Regulatory proteins for the activated third and fourth components of complement (C3b and C4b) in mice. II....

We identified on the membrane of mouse spleen cells a polypeptide of Mr 190,000 (S190), with binding affinity for the mouse third component of the complement system (C3). S190, purified by affinity chromatography on C3-Sepharose, has properties resembling those of the human C3 receptor type 1 (CR1). Thus, S190, like CR1, served as a cofactor for the C3b inactivator (I)-mediated cleavage of fluid-phase C3b into iC3b, and had cofactor activity comparable to that of serum factor H (H). S190 also acted as a cofactor for the cleavages of membrane-bound C3b or membrane-bound iC3b into C3c (Mr 140,000) and C3dg (Mr 40,000) by serum factor I. As is the case with CR1, the specific activity of S190 for the cleavages leading to C3c-C3dg formation was approximately 100-fold greater than that of H. We therefore conclude that S190 and CR1 are analogous proteins.