We have studied the biosynthesis of the insulin receptor in human IM-9 lymphocytes. The alpha (135,000) and beta (95,000) subunits of the receptor are synthesized in the endoplasmic reticulum as a single M/r 190,000 glycoprotein with only high mannose oligosaccharide chains, i.e. proreceptor. This proreceptor is then transported to the Golgi complex 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 found in control cells is GLc1Man9GlcNac2 which represents only a small fraction (3%) of the total. The predominant proreceptor oligosaccharides are MAN9GLCNAc2 (25%) and MAN8GLCNAc2 (48%). Since a GLc3MAN9GLNAc2 species is transferred cotranslationally, carbohydrate processing of the proreceptor is very rapid and limited to removal of the three glucoses and one mannose. Furthermore, in the presence of glucosidase inhibitors, castanospermine and l-deoxynojirimycin, an abnormal precursor of M/r 205,000 is synthesized. The processing of this precursor to mature subunits is delayed and there is a reduction in cell surface insulin receptors. Thus, glucose removal is an important signal for processing of the insulin receptor. Additionally, we have found that the insulin receptor contains covalently linked atty acids. Both the alpha and the beta subunits incorporate (3H)myristic and (3H)palmitic acids. The incorporation of fatty acid is dependent on protein synthesis and is found in the M/r 190,000 precursor. Thus, fatty acylation is a newly identified post-translational modification of the insulin receptor.
