Multisite serine phosphorylation of the insulin and IGF‐I receptors in transfected cells

Abstract
Serine phosphorylation of insulin/IGF‐I receptors in transfected fibroblasts was analysed by peptide mapping, PMA stimulated the phosphorylation of 5 distinct insulin receptor phosphopeptides: a single major phosphothreonine peptide containing Thr‐1348, one major and 3 minor phosphoserine peptides. The major insulin‐stimulated phosphoserine peptides were the same as those after PMA, with the exception of 2 minor phosphoserine peptides. PMA stimulated phosphorylation of a single major IGF‐I receptor phosphoserine peptide which was phosphorylated to a lesser extent after IGF‐I. We conclude that insulin/IGF‐I and PMA stimulate phosphorylation of the same sites, but differ in the extents of phosphorylation.