[In vitro evolutional selection of a combinatorial phage library displaying randomly-rearranged various binding domains of SpA and SpG with four human IgG subclasses].

Protein A and protein G are two well-defined immunoglobulin (Ig)-binding proteins (IBPs), which show affinity for specific sites on Ig of mammalian hosts. Protein A and protein G contained several highly homologous IgG-binding domains which had been demonstrated to have function to bind to IgG. Whether combinations of Ig-binding domains of various IBPs could produce useful novel binding properties remains interesting. We constructed a combinatorial phage library which displayed randomly-rearranged A, B, C, D and E domains of protein A, B2 and B3 domains of protein G. Four rounds molecular evolution of this library directed by all four human IgG subclasses respectively generated a common arrangement of D-C respectively which didn't exist in SpA. The dynamic loss of control phages and increase of the phages displaying two or more binding domains, especially the selective enrichment of D-C and strict selection of its linking peptides demonstrated the efficient molecular evolutions and the significance of the selected D-C arrangement. The phage binding assays confirmed that D-C possessed a binding advantage with four human IgG subclasses compared to SpA. In this work, a novel combination of Ig-binding domains, D-C, was obtained and presented the novel Ig binding properties which provided a novel candidate molecule for the purification, production and detection of IgG antibodies and a new approach for the further study of structures and functions of IBPs.