This invention demonstrates the utility of a
yeast expression
system for the expression of functional
heterologous multi-domain proteins in
yeast. The
yeast expression
system allows for the inclusion of a plurality of (up to three) modular expression cassettes which may
encode multiple polypeptide chains of a
heterologous multi-domain
protein on a single
plasmid (Twin Cassette). Because multiple polypeptide chains may be encoded for by the expression cassettes of the present invention in a single vector, the
system can produce equivalent amounts of the multiple polypeptide chains, thereby enhancing the yield of a functional
heterologous multi-domain
protein. For example, functional
monoclonal antibodies (MAbs) comprising a
heavy chain and a light chain of an immunoglobulin (IgG), and functional immunotoxins comprising an
antibody domain and an oxidase
toxin may be produced using the
Yeast expression system of the present invention. In addition, functional
single chain antibodies,
antibody fragments and chimeric antibodies may also be produced.