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Mitogen-activated protein (MAP) kinases are important players in signal transduction pathways activated by a range of stimuli and mediate a number of physiological and pathological changes in cell function. There are three major subgroups in the MAPK family: ERK, p38, and JNK/SAPK. ERK is activated mainly by mitogenic stimuli, whereas p38 and JNK/SAPK are activated mainly by stress stimuli or inflammatory cytokines. MAP kinases are part of a three-tiered phosphorylation cascade and MAP kinase phosphorylation on a threonine and tyrosine residue located within the activation loop of kinase subdomain VIII results in activation. However, this process is reversible even in the continued presence of activating stimuli, indicating that protein phosphatases provide an important mechanism for MAP kinase control. Dual specificity phosphatases (DSPs) from the tyrosine phosphatase (PTP) gene superfamily are selective for dephosphorylating the critical phosphothreonine and phosphotyrosine residues within MAP kinases. Ten members of dual specificity phosphatases specifically acting on MAPKs, termed MAPK phosphatases (MKPs), have been reported. They share sequence homology and are highly specific for MAPKs but differ in the substrate specificity, tissue distribution, subcellular localization, and inducibility by extracellular stimuli. MKPs have been shown to play important roles in regulating the function of the MAPK family. DSP gene expression is induced strongly by various growth factors and/or cellular stresses. Expression of some gene family members, including CL100/MKP-1, hVH-2/MKP-2, and PAC1, is dependent at least in part on MAP kinase activation providing negative feedback for the inducing MAP kinase or for regulatory cross talk between parallel MAP kinase pathways. DSPs are localized to different subcellular compartments and certain family members appear highly selective for inactivating distinct MAP kinase isoforms. This enzymatic specificity is due to catalytic activation of the DSP phosphatase after tight binding of its amino-terminal to the target MAP kinase. Thus, DSP phosphatases provide a sophisticated mechanism for targeted inactivation of selected MAP kinase activities. (This definition may be outdated - see the DesignNote.)
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