Agglutination of complement-coated erythrocytes by serum amyloid P-component

Serum amyloid P-component (SAP) is a normal plasma glycoprotein apparently identical with the P-component associated with amyloid deposits. SAP shows extensive amino acid sequence homology with the C-reactive protein, and both have a similar molecular configuration. SAP undergoes calcium-dependent binding to zymosan, agarose, and amyloid fibrils, but its functional properties are not yet known. We report here that SAP agglutinates complement-(C) coated antibody-sensitized erythrocytes by a calcium-dependent reaction. SAP was found to interact predominantly with a modified form of bound C3b. This modification was achieved by prolonged treatment of EAC43 with heated normal human serum or with isolated C3bINA and beta 1H, and reactivity was reduced upon treatment of the cells with trypsin. SAP thus seems to react with fixed C3 in a manner similar to the reaction of C3 with bovine conglutinin, a molecule that also undergoes calcium-dependent binding to zymosan and agarose. These studies identify a new reactivity for SAP and demonstrate an interaction between an amyloid protein and the C system. The close similarity between the calcium-dependent binding specificities of SAP and of bovine conglutinin may assist in characterization of these molecules and in investigation of their function.