The present invention relates to the design of trimeric polypeptides using polypeptide structural elements derived from the tetranectin
protein family, and their use in rational de novo design and production of multi-functional molecules including the application of the multi-functional molecules in
protein library technology, such as
phage display technology, diagnostic and therapeutic systems, such as human
gene therapy and imaging. The trimeric polypeptides being constructed as a
monomer polypeptide construct comprising at least one tetranectin trimerising
structural element (TTSE) which is covalently linked to at least one
heterologous moiety, said TTSE being capable of forming a stable complex with two other TTSEs; or as an
oligomer which is comprised of two
monomer polypeptide constructs as mentioned above, and which comprises three TTSEs or a multiplum of three TTSEs, or which is comprised of three
monomer polypeptide constructs.