Replacement of insulin receptor tyrosine residues 1162 and 1163 does not alter the mitogenic effect of the hormone.

Chinese hamster ovary transfectants that express insulin receptors in which tyrosine residues 1162 and 1163 were replaced by phenylalanine exhibit a total inhibition of the insulin-mediated tyrosine kinase activity toward exogenous substrates [histone, casein, and poly(Glu/Tyr)]; this latter activity is associated with total inhibition of the hypersensitivity reported for insulin in promoting 2-deoxyglucose uptake. We now present evidence that the twin tyrosines also control the insulin-mediated stimulation of glycogen synthesis. Surprisingly, this type of Chinese hamster ovary transfectant is as hypersensitive to insulin for its mitogenic effect as are Chinese hamster ovary cells expressing many intact insulin receptors. Such data suggest that (i) the insulin mitogenic effect routes through a different pathway than insulin uses to activate the transport and metabolism of glucose and (ii) the mitogenic effect of insulin is not controlled by the twin tyrosines. At the molecular level, the solubilized mutated receptor has no insulin-dependent tyrosine kinase activity, whereas this receptor displays measurable insulin-stimulated phosphorylation of its beta subunit in 32P-labeled cells. We therefore propose that the autocatalytic phosphorylating activity of the receptor reports a cryptic tyrosine kinase activity that cannot be visualized by the use of classical exogenous substrates.