Specificity of the binding interaction between human serum amyloid P-component and immobilized human C-reactive protein.

C-reactive protein (CRP) and serum amyloid P-component (SAP) are two members of a group of plasma proteins termed pentraxins, which are composed of five identical noncovalently linked subunits that display Ca(2+)-dependent binding to a wide variety of substrates. Purified human SAP binds to CRP, only when the latter is immobilized, in a Ca(2+)-dependent manner under physiological conditions. Externally labeled SAP rapidly binds to two distinct forms of immobilized CRP (direct and phosphorylcholine captured) with a relatively high affinity (KD = 5 nM) at a molar ratio of specifically bound SAP/CRP = 0.3. Studies of binding inhibition using monoclonal antibodies to CRP or synthetic peptides of CRP revealed that residues 134-148 and the COOH-terminal region (residues 191-206) were recognized by SAP. A fragment of CRP consisting of the COOH-terminal 60 residues within each subunit was also selectively bound by SAP. The ability of immobilized CRP to bind SAP was distinguished from CRP's lectin-like binding reactivity since deglycosylated SAP retained its binding reactivity for CRP and sugars that inhibit CRP's lectin-like binding activity failed to inhibit binding. A peptide from trypsin digested SAP composed of residues 144-199 retained CRP binding activity, implicating the COOH-terminal region of SAP as the CRP recognition site.