CHARACTERIZATION OF THE BINDING OF SERUM AMYLOID-P TO TYPE-IV COLLAGEN

Serum amyloid P (SAP), a member of the evolutionarily conserved pentra xin family, is a normal component of a number of basement membranes, i ncluding glomerular and alveolar, In vitro SAP binds to a variety of p roteins including fibronectin, proteoglycans, and the collagen-like re gion of the complement component C1q. In these studies, binding of SAP to type IV collagen, a major component of basement membrane, was exam ined, Purified SAP binds to human and mouse type IV collagen but not t ype I, II, or III collagens, Binding of SAP to type IV collagen is dep endent on the presence of Ca2+. This binding is saturable with a K-d a pproximate to 1.2 x 10(-7) M based on solid phase binding and 4 x 10(- 8) M based on the IC50 value from fluid phase binding data, Binding of SAP to type IV collagen was inhibited by both SAP and C-reactive prot ein (CRP), However, a 5-fold molar excess of CRP as compared with SAP was required to inhibit the SAP binding by 50%. Binding of SAP to type TV collagen was inhibited by both collagen IV and C1q but not by phos phatidylethanolamine or bovine serum albumin, The inhibition data indi cate that SAP may bind to the triple helical region of type IV collage n via a site distinct from its galactan binding site, The most likely site of SAP involved in its interaction with type TV collagen may be t he region spanning amino acid residues 108-120, which shows a great de al of sequence homology (60% strict identity) with the CRP region impl icated in its binding to the collagen-like region of the C1q molecule.