Biosynthesis and intracellular transport of the mouse macrophage Fc receptor.

The membrane insertion, processing, and intracellular transport of the mouse macrophage Fc receptor for IgG1/IgG2b was studied using specific mono- and polyclonal anti-receptor antibodies. By immunoprecipitation from Triton X-114 lysates of radiolabeled J774 cells, we determined that the mature 60-kDa receptor is a transmembrane glycoprotein which is synthesized in the rough endoplasmic reticulum as a 53-kDa precursor. Digestion of the precursor with endo-beta-N-acetylglucosaminidase F demonstrated that the receptor consisted of a 37-kDa polypeptide to which four asparagine-linked oligosaccharides were attached. Proteinase K treatment of isolated microsomes indicated that the receptor also has a putative 15-kDa cytoplasmic domain apparently recognized by at least one anti-Fc receptor monoclonal antibody. An additional 15-kDa domain was found to be inaccessible to proteolysis from either side of the membrane. Pulse-chase experiments using [35S]methionine-, [3H]mannose-, and [3H]galactose-labeled cells showed that processing of the receptor's N-linked oligosaccharides occurred rapidly (t1/2 = 15 min) and resulted in the conversion of at least three of the chains to complex endo-beta-N-acetylglucosaminidase H-resistant forms. O-Linked oligosaccharides were not detected. Fc receptor was detected on the plasma membrane 30 min after its synthesis. Transport of newly synthesized receptors to the plasma membrane was slowed but not blocked by incubation of J774 cells at 20 degrees C or by the carboxylic ionophore monensin, although monensin completely inhibited the galactosylation of the receptor.