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

We have studied the structure and the function of a truncated human insulin receptor in which 113 amino acids (aa 1231-1343) at the C-terminus of the beta-subunit were deleted. In this study, wild-type and truncated insulin receptors were expressed by stable transfection in NIH-3T3 cells and CHO cells. The mutation impairs post-translational processing of the insulin receptor; proteolytic cleavage is retarded, and degradation of the truncated receptor is accelerated. Furthermore, insulin-stimulated autophosphorylation of the mutant insulin receptor is impaired. This is associated with a defect in insulin-stimulated endocytosis. 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 mitogenic response to insulin. It has previously been shown that deletion of 43 amino acids at the C-terminus of the beta-subunit did not affect insulin receptor tyrosine kinase activity. Our data suggest that the structural domain located 43-113 amino acids from the C-terminus appears to have several functional roles. First, the domain appears to promote folding of receptor into the optimal conformation for post-translational processing. Second, the presence of this domain appears to promote the stability of the receptor beta-subunit in intact cells. Finally, perhaps as a consequence of the effects upon the stability of the receptor, this domain is required in intact cells for insulin-stimulated autophosphorylation and signal transmission.