Threonine 1336 of the human insulin receptor is a major target for phosphorylation by protein kinase C

Abstract

The ability of tumor-promoting phorbol diesters to inhibit both insulin receptor tyrosine kinase activity and its intracellular signaling correlates with the phosphorylation of the insulin receptor .beta. subunit on serine and threonine residues. In the present studies, mouse 3T3 fibroblasts transfected with a human insulin receptor cDNA and expressing greater than one million of these receptors per cell were labeled with [32P]phosphate and treated with or without 100 nM 4.beta.-phorbol 12.beta.-myristate 13.alpha.-acetate (PMA). Phosphorylated insulin receptors were immunoprecipitated and digested with trypsin. Alternatively, insulin receptors affinity purified from human term placenta were phosphorylated by protein kinase C prior to trypsin digestion of the 32P-labeled .beta. subunit. Analysis of the tryptic phosphopeptides from both the in vivo and in vitro labeled receptors by reversed-phase HPLC and two-dimensional thin-layer separation revealed that PMA and protein kinase C enhanced the phosphorylation of a peptide with identical chromatographic properties. Partial hydrolysis and radiosequence analysis of the phosphopeptide derived from insulin receptor phosphorylated by protein kinase C indicated that the phosphorylation of this tryptic peptide occurred specifically on a threonine, three amino acids from the amino terminus of the tryptic fragment. Comparison of these data with the known, deduced receptor sequence suggested that the receptor-derived tryptic phosphopeptide might be Ile-Leu-Thr(P)-Leu-Pro-Arg. Comigration of a phosphorylated synthetic peptide containing this sequence with the receptor-derived phosphopeptide confirmed the identity of the tryptic fragment. The phosphorylation site corresponds to threonine 1336 in the human insulin receptor .beta. subunit. This threonine, which resides in a receptor domain also containing tyrosine phosphorylation sites, is located eight amino acids from the carboxyl terminus of the .beta. subunit and may play a role in the protein kinase C induced inhibition of insulin receptor tyrosine kinase activity.