Immunoglobulin Binding Specificities of the Homology Regions (Domains) of Protein A

Protein A binds immunoglobulins and it has two target structures, one in Feγ (C_H) and the other in selected V_H regions. The protein has five homology regions (domains). A, B, C, D, and E. Fc‐binding and V_H‐binding have been reported to be non‐competitive, suggesting that different domains are responsible for the binding of the two ligands. On the other hand, all five domains have been reported to bind Fc. I studied binding of different immunoglobulins by protein A or its domain B (rBB). The results show that separate domains bind V_H and Fc. If all five domains are capable of binding Fc, the ones that bind V_h have low affinity for Fc. Furthermore, the number of Fc‐binding domains varies depending on the type of the IgG being bound. Human IgG1 or lgG2 or rabbit IgG (Fc) seem to be bound by several domains (possibly four), and domain B is one of them. Mouse IgG1 or lgG2b are bound by fewer domains not including B. Murine lgG2a is also bound by fewer domains but B is one of them.