Human C4b-binding protein has overlapping, but not identical, binding sites for C4b and streptococcal M proteins.

Many strains of Streptococcus pyogenes bind C4b-binding protein (C4BP), an inhibitor of complement activation. The binding is mediated by surface M proteins in a fashion that has been suggested to mimic the binding of C4b. We have previously shown that a positively charged cluster at the interface between complement control protein domains 1 and 2 of C4BP alpha-chain is crucial for the C4b-C4BP interaction. To extend this observation, and to investigate the interaction with M proteins, we constructed and characterized a total of nine mutants of C4BP. We identified a key recognition surface for M proteins that overlaps with the C4b binding site because substitution of R64 and H67 by Gln dramatically reduces binding to both ligands. However, the analysis of all mutants indicates that the binding sites for C4b and M proteins are only overlapping, but not identical. Furthermore, M proteins were able to displace C4BP from immobilized C4b, whereas C4b only weakly affected binding of C4BP to immobilized M proteins. We found that the molecular mechanisms involved in these two interactions differ because the binding between M proteins and C4BP is relatively insensitive to salt in contrast to the C4BP-C4b binding. In addition, six mAbs directed against the alpha-chain interfered with C4b-C4BP interaction, whereas only two of them efficiently inhibited binding of C4BP to M proteins. Collectively, our results suggest that binding between C4b and C4BP is governed mostly by electrostatic interactions, while additional noncovalent forces cause tight binding of C4BP to streptococcal M proteins.