Mitotic Activation of Protein-tyrosine Phosphatase α and Regulation of Its Src-mediated Transforming Activity by Its Sites of Protein Kinase C Phosphorylation*

During mitosis, the catalytic activity of protein-tyrosine phosphatase (PTP) α is enhanced, and its inhibitory binding to Grb2, which specifically blocks Src dephosphorylation, is decreased. These effects act synergistically to activate Src in mitosis. We show here that these effects are abrogated by mutation of Ser¹⁸⁰ and/or Ser²⁰⁴, the sites of protein kinase C-mediated phosphorylation within PTPα. Moreover, either a Ser-to-Ala substitution or serine dephosphorylation specifically eliminated the ability of PTPα to dephosphorylate and activate Src even during interphase. This explains why the substitutions eliminated PTPα transforming activity, even though PTPα interphase dephosphorylation of nonspecific substrates was only slightly decreased. This occurred without change in the phosphorylation of PTPα at Tyr⁷⁸⁹, which is required for “phosphotyrosine displacement” during Src dephosphorylation. Thus, in addition to increasing PTPα nonspecific catalytic activity, Ser¹⁸⁰ and Ser²⁰⁴ phosphorylation (along with Tyr⁷⁸⁹ phosphorylation) regulates PTPα substrate specificity. This involves serine phosphorylation-dependent differential modulation of the affinity of Tyr(P)⁷⁸⁹for the Src and Grb2 SH2 domains. The results suggest that protein kinase C may participate in the mitotic activation of PTPα and Src and that there are intramolecular interactions between the PTPα C-terminal and membrane-proximal regions that are regulated, at least in part, by serine phosphorylation.