STRUCTURAL INVESTIGATION OF C4B-BINDING PROTEIN BY MOLECULAR MODELING- LOCALIZATION OF PUTATIVE BINDING-SITES

C4b-binding protein (C4BP) contributes to the regulation of the classi cal pathway of the complement system and plays an important role in bl ood coagulation. The main human C4BP isoform is composed of one beta-c hain and seven alpha-chains essentially built from three and eight com plement control protein (CCP) modules, respectively, followed by a non repeat carboxy-terminal region involved in polymerization of the chain s. C4BP is known to interact with heparin, C4b, complement factor I, s erum amyloid P component, streptococcal Arp and Sir proteins, and fact or VII/VIIIa via its alpha-chains and with protein S through its beta- chain. The principal aim of the present study was to localize regions of C4BP involved in the interaction with C4b, Arp, and heparin. For th is purpose, a computer model of the 8 CCP modules of C4BP alpha-chain was constructed, taking into account data from previous electron micro scopy (EM) studies. This structure was investigated in the context of known and/or new experimental data. Analysis of the alpha-chain model, together with monoclonal antibody studies and heparin binding experim ents, suggests that a patch of positively charged residues, at the int erface between the first and second CCP modules, plays an important ro le in the interaction between C4BP and C4b/Arp/Sir/heparin. Putative b inding sites, secondary-structure prediction for the central core, and an overall reevaluation of the size of the C4BP molecule are also pre sented. An understanding of these intermolecular interactions should c ontribute to the rational design of potential therapeutic agents aimin g at interfering specifically some of these protein-protein interactio ns. (C) 1998 Wiley-Liss, Inc.