Biochemistry

To study whether protein kinase C (PKC) isoforms can interact with protein-tyrosine-phosphatases (PTPs) which are connected to the insulin signaling pathway, we co-overexpressed PKC isoforms together with insulin receptor, docking proteins, and the PTPs SHP1 and SHP2 in human embryonic kidney (HEK) 293 cells. After phorbol ester induced activation of PKC isoforms alpha, beta1, beta2, and eta, we could show a defined gel mobility shift of SHP2, indicating phosphorylation on serine/threonine residues. This phosphorylation was not dependent on insulin receptor or insulin receptor substrate- 1 (IRS-1) overexpression and did not occur for the closely related phosphatase SHP1. Furthermore, PKC phosphorylation of SHP2 was completely blocked by the PKC inhibitor bisindolylmaleimide and was not detectable when SHP2 was co- overexpressed with kinase negative mutants of PKCbeta1 and - beta2. The phosphorylation also occurred on endogenous SHP2 in Chinese hamster ovary (CHO) cells stably overexpressing PKCbeta2. Using point mutants of SHP2, we identified serine residues 576 and 591 as phosphorylation sites for PKC. However, no change of phosphatase activity by TPA treatment was detected in an in vitro assay. In summary, SHP2 is phosphorylated on serine residues 576 and 591 by PKC isoforms alpha, beta1,beta2, and eta