C1q, the collagen-like subcomponent of the first component of complement C1, is a membrane protein of guinea pig macrophages.

C1q, a subcomponent of C1--the first component of complement, is synthesized by macrophages (M phi). Immunofluorescence and immunoperoxidase studies first indicated the presence of C1q on the surface of guinea pig (gp) and human peritoneal M phi (Loos, M., Storz, R., Müller, W. and Lemmel, E. M., Immunobiology 1981. 158: 213). In our study different methods for labeling of gp serum and gp M phi C1q were employed. The presence of C1q protein on the surface of gp peritoneal M phi is shown by cell surface labeling with the biotin derivative sulfosuccinimdyl-6-(biotinamido)-hexanoate and subsequent immunoprecipitation. The mechanism by which C1q is attached to the cell membrane was also investigated. Intact cells were treated with acid stripping-buffers or phosphatidylinositol-specific phospholipase C and separated membranes were extracted with a buffer containing 1 M KCl and 3 M urea. Regardless of which method was used, C1q remained attached to the membrane. When surface-labeled cells were cultured, they were found to release the C1q from their surface membrane into the culture medium. Lysates of biosynthetically labeled cells were used to show that, like secreted or serum C1q, cellular M phi C1q binds to immobilized homologous IgG. This implies that the globular regions of the cellular C1q are functionally active. The results reveal that (i) cellular M phi C1q is firmly located in the membrane throughout the biosynthetic pathway, such that it is comparable with an integral membrane protein, (ii) cellular M phi C1q is not reversibly bound to the cell surface via a receptor. We suggest that C1q, as a membrane protein of M phi, serves as an Fc binding factor that also is secreted into the environment.