XCL100, AN INDUCIBLE NUCLEAR MAP KINASE PHOSPHATASE FROM XENOPUS-LAEVIS - ITS ROLE IN MAP KINASE INACTIVATION IN DIFFERENTIATED CELLS AND ITS EXPRESSION DURING EARLY DEVELOPMENT

We have cloned the Xenopus laevis homologue (XCL100) of the human CL10 0 (Thr/Tyr) MAP kinase phosphatase. Expression of the XCL100 mRNA and protein is inducible by serum stimulation and oxidative/heat stress in a X. laevis kidney cell line, In contrast, XCL100 is constitutively e xpressed in growing Xenopus oocytes. Recombinant XCL100 protein is abl e to dephosphorylate both tyrosine and threonine residues of activated p42 MAP kinase in vitro and both the Xenopus and human CL100 proteins were localised predominantly in the nucleus in transfected COS-1 cell s, As nuclear translocation of activated MAP kinase is necessary for s ome of its essential functions in proliferation and cell differentiati on our results indicate a role for CL100 in the regulation of these nu clear signalling events. In Xenopus kidney cells both heat shock and s erum stimulation lead to transient activation of MAP kinase. However, in contrast to results previously reported from studies on mammalian f ibroblasts the inactivation of MAP kinase in these epitheloid cells is rapid and is not dependent on synthesis of new protein. These results indicate that the induction of CL100 (or CL100-like enzymes) may not be required for MAP kinase inactivation in all cell types. Finally, du ring early embryogenesis, levels of XCL100 mRNA are greatly increased at the mid-blastula transition, suggesting that this enzyme may be inv olved in the regulation of MAP kinase activity during early developmen t.