Dephosphorylation of clustered phosphoserine residues in human Grb14 by protein phosphatase 1 and its effect on insulin receptor complex formation.

The physical interaction of the human growth factor receptor-bound protein 14 (hGrb14) and the insulin receptor (IR) represses insulin signaling. With respect to the recruiting mechanism of hGrb14 to IR respond to insulin stimulus, our previous reports have suggested that phosphorylation of Ser(358), Ser(362), and Ser(366) in hGrb14 by glycogen synthase kinase-3 repressed hGrb14-IR complex formation. In this study, we investigated phosphatase-mediated dephosphorylation of the hGrb14 phosphoserine residues. An in vitro phosphatase assay with hGrb14-derived synthetic phosphopeptides suggested that protein phosphatase 1 (PP1) is involved in the dephosphorylation of Ser(358) and Ser(362). Furthermore, coimmunoprecipitation experiments suggested that insulin-induced hGrb14-IR complex formation was repressed by the substitution of Ser(358) or Ser(362) with glutamic acid. These findings suggested that phosphate groups on Ser(358) and Ser(362) in hGrb14 are dephosphorylated by PP1, and the dephosphorylation facilitates hGrb14-IR complex formation.