PHOSPHORYLATION OF PHOSPHATASE-1-ALPHA IN CELLS EXPRESSING V-SRC

Phosphatase-1 (PP1) is phosphorylated ''in vitro'' by the tyrosine-kin ases c-src, v-src and v-abl. In the case of src, this induces enzyme i nactivation. We investigated whether in NIH-3T3 cells expressing v-src (A4 cells) PP1 was phosphorylated on Tyr and inactivated. In mammalia n cells, three PP1 isoforms are present: PP1 alpha, PP1 gamma 1 and PP 1 delta. In A4 cells the three PP1 isoforms were all phosphorylated on Ser, but only PP1 alpha was also phosphorylated on Tyr. A lower level of PP1 phosphorylation, and on Ser only, was found also in wild-type NIH-3T3 cells. In A4 cells most of the Tyr-phosphorylated PP1 alpha wa s cytosolic. Also the PPI activity was decreased in the cytosol of the A4 cells. Assay of the three immunoprecipitated PP1 isoforms indicate d that only PP1 alpha was inactivated. Altogether the data suggest tha t PP1 alpha might be a target of v-src ''in vivo.'' (C) 1996 Academic Press, Inc.