Serine/threonine phosphorylation of ShcA. Regulation of protein-tyrosine phosphatase-pest binding and involvement in insulin signaling.

Serine phosphorylation of the ShcA signaling molecule has been reported recently. In this work, we have identified 12-O-tetradecanoylphorbol-13-acetate (TPA)- and growth factor-induced serine/threonine phosphorylation sites in p52(Shc) and p66(Shc). Among them, Ser(29) in p52(Shc) (equivalent to Ser(138) in p66(Shc)) was phosphorylated only after TPA stimulation. Phosphorylation of this site together with the intact phosphotyrosine-binding domain was essential for ShcA binding to the protein-tyrosine phosphatase PTP-PEST. TPA-induced ShcA phosphorylation at this site (and hence, its association with PTP-PEST) was inhibited by a protein kinase C-specific inhibitor and was induced by overexpression of constitutively active mutants of protein kinase Calpha, -epsilon, and -delta isoforms. Insulin also induced ShcA/PTP-PEST association, although to a lesser extent than TPA. Overexpression of a PTP-PEST binding-defective mutant of p52(Shc) (S29A) enhanced insulin-induced ERK activation in insulin receptor-overexpressing HIRc-B cells. Consistent with this, p52(Shc) S29A was more tyrosine-phosphorylated than wild-type p52(Shc) after insulin stimulation. Thus, we have identified a new mechanism whereby serine phosphorylation of ShcA controls the ability of its phosphotyrosine-binding domain to bind PTP-PEST, which is responsible for the dephosphorylation and down-regulation of ShcA after insulin stimulation.