CONTROL OF MAP KINASE ACTIVATION BY THE MITOGEN-INDUCED THREONINE TYROSINE PHOSPHATASE PAC1/

INTRACELLULAR Signalling following mitogenic stimulation of quiescent cells involves the initiation of a phosphorylation cascade that leads to the rapid and reversible activation of the mitogen-activated protei n (MAP) kinases ERK1 and ERK2 (refs 1, 2). MAP kinase activation is me diated by dual phosphorylation within the motif Thr-Glu-Tyr by MAP kin ase kinase (MEK)(3). Following activation, the MAP kinases translocate into the nucleus where they phosphorylate several transduction target s, including transcription factors(4-7). We have previously identified PAC1 as an immediate-early mitogen-inducible tyrosine phosphatase in nuclei of T cells(8). Here we present several lines of evidence indica ting that PAC1 is a physiologically relevant MAP kinase phosphatase. R ecombinant PAC1 in vitro is a dual-specific Thr/Tyr phosphatase with s tringent substrate specificity for MAP kinase. Constitutive expression of PAC1 in vivo leads to inhibition of MAP kinase activity normally s timulated by epidermal growth factor, phorbol myristyl acetate, or T-c ell receptor crosslinking. The inactivation of MAP kinase by PAC1 resu lts in inhibition of MAP kinase-regulated reporter gene expression.