ALTERED EXPRESSION OF PROTEIN-TYROSINE-PHOSPHATASE 2C IN 293 CELLS AFFECTS PROTEIN-TYROSINE PHOSPHORYLATION AND MITOGEN-ACTIVATED PROTEIN-KINASE ACTIVATION

PTP2C, an SH2 domain-containing protein-tyrosine phosphatase, is recru ited to the growth factor receptors upon stimulation of cells. To inve stigate its role in growth factor signaling, we have overexpressed by approximately 6-fold the native PTP2C and a catalytically inactive mut ant of the enzyme in 293 human embryonic kidney cells. The native PTP2 C was located entirely in the cytosol, while the inactive mutant was n early equally distributed in cytosolic and membrane fractions. Express ion of the latter caused hyperphosphorylation on tyrosine of a 43-kDa protein, which was coimmunoprecipitated and co-partitioned in the plas ma membrane fraction with the inactive PTP2C mutant. This protein may represent a physiological substrate of PTP2C. Overexpression of the na tive PTP2C enhanced epidermal growth factor (EGF)-stimulated mitogen-a ctivated protein (MAP) kinase kinase activity by 30%, whereas expressi on of the inactive mutant reduced the stimulated activity by 50%. Simi lar effects were observed for the activation of MAP kinase as determin ed by activity assay, gel mobility shift, and tyrosine phosphorylation . The data suggest that the phosphatase activity of PTP2C is partly re quired for MAP kinase activation by EGF and that PTP2C may function by dephosphorylating the 43-kDa membrane protein.