The amino-terminal module of the C4b-binding protein β-chain contains the protein S-binding site

Abstract

Human C4b-binding protein (C4BP) is composed of multiple α-chains associated with a single β-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 β-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 β-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 β-chain (β1,2 and β2,3) were expressed in a prokaryotic expression system. The β1,2 construct (SCR-1 + SCR-2) contained the high affinity binding site for protein S in contrast to β2,3 (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 α/β-chain chimeras were constructed. These chimeras were composed of α-chains with one, two, or three of the amino-terminal SCR modules replaced by the β-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 α/β-chain chimeras bound protein S equally well, with a K(α) of ~2.3 x 108 ± 0.2 M-1 as compared with 2.1 x 108 ± 0.3 M-1 for plasma- purified C4BP. These results show that the entire protein S-binding site on C4BP is contained within β-chain SCR-1.