The Amino-terminal Module of the C4b-binding Protein β-Chain Contains the Protein S-binding Site

Human C4b-binding protein (C4BP) is composed of multiple alpha-chains associated with a single beta-chain. Each chain is composed of homologous, tandemly arranged repeats of so-called short consensus repeats (SCRs). We have previously shown that the three SCR modules of the beta-chain contain a high affinity binding site for anticoagulant vitamin K-dependent protein S. On the basis of experiments using synthetic peptides, residues 31-45 of the amino-terminal SCR (SCR-1) in the beta-chain were suggested to be involved in protein S binding, but it is not known whether SCR-1 contains the entire protein S-binding site. To address this question, two different truncated forms of the beta-chain (beta1,2 and beta2, 3) were expressed in a prokaryotic expression system. The beta1,2 construct (SCR-1 + SCR-2) contained the high affinity binding site for protein S in contrast to beta2,3 (SCR-2 + SCR-3), which did not bind protein S. Unfortunately, it was not possible to express SCR-1 alone in this system. To further elucidate whether the protein S-binding site is fully contained in SCR-1 or whether SCR-2 is also required, recombinant alpha/beta-chain chimeras were constructed. These chimeras were composed of alpha-chains with one, two, or three of the amino-terminal SCR modules replaced by the beta-chain counterpart and were expressed in a eukaryotic expression system. All recombinant variants were retained within the cells and could be extracted in biologically active forms. The three alpha/beta-chain chimeras bound protein S equally well, with a Ka of approximately 2.3 x 10(8) +/- 0.2 M-1 as compared with 2.1 x 10(8) +/- 0.3 M-1 for plasma-purified C4BP. These results show that the entire protein S-binding site on C4BP is contained within beta-chain SCR-1.