Studies involving the biosynthesis and other aspects of the life cycle of the insulin receptor in human IM-9 lymphocytes as well as in isolated rat adipose cells are under investigation. The receptors are labeled by either biosynthetic incorporation of radioactive sugars or amino acids or by techniques that label proteins or glycoproteins at the cell surface. After labeling, the receptors are isolated by immunoprecipitation with anti-receptor antibodies and analyzed by sodium dodecyl suberate/polyacrylamide gel electrophoresis. In addition, in order to characterize the subcellular sites responsible for the biosynthetic as well as for other processes of the life cycle of the receptor, three different subcellular fractions were prepared from rat adipose cells: 1) plasma membrane fraction; 2) high density microsomal fraction (enriched in endoplasmic reticulum; 3) low density microsomal fraction (enriched in Golgi apparatus. Our results demonstrate that the insulin receptor is composed of two major subunits, Alpha and Beta, of apparent molecular weight of 135,000 and 95,000, respectively. Both Alpha and Beta subunits are exposed at the cell surface but their orientation in the plasma membrane is asymmetric. The a subunit transverses the membrane and has a portion of its chain exposed on the cytoplasm. Both subunits are synthesized in the endoplasmic reticulum as a Mr = 190,000 single polypeptide which contains exclusively high mannose type oligosaccharides. The proreceptor is then transported to the Golgi apparatus where it is cleaved by a tryspin-like enzyme yielding subunits of Mr = 120,000 and 80,000. Also in the Golgi apparatus, some of the carbohydrate chains of these """"""""immature"""""""" subunits are converted into complex type generating the mature Alpha and Beta subunits which are immediately inserted in the plasma membrane.
