THE HUMAN CL100 GENE ENCODES A TYR THR-PROTEIN PHOSPHATASE WHICH POTENTLY AND SPECIFICALLY INACTIVATES MAP KINASE AND SUPPRESSES ITS ACTIVATION BY ONCOGENIC RAS IN XENOPUS-OOCYTE EXTRACTS/

The expression of the human CL100 gene and its mouse homologue 3CH134 is increased up to 40-fold in fibroblasts exposed to oxidative/heat st ress and growth factors. CL100 is a member of an expanding family of p rotein tyrosine phosphatases with amino acid sequence similarity to a Tyr/Ser-protein phosphatase encoded by the late H1 gene of vaccinia vi rus. Here we show that the CL100 phosphatase, expressed and purified i n bacteria, rapidly and potently inactivates recombinant MAP kinase in vitro by the concomitant dephosphorylation of both its phosphothreoni ne and phosphotyrosine residues. Furthermore, CL100 suppresses the [va l12] ras-induced activation of MAP kinase in a cell-free system from X enopus oocytes. Both activities are abolished by mutagenesis of the hi ghly conserved cysteine (Cys-258) within the phosphatase active site. In contrast to the vaccinia H1 phosphatase, CL100 shows no measurable catalytic activity towards a remember of other substrate proteins modi fied on serine, threonine or tyrosine residues. Our results demonstrat e that CL100 is a dual specificity phosphatase and indicate that MAP k inase is one of its physiological targets. CL100 may be the first exam ple of a new class of protein phosphatases responsible for modulating the activation of MAP kinase following exposure of quiescent cells to growth factors and further implicates MAP kinase activation/deactivati on in the cellular response to stress.