Receptor-mediated internalization of insulin requires a 12-amino acid sequence in the juxtamembrane region of the insulin receptor beta-subunit

The juxtamembrane region of the insulin receptor (IR) beta-subunit contains an unphosphorylated tyrosyl residue (Tyr960) that is essential for insulin-stimulated tyrosyl phosphorylation of some endogenous substrates and certain biological responses (White, M.F., Livingston, J.N., Backer, J.M., Lauris, V., Dull, T.J., Ullrich, A., and Kahn, C.R. (1988) Cell 54, 641-649). Tyrosyl residues in the juxtamembrane region of some plasma membrane receptors have been shown to be required for their internalization. In addition, a juxtamembrane tyrosine in the context of the sequence NPXY [corrected] is required for the coated pit-mediated internalization of the low density lipoprotein receptor. To examine the role of the juxtamembrane region of the insulin receptor during receptor-mediated endocytosis, we have studied the internalization of insulin by Chinese hamster ovary (CHO) cells expressing two mutant receptors: IRF960, in which Tyr960 has been substituted with phenylalanine, and IR delta 960, in which 12 amino acids (Ala954-Asp965), including the putative consensus sequence NPXY [corrected], were deleted. Although the in vivo autophosphorylation of IRF960 and IR delta 960 was similar to wild type, neither mutant could phosphorylate the endogenous substrate pp185. CHO/IRF960 cells internalized insulin normally whereas the intracellular accumulation of insulin by CHO/IR delta 960 cells was 20-30% of wild-type. However, insulin internalization in the CHO/IR delta 960 cells was consistently more rapid than that occurring in CHO cells expressing kinase-deficient receptors (CHO/IRA1018). The degradation of insulin was equally impaired in CHO/IR delta 960 and CHO/IRA1018 cells. These data show that the juxtamembrane region of the insulin receptor contains residues essential for insulin-stimulated internalization and suggest that the sequence NPXY [corrected] may play a general role in directing the internalization of cell surface receptors.     

Augmented desensitization to epidermal growth factor (EGF) immediate actions: a novel mechanism for altered EGF growth response in mutant A431 cells.

Epidermal growth factor (EGF) may either stimulate or inhibit cell growth. To elucidate the mechanism of these varied effects, we compared EGF action in parental A431 cells in which cell growth is inhibited, and clone 15, a mutant of these cells resistant to EGF growth inhibition. In both lines, EGF receptor was present in similar concentrations and underwent tyrosine phosphorylation to the same extent. Likewise, in both lines, acute exposure to EGF stimulated an increase in free cytoplasmic [Ca2+], as well as a similar increase in phosphorylation of lipocortin 1, a major substrate for the EGF receptor kinase whose phosphorylation is calcium-dependent. On the other hand, pretreatment of clone 15 cells with EGF for 72 h abolished EGF-induced phosphorylation of lipocortin 1 and led to a loss of the increase in cytoplasmic free [Ca2+], whereas no such desensitization was seen in the parental A431 cells. These data indicate a link between EGF-induced increase in cytoplasmic calcium, lipocortin phosphorylation, and cell growth and suggest that differences in mechanisms of desensitization to these immediate actions of EGF may lead to altered growth response to this hormone.     

Evidence for the presence of diacylglycerol kinase in rat brain myelin

The previous demonstration that incubation of brain slices with [³²P]phosphate brings about rapid tabeling of phosphatidic acid in myelin suggests that the enzyme involved should be present in this specialized membrane. DAG kinase (ATP:1,2-diacyglycerol 3-phosphotransferase, E.C. 2.7.1.107) is present in rat brain homogenate at a specific activity of 2.5 nmol phosphatidic acid formed/min/mg protein, while highly purified myelin had a much lower specific activity (0.29 nmol/min/mg protein). Nevertheless, the enzyme appears to be intrinsic to this membrane since it can not be removed by washing with a variety of detergents or chelating agents, and it could not be accounted for as contamination by another subcellular fraction. Production of endogenous, membrane-associated, diacylglycerol (DAG) by PLC (phospholipase C) treatment brought about translocation from soluble to particulate fractions, including myelin. Another level of control of activity involves inactivation by phosphorylation; a 10 min incubation of brain homogenate with ATP resulted in a large decrease in DAG kinase activity in soluble, particulate and myelin fractions.     

Conformational changes in the alpha- and beta-subunits of the insulin receptor identified by anti-peptide antibodies

The structure of the insulin receptor was studied with polyclonal antibodies obtained from rabbits which were immunized with synthetic peptides having a sequence identity to three regions of the alpha-subunit and five regions of the beta-subunit. None of the alpha-subunit antibodies including alpha-Pep8 (residues 40-49 (Ullrich, A., Bell, J.R., Chen, E.Y., Herrera, R., Petruzzelli, L.M., Dull, T.J., Gray, A., Coussens, L., Liao, Y.-C., Tsubokawa, M., Mason, A., Seeburg, P.H., Grunfeld, C., Rosen, O.M., and Ramachandran, J. (1985) Nature 313, 756-761), alpha-Pep7 (12 amino acid C-terminal extension (Ebina, Y., Ellis, L., Jarnagin, K., Ederly, M., Graf, L., Clauser, E., Ou, J.-H., Masiar, F., Kan, Y.W., Goldfine, I.D., Roth, R.A., and Rutter, W.J. (1985) Cell 313, 747-758], or alpha-Pep6 (residues 1-7, 9) immunoprecipitated the human insulin receptor solubilized from IM-9 lymphocytes; however, alpha-Pep8 immunoprecipitated the dithiothreitol-reduced receptor. Antibodies prepared against the N terminus of the beta-subunit (alpha-Pep5, residues 780-790) and the ATP binding site (alpha-Pep3, residues 1013-1022) did not react with the intact receptor under any conditions; however, antibodies to the C terminus of the beta-subunit (alpha-Pep1, residues 1314-1324) and to the juxta-membrane region (alpha-Pep3, residues 952-962) immunoprecipitated the solubilized receptor in both its phosphorylated and nonphosphorylated forms. In contrast, the antibody reactive with the regulatory region of the beta-subunit which contains the major autophosphorylation sites (alpha-Pep2, residues 1143-1154) only precipitated the phosphorylated form. Thus the conformation of the extracellular domain of the receptor is rigid and stabilized by disulfide bonds, whereas several regions of the intracellular domain are accessible to antibodies and undergo conformational changes during autophosphorylation.