / mL commercial transferrin) and no Pichia culture supernatant were added to each block of experiments shown in the figure and a positive control featuring C2C12 cells grown in DMEM F12 with 10% FBS are included in Figure 5. There is also a negative control (B9 media only) included in Figure 5. Table 1. Sample treatments for recombinant yeast-derived transferrin prior to functional assays Treatment Production vessel Post harvest treatment of supernatant name type d, ed n ed Use of the direct (raw) lyophilised recombinant transferrin product generally resulted in some toxicity to the C2C12 cells, particularly for the bioreactor-derived (ferment-derived) recombinant transferrin (Tubes A-C in figure 5), but both lyophilised recombinant transferrin derived from flask cultures, and ferment-derived recombinant transferrin that had been filtered and dialysed after lyophilisation were functionally active and enhanced proliferation of C2C12 cells. Growth performance of cell cultures with these partially purified recombinant bovine transferrin samples equalled or exceeded that of cell cultures supplemented with commercially-source transferrin (Figure 5, Flasks A & B, Tube F compared to B9+ Apo20). Optimal growth performance was seen with the addition of 10 µg / mL of yeast-derived recombinant bovine transferrin. The data demonstrating functionality of partially purified bovine transferrin are simplified to provide direct comparison between the optimal recombinant Pichia transferrin samples and the controls in Figure 6. Again, these results illustrate that low purity extracts of transferrin from yeast culture supernatant are functionally active and thus provide a viable option for media supplement use.

Claims

CLAIMS:

1. A method of producing a component protein, said method including the steps of: (a) providing a culture medium in which a yeast cell configured to express the component protein has been cultured under suitable conditions to express the component protein, wherein the component protein is of non-human origin; and (b) extracting a low purity extract of the component protein from the culture medium.

2. A method of producing a component protein, said method including the steps of: (a) providing a yeast cell configured to express the component protein, wherein the component protein is of non-human origin; (b) culturing the yeast cell in a culture medium under suitable conditions to express the component protein; and (c) extracting a low purity extract of the component protein from the culture medium and / or the yeast cell.

3. The method of Claim 1 or Claim 2, wherein the component protein is present in an amount less than about 95% of a total protein content of the low purity extract.

4. The method of any one of the preceding claims, wherein the component protein is present in an amount of less than about 80% of a total protein content of the low purity extract.

5. The method of any one of the preceding claims, wherein the extraction step comprises a dialysis and / or filtration step.

6. The method of Claim 5, wherein the dialysis and / or filtration step includes dialysing and / or filtering the culture medium with a membrane having a molecular weight cut off of at least about 2.5 kDa.

7. The method of Claim 6, wherein the membrane has a molecular weight cut off of between about 5 kDa and about 15 kDa.

8. The method of Claim 6, wherein the membrane has a molecular weight cut off of about 10 kDa or about 30 kDa.

9. The method of any one of the preceding claims, further include the step of lyophilising the low purity extract of the component protein.

10. The method of any one of the preceding claims, wherein the yeast cell is or comprises a Pichia pastoris cell.

11. The method of any one of the preceding claims, wherein the component protein is of an origin selected from the group consisting of a non-human mammalian species, an avian species and a teleost species, such as a bovine, a chicken or a salmon.

12. The method of any one of the preceding claims, wherein the component protein is selected from the group consisting of transferrin, albumin, insulin, insulin-like growth factor 1 (IGF-1), insulin-like growth factor 2 (IGF-2), fibroblast growth factor 2 (FGF2), activin, transforming growth factor β (TGF-β), neuregulin 1 (NRG1) and any combination thereof.

13. The method of any one of the preceding claims, wherein the component protein is transferrin.

14. An isolated component protein produced according to the method of any one of Claims 1 to 13.

15. A low purity extract of a component protein, wherein the component protein has been produced by a yeast cell configured to express the component protein and wherein the component protein is of non-human origin.

16. A kit for producing a culture medium for culturing non-human animal cells, said kit comprising a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to the non-human animal cells.

17. The kit of Claim 16, further comprising a base medium.

18. A culture medium for culturing non-human animal cells comprising: (a) a base medium; and (b) a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to the non-human animal cells.

19. The culture medium of Claim 18, wherein the culture medium is free or substantially free of serum.

20. A method of producing a culture medium for culturing non-human animal cells, said method including the steps of: (a) providing a base medium; (b) adding a low purity extract of a component protein to the base medium, wherein the component protein has been produced by a yeast and is of a same or similar species to the non- human animal cells.

21. A method of culturing non-human animal cells, said method including the step of culturing the non-human animal cells in a culture medium comprising a base medium and a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to that of the non-human animal cells.

22. A method of in vitro meat production, said method including the steps of: (a) providing a plurality of non-human animal cells; and (b) culturing the non-human animal cells in a culture medium comprising a base medium and a low purity extract of a component protein, wherein the component protein has been produced by a yeast and is of a same or similar species to that of the non-human animal cells.

23. The method of Claim 21 or Claim 22, further including the earlier step of isolating the plurality of non-human cells from an animal source.

24. The method of any one of Claims 21 to 23, wherein the component protein is of an origin selected from the group consisting of a non-human mammalian species, an avian species or a teleost species.

25. The method of any one of Claims 21 to 24, further including the step of harvesting a meat product produced from culturing the non-human animal cells.

26. The kit of Claim 16 or Claim 17, the culture medium of Claim 18 or Claim 19 or the method of any one of Claims 20 to 25, wherein the low purity extract of the component protein has been produced by the method of any one of Claims 1 to 13.

27. The kit, the culture medium or the method of any one of Claims 16 to 26, wherein the component protein is present in the low purity extract in an amount less than about 95% of a total protein content of the low purity extract.

28. The kit, the culture medium or the method of any one of Claims 16 to 27, wherein the component protein is present in the low purity extract in an amount less than about 80% of a total protein content of the low purity extract.

29. A meat product produced according to the method of any one of Claims 21 to 28.

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