Binding of human serum amyloid P-component to phosphocholine

Human serum amyloid P-component (SAP) and C-reactive protein (CRP) are structurally similar pentraxins composed of five identical subunits in a disc-like configuration and display Ca(2+)-dependent binding reactivity to a variety of unrelated ligands. CRP is generally classified and defined as a phosphocholine (PC)-binding protein, whereas SAP is identified as a polysaccharide-binding protein. We examined the PC-binding activity of human SAP and compared it to human CRP since many of the biological activities of CRP are triggered upon PC-binding. SAP was able to bind to immobilize PC in a saturable, Ca(2+)-dependent manner but with lower avidity than CRP in direct competitive binding assays. The affinity of the binding of SAP to soluble [14C]PC was slightly lower than the affinity of CRP; however, the valence of SAP was only one PC-binding site/pentraxin or 2/protein vs 5 such sites per CRP molecule. Both SAP and CRP displayed a similar binding preference for PC vs phosphoethanolamine (PE). Two monoclonal antibodies (mAb) generated against the PC-binding site of SAP also reacted with the PC-binding site of CRP and inhibited PC-binding by both pentraxins. A mAb specific for the PC-binding site on CRP also inhibited SAP binding to PC. SAP was also recognized by two anti-idiotypic mAb that shared reactivity with the TEPC-15 PC-binding myeloma protein and the PC-binding site of CRP. Both pentraxins could be isolated from human serum by affinity chromatography on either PC- or PE-substituted agarose beads. The findings indicate that SAP is also a PC-binding protein.