Insulin-induced surface redistribution regulates internalization of the insulin receptor and requires its autophosphorylation

Optimizing transmembrane domain helicity accelerates insulin receptor internalization and lateral mobility

Transmembrane (TM) domains of integral membrane proteins are generally thought to be helical. However, a Gly-Pro sequence within the TM domain of the insulin receptor is predicted to act as a helix breaker. CD analyses of model TM peptides in a lipid-like environment show that substitution of Gly and Pro by Ala enhances helicity. On this basis, Gly933 and Pro934 within the TM domain of the intact human insulin receptor were mutated to Ala (G-->A, P-->A, GP-->AA) to assess effects of altered helicity on receptor functions. Mutated and wild-type receptors, expressed stably in cultured CHO cells at equivalent levels, were properly assembled, biosynthetically processed, and exhibited similar affinities for insulin. Receptor autophosphorylation and substrate kinase activity in intact cells and soluble receptor preparations were indistinguishable. In contrast, insulin-stimulated receptor internalization was accelerated 2-fold for the GP-->AA mutant, compared to a wild-type control or the G-->A and P-->A mutants. Insulin degradation, which occurs during receptor endocytosis and recycling, was similarly elevated in cells transfected with GP-->AA mutant receptors. Fluorescence photobleaching recovery measurements showed that the lateral mobility of GP-->AA mutant receptors was also increased 2- to 3-fold. These results suggest that lateral mobility directly influences rates of insulin-mediated receptor endocytosis and that rates of endocytosis and lateral mobility are retarded by a kinked TM domain in the wild-type receptor. Invariance of Gly-Pro within insulin receptor TM domain sequences suggests a physiologic advantage for submaximal rates of receptor internalization.

Transmembrane signaling by an insulin receptor lacking a cytoplasmic beta-subunit domain

To assess the function of the cytoplasmic domain of the insulin receptor (IR) beta subunit, we have studied a mutant IR truncated by 365 aa (HIR delta 978), thereby deleting > 90% of the cytoplasmic domain. HIR delta 978 receptors were processed normally to homodimers that were expressed at the cell surface where they bind insulin with normal affinity. Although these truncated IRs were inactive with respect to ligand-induced internalization and autophosphorylation, insulin stimulated endogenous substrate (pp185) phosphorylation significantly more in HIR delta 978 cells than in untransfected Rat1 cells. Importantly, despite absence of the beta-subunit cytoplasmic domain, fibroblasts expressing HIR delta 978 receptors displayed enhanced sensitivity to insulin for stimulation of glucose incorporation into glycogen, alpha-aminoisobutyric acid uptake, thymidine incorporation, and S6 kinase activity compared with parental fibroblasts. Insulin also induced the expression of the protooncogene c-fos and the early growth response gene Egr-1 in HIR delta 978 cells far greater than in parental Rat1 fibroblasts. Furthermore, an agonistic monoclonal antibody specific for the human IR stimulated insulin action in fibroblasts expressing wild-type human IR but had no effect on HIR delta 978 cells. In conclusion, the HIR delta 978 truncated IRs appear to confer enhanced insulin sensitivity by augmenting the signaling properties of the endogenous rodent IRs.

The insulin receptor juxtamembrane region contains two independent tyrosine/beta-turn internalization signals

We have investigated the role of tyrosine residues in the insulin receptor cytoplasmic juxtamembrane region (Tyr953 and Tyr960) during endocytosis. Analysis of the secondary structure of the juxtamembrane region by the Chou-Fasman algorithms predicts that both the sequences GPLY953 and NPEY960 form tyrosine-containing beta-turns. Similarly, analysis of model peptides by 1-D and 2-D NMR show that these sequences form beta-turns in solution, whereas replacement of the tyrosine residues with alanine destabilizes the beta-turn. CHO cell lines were prepared expressing mutant receptors in which each tyrosine was mutated to phenylalanine or alanine, and an additional mutant contained alanine at both positions. These mutations had no effect on insulin binding or receptor autophosphorylation. Replacements with phenylalanine had no effect on the rate of [125I]insulin endocytosis, whereas single substitutions with alanine reduced [125I]insulin endocytosis by 40-50%. Replacement of both tyrosines with alanine reduced internalization by 70%. These data suggest that the insulin receptor contains two tyrosine/beta-turns which contribute independently and additively to insulin-stimulated endocytosis.