We have studied the biosynthesis of the insulin receptor in human IM-9 lymphocytes and isolated rat adipocytes. The alpha (Mr=135,000) and beta (Mr=95,000) subunits of the receptor are synthesized in the endoplasmic reticulum as a Mr=190.000 single polypeptide, high mannose glycoprotein. This proreceptor is then transported to the Golgi apparatus where it undergoes proteolytic cleavage and carbohydrate processing. Direct analysis by high performance liquid chromatography of the carbohydrate chains of the insulin proreceptor demonstrate that the largest oligosaccharide present is Glc1Man9GlcNAc2. This structure represents only a small fraction (3%) of the total. The predominant proreceptor oligosaccharides are Man9GlcNAc2 (25%) and Man8GlcNAc2 (48%). Assuming that a Glc3Man9GlcNAc2 species is transferred co-translationally, carbohydrate processing of the proreceptor appears to be very rapid and limited to the removal of three glucoses and one mannose residue. We have also investigated the role of this early carbohydrate processing by treating cells with two different inhibitors of glucosidases: castanospermine and 1-deoxynojirimycin. In the presence of these agents an abnormal precursor (Mr=205,000) is synthesized. The processing of this proreceptor to mature subunits is markedly delayed and there is a reduction of cell surface insulin receptors. Thus glucose removal is probably an important signal for further processing and transport of the receptor. In addition the regulation of the proreceptor synthesis has been studied. Hydrocortisone treatment of IM-9 lymphocytes induces a 2-3 fold increase in the proreceptor concentration which leads to a similar increase in cell surface receptors.
