Studies were initiated to localize the C-reactive protein (CRP) binding site on the collagen-like region (CLR) of C1q. CRP bound preferentially to the A chain of reduced C1q, in contrast to aggregated immunoglobulin G (Agg-IgG), which reacted preferentially with the C chain. A group of C1q A chain peptides, including peptides identical to residues 81-97, 76-92, and 14-26, respectively, were synthesized from predicted binding regions. Peptide 76-92 contained two proximal lysine groups, and peptide 14-26 contained four proximal arginine groups. CRP-trimers and CRP-ligand complexes did not bind to immobilized peptide 81-97, but bound avidly to immobilized peptides 76-92 and 14-26. Agg-IgG did not bind to any of the peptides. Peptide 76-92 partially, and peptide 14-26 completely, inhibited binding of CRP to intact C1q. Peptide 14-26 also blocked C consumption initiated by CRP, but not by IgG. Replacement of the two prolines with alanines, or scrambling the order of the amino acids, resulted in loss of ability of peptide 14-26 to inhibit C1q binding and C activation by CRP, indicating a sequence specificity, and not a charge specificity alone, as the basis for the inhibitory activity of the peptide. Similar investigations with scrambled peptides showed a sequence specificity for the effects of peptide 76-92 as well. DNA and heparin inhibited binding of CRP trimers to intact C1q, as well as to each peptide 14-26 and 76-92, suggesting involvement of these regions in C1q-CLR binding reactions generally. Collectively, these data identify two cationic regions within residues 14-26 and 76-92 of the C1q A chain CLR as sites through which CRP binds and activates the classical C pathway, and suggest that these residues represent significant regions for C1q CLR binding reactions generally. To our knowledge, this represents the first delineation of sites on C1q through which binding and activation of the classical C pathway can occur.

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