DIFFERENTIAL REGULATION OF THE MAP, SAP AND RK P38 KINASES BY PYST1, A NOVEL CYTOSOLIC DUAL-SPECIFICITY PHOSPHATASE/

The Pyst1 and Pyst2 mRNAs encode closely related proteins, which are n ovel members of a family of dual-specificity MAP kinase phosphatases t ypified by CL100/MKP-1. Pyst1 is expressed constitutively in human ski n fibroblasts and, in contrast to other members of this family of enzy mes, its mRNA is not inducible by either stress or mitogens. Furthermo re, unlike the nuclear CL100 protein, Pyst1 is localized in the cytopl asm of transfected Cos-1 cells. Like CL100/MKP-1, Pyst1 dephosphorylat es and inactivates MAP kinase in vitro and in vivo. In addition, Pyst1 is able to form a physical complex with endogenous MAP kinase in Cos- 1 cells. However, unlike CL100, Pyst1 displays very low activity towar ds the stress-activated protein kinases (SAPKs) or RK/p38 in vitro, in dicating that these kinases are not physiological substrates for Pyst1 . This specificity is underlined by the inability of Pyst1 to block ei ther the stress-mediated activation of the JNK-1 SAP kinase or RK/p38 in vivo, or to inhibit nuclear signalling events mediated by the SAP k inases in response to UV radiation. Our results provide the first evid ence that the members of the MAP kinase family of enzymes are differen tially regulated by dual-specificity phosphatases and also indicate th at the MAP kinases may be regulated by different members of this famil y of enzymes depending on their subcellular location.