Endocytosis, membrane recycling and Fc receptor function

We have studied the composition and fate of plasma membrane internalized during both fluid-phase and receptor-mediated endocytosis in mouse macrophages. Particular attention has been paid to the macrophage Fc receptor, an intrinsic membrane glycoprotein that we have isolated and characterized biochemically and immunologically. Monoclonal and polyclonal antibodies directed against the receptor and against a series of other unrelated plasma membrane proteins have been used. In addition, we have used radioiodination techniques to label selectively the polypeptides of pinocytic vesicle membrane from within intact cells. Our results indicate that fluid pinocytosis in macrophages involves the internalization of a largely representative sample of plasma membrane polypeptides. Significantly, the Fc receptor seems to be internalized at a rate similar to that of most other membrane proteins. However, selective internalization of the receptor is induced during the endocytosis of certain ligands. The phagocytosis of immunoglobulin G (IgG)-coated erythrocyte ghosts results in the selective and largely irreversible removal of Fc receptors from the macrophage surface. Selectively internalized receptors do not recycle but are rapidly degraded. Fc receptors also appear to be preferentially interiorized during the rapid pinocytosis of IgG-containing soluble immune complexes. Uptake is accompanied by a sharp decrease in the number of surface receptors, which is partially reversed after the removal of ligand.