MKP-3, A NOVEL CYTOSOLIC PROTEIN-TYROSINE-PHOSPHATASE THAT EXEMPLIFIES A NEW CLASS OF MITOGEN-ACTIVATED PROTEIN-KINASE PHOSPHATASE

MKP-1 (also known as CL100, 3CH134, Erp, and hVH-1) exemplifies a clas s of dual-specificity phosphatase able to reverse the activation of mi togen-activated protein (MAP) kinase family members by dephosphorylati ng critical tyrosine and threonine residues. We now report the cloning of MKP-3, a novel protein phosphatase that also suppresses MAP kinase activation state. The deduced amino acid sequence of MKP-3 is 36% ide ntical to MKP-1 and contains the characteristic extended active-site s equence motif VXVHCXXGXSRSXTXXXAYLM (where X is any amino acid) as wel l as two N-terminal CH2 domains displaying homology to the cell cycle regulator Cdc25 phosphatase. When expressed in COS-7 cells, MKP-3 bloc ks both the phosphorylation and enzymatic activation of ERK2 by mitoge ns. Northern analysis reveals a single mRNA species of 2.7 kilobases w ith an expression pattern distinct from other dual-specificity phospha tases. MKP-3 is expressed in lung, heart, brain, and kidney, but not s ignificantly in skeletal muscle or testis. In situ hybridization studi es of MKP-3 in brain reveal enrichment within the CA1, CA3, and CA4 la yers of the hippocampus. Metrazole-stimulated seizure activity trigger s rapid (<1 h) but transient up-regulation of MKP-3 mRNA in the cortex , piriform cortex, and some amygdala nuclei. Metrazole stimulated simi lar regional up-regulation of MKP-1, although this was additionally in duced within the thalamus. MKP-3 mRNA also undergoes powerful inductio n in PC12 cells after 3 h of nerve growth factor treatment. This respo nse appears specific insofar as epidermal growth factor and dibutyryl cyclic AMP fail to induce significant MKP-3 expression. Subcellular lo calization of epitope-tagged MKP-3 in sympathetic neurons reveals expr ession in the cytosol with exclusion from the nucleus. Together, these observations indicate that MKP-3 is a novel dual-specificity phosphat ase that displays a distinct tissue distribution, subcellular localiza tion, and regulated expression, suggesting a unique function in contro lling MAP kinase family members. Identification of a second partial cD NA clone (MKP-X) encoding the C-terminal 280 amino acids of an additio nal phosphatase that is 76% identical to MKP-3 suggests the existence of a distinct structurally homologous subfamily of MAP kinase phosphat ases.