PTP-PEST - A PROTEIN-TYROSINE-PHOSPHATASE REGULATED BY SERINE PHOSPHORYLATION

The protein tyrosine phosphatase PTP-PEST is an 88 kDa cytosolic enzym e which is ubiquitously expressed in mammalian tissues. We have expres sed PTP-PEST using recombinant baculovirus, and purified the protein e ssentially to homogeneity in order to investigate phosphorylation as a potential mechanism of regulation of the enzyme. PTP-PEST is phosphor ylated in vitro by both cyclic AMP-dependent protein kinase (PKA) and protein kinase C (PKC) at two major sites, which we have identified as Ser39 and Ser435. PTP-PEST is also phosphorylated on both Ser39 and S er435 following treatment of intact HeLa cells with TPA, forskolin or isobutyl methyl xanthine (IBMX). Phosphorylation of Ser39 in vitro dec reases the activity of PTP-PEST by reducing its affinity for substrate . In addition, PTP-PEST immunoprecipitated from TPA-treated cells disp layed significantly lower PTP activity than enzyme obtained from untre ated cells. Our results suggest that both PKC and PKA are capable of p hosphorylating, and therefore inhibiting, PTP-PEST in vivo, offering a mechanism whereby signal transduction pathways acting through either PKA or PKC may directly influence cellular processes involving reversi ble tyrosine phosphorylation.