ABSTRACT
We have recently reported that the Arg1152-->Gln insulin receptor mutation (QK single mutant) alters a conserved motif (RK motif) immediately next to the key tyrosine phosphorylation sites (Tyr1146, Tyr1150, Tyr1151) of the receptor and constitutively activates its kinase and metabolic signaling. To investigate further the function of the RK motif, we have expressed two additional mutant insulin receptors: a single mutant, in which the second basic residue in the RK motif (Lys1153) was substituted (RA mutant); and a double mutant, in which both the Arg and the Lys residues were replaced with noncharged amino acids (QA mutant). As compared with the transfected wild-type receptors (WT), both the single and the double mutant receptors were normally synthetized and transported to the plasma membrane and bound insulin normally. Whereas the double mutant receptor exhibited preserved insulin-dependent autophosphorylation, kinase activity, and 2-deoxyglucose uptake, all of these functions were grossly impaired in the two single mutant receptors. Two-dimensional analysis of tryptic phosphopeptides from receptor beta-subunits revealed that decreased autophosphorylation of the single mutant receptors mainly involved regulatory Tyr1150,1151 and carboxyl-terminal Tyr1316,1322. At variance with the insulin-stimulated, insulin-independent tyrosine kinase activity toward poly(Glu-Tyr) 4:1 was increased 3-fold in both the double and the single mutants. All mutant receptors induced a 2-fold increase in basal 2-deoxyglucose uptake in NIH-3T3 cells. Treatment of WT transfected cells with 12-O-tetradecanoyl-phorbol-13-acetate or 8-bromo-cAMP increased insulin receptor phosphorylation by 3-fold. No phosphorylation was observed in cells expressing the two single or the double mutant receptor. Consistently, purified preparations of PKC and PKA phosphorylated the WT but not the mutant receptors in vitro. A 17-amino acid synthetic peptide encoding the receptor sequence surrounding the RK motif inhibited phosphorylation of WT insulin receptors by both protein kinases A and C. A mutant peptide in which the RK sequence was replaced by QK (to mimic the mutation in the QK receptor) exhibited no inhibitory effect. Thus, the RK insulin receptor motif is required for insulin receptor phosphorylation by protein kinases C and A and may modulate insulin-independent receptor activity. The RK motif may also have an important structural role in allowing normal insulin regulation of the kinase.
