Mutational analysis of the N-linked glycosylation sites of the human insulin receptor

Site-directed mutagenesis has been used to remove 15 of the 18 potential N- linked glycosylation sites, in 16 combinations, from the human exon 11-minu s receptor isoform. The three glycosylation sites not mutated were asparagi ne residues 25, 397 and 894, which are known to be important in receptor bi osynthesis or function. The effects of these mutations on proreceptor proce ssing into alpha and beta subunits, cell-surface expression, insulin bindin g and receptor autophosphorylation were assessed in Chinese hamster ovary c ells. The double mutants 16+78, 16+111, 16+215, 16+255, 337+418, the triple mutants 295+337+418, 295+418+514, 337+418+514 and 730+743+881 and the quad ruple mutants 606+730+743+881 and 671+730+743+881 seemed normal by all crit eria examined. The triple mutant 16+215+255 showed only low levels of corre ctly processed receptor on the cell surface, this processed receptor being autophosphorylated in response to insulin. The quadruple mutant 624+730+743 +881 showed normal processing and ligand binding but exhibited a constituti vely active tyrosine kinase as judged by autophosphorylation. Three higher- order mutants were constructed, two of which, 16+337+418+730+743 (Delta 6) and 16+295+337+418+730+743+881 (Delta 7a), seemed normal. The third constru ct, 16+337+418+514+730+743+881 (Delta 7b), was expressed at high levels on the cell surface, essentially as uncleaved proreceptor with only the small proportion of Delta 7b that was correctly processed showing insulin-stimula ted autophosphorylation. The mutations of Delta 6 and Delta 7a were incorpo rated into soluble ectodomains, which had affinities for insulin that were 4-fold that of wild-type ectodomain. The Delta 6 ectodomain expressed in Le c8 cells was produced in quantity in a bioreactor for subsequent structural analysis.