Macrophage C1q: characterization of a membrane form of C1q and of multimers of C1q subunits.

It has been shown recently that C1q, a subcomponent of the first component of the classical complement pathway, is synthesized by macrophages and that endogenous C1q is detectable on the macrophage membrane. In this report, we demonstrate that membrane-associated C1q, which contains the A, B, and C chains of C1q, is structurally distinct from fluid-phase C1q in that the B chain of the membrane species is approximately 1000 m.w. less than its fluid-phase counterpart. By using biosynthetically ([3H]proline) labeled C1q from guinea pig peritoneal macrophages, we found that the membrane form of C1q is derived from already secreted C1q. The demonstration of a distinct membrane form of C1q supports earlier functional studies which implicated C1q as a membrane-associated molecule with receptor functions for those molecules which also interact with fluid-phase C1q, such as polyanions, immune complexes, and bacteria. Furthermore, we show that, in the vicinity of macrophages, C1q is very susceptible to oxidation manifested by the formation of disulfide bonds. By SDS-PAGE (nonreduced and reduced), we demonstrate the existence of disulfide-linked multimers (180,000 m.w., 360,000 m.w.) which are composed of the A, B, and C chains of C1q.