DELETION OF C-TERMINAL 113 AMINO-ACIDS IMPAIRS PROCESSING AND INTERNALIZATION OF HUMAN INSULIN-RECEPTOR - COMPARISON OF RECEPTORS EXPRESSEDIN CHO AND NIH-3T3 CELLS

We have studied the structure and the function of a truncated human in sulin receptor in which 113 amino acids (aa 1231-1343) at the C-termin us of the beta-subunit were deleted. In this study, wild-type and trun cated insulin receptors were expressed by stable transfection in NIH-3 T3 cells and CHO cells. The mutation impairs post-translational proces sing of the insulin receptor; proteolytic cleavage is retarded, and de gradation of the truncated receptor is accelerated. Furthermore, insul in-stimulated autophosphorylation of the mutant insulin receptor is im paired. This is associated with a defect in insulin-stimulated endocyt osis. Finally, in NIH-3T3 cells, the mutant insulin receptor failed to mediate the mitogenic effects of insulin. In CHO cells, transfection of insulin receptor cDNA (either wild-type or mutant) did not alter mi togenic response to insulin. It has previously been shown that deletio n of 43 amino acids at the C-terminus of the beta-subunit did not affe ct insulin receptor tyrosine kinase activity. Our data suggest that th e structural domain located 43-113 amino acids from the C-terminus app ears to have several functional roles. First, the domain appears to pr omote folding of receptor into the optimal conformation for post-trans lational processing. Second, the presence of this domain appears to pr omote the stability of the receptor beta-subunit in intact cells. Fina lly, perhaps as a consequence of the effects upon the stability of the receptor, this domain is required in intact cells for insulin-stimula ted autophosphorylation and signal transmission.