The structure of the insulin receptor: studies using external and internal labeling techniques

Using both biosynthetic and external labeling techniques, we have demonstrated that the insulin receptor consists of two major subunits with a molecular weight of approximately 135,000 and 95,000, respectively. These two major subunits are glycoproteins. Our data and observations from other laboratories have led to the suggestion that the insulin receptor is a heterodimer of the 134,000 and 95,000 subunits and, in fact, has an immunoglobulin-like structure with heavy and light chains held together by disulfide bonds. Biosynthetic studies showing the incorporation of all four labeled monosaccharides (fucose, mannose, galactose and glucosamine) into the two major subunits of the insulin receptor suggested that both subunits were likely to contain carbohydrate chains of the complex, N-linked type. External labeling techniques demonstrated that a portion of the protein and carbohydrate moiety of both subunits was exposed at the external cell surface. Further, labeling of the external oriented carbohydrates revealed remarkable differences in the nonreducing termini of the carbohydrate chains of both major subunits. Indeed, in the Mr 134,000 subunit, there appeared to be more exposed galactosyl or N-acetylgalactosaminyl (or both) residues, whereas the Mr 95,000 subunit seemed to have a higher degree of sialylation.