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

Abstract

C1q, a subcomponent of C1 ‐ the first component of complement, is synthesized by macrophages (Mϕ). Immunofluorescence and immunoperoxidase studies first indicated the presence of Clq on the surface of guinea pig (gp) and human peritoneal Mϕ (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 Mϕ C1q were employed. The presence of C1q protein on the surface of gp peritoneal Mϕ is shown by cell surface labeling with the biotin derivative sulfosuccinimdyl–6–(biotinamido)‐hexanoate and subsequent immunoprecipitation. The mechanism by which Clq is attached to the cell membrane was also investigated. Intact cells were treated with acid stripping‐buffers or phosphati‐dylinositol‐specific phospholipase C and separated membranes were extracted with a buffer containing 1 M KC1 and 3 M urea. Regardless of which method was used, Clq remained attached to the membrane. When surface‐labeled cells were cultured, they were found to release the Clq from their surface membrane into the culture medium. Lysates of biosynthetically labeled cells were used to show that, like secreted or serum Clq, cellular Mϕ Clq binds to immobilized homologous IgG. This implies that the globular regions of the cellular Clq are functionally active. The results reveal that (i) cellular Mϕ C1q is firmly located in the membrane throughout the biosynthetic pathway, such that it is comparable with an integral membrane protein, (ii) cellular Mϕ C1q is not reversibly bound to the cell surface via a receptor. We suggest that C1q, as a membrane protein of Mϕ, serves as an Fc binding factor that also is secreted into the environment.