One of the post translational modifications, N-linked glycosylation, of the insulin receptor has been studied in cultured cells. Using an experimental approach based on mutagenesis of the insulin receptor complimentary DNA (cDNA) at specific sites of potential modification, the mutated cDNA molecules are stably transfected into cultured cells, and then the structure and function of these mutant receptors can be studied. Processing of these mutants was investigated by biosynthetic labeling. Cells unable to glycosylate their receptors in the first four potential glycosylation sites are abnormally processed. These receptors do not appear on the cell surface and remain in proreceptor form in the endoplasmic reticulum. Mutants for each of these sites, individually and in combination, also are abnormally processed. However, the severity of the defect is dependent on the site of the mutation and the number of sites mutated. The first or the second site can be mutated with very little effect on the concentration of receptor processed. However, combination of mutations in both of these sites has a dramatic effect on the processing of the receptor, whereas the combination of mutations in the third and fourth sites has less of an effect on the processing of the receptor.
