A ser for cys mutation in the extracellular portion of insulin receptor β subunit impairs the insulin-insulin receptor complex internalization in CHO cells

We investigated the effects of a ser for cys 860 mutation, located in the extracellular portion of insulin receptor β subunit, on several receptor functions. CHO cells, stably transfected with the mutated cDNA, were used for this study. In the present paper, we show that the ser 860 mutation does not affect the ¹²⁵I-insulin binding, but severely impairs the insulin-insulin receptor complex internalization. The kinetic analysis of internalization indicates that this process is inhibited at steps preceding the coated pit endocytosis. The β subunit autophosphorylation of the mutated receptor is higher both in the basal and insulin stimulated states, compared with autophosphorylation measured in wild type insulin receptors. The ser 860 mutation impairs also the insulin receptor down regulation, thus suggesting an effect on the intracellular sorting of insulin-insulin receptor complex. On the basis of these results we suggest that the cys 860 plays an important role in insulin receptor lateral moving on cell surface, after insulin binding, and on the intracellular sorting to degradation pathways.