Regulation of MAPK Cascades by Protein Tyrosine Phosphatases

The degree of activation of mitogen-activated protein kinases (MAPKs) in response to extracellular stimuli is tightly regulated in vivo in a cell type- and stimulus-dependent manner, as the result of the coordinated function of protein kinases and phosphatases (1 ). Activation of MAPKs requires phosphorylation by MAPK kinases on both the threonine and tyrosine MAPKs regulatory residues, located in the activation loop within the kinase domain VII (2 ). On the other hand, dephosphorylation of any of these two residues is sufficient to inactivate the MAPKs. Thus, the protein serine/threonine phosphatases PP2A and PP2C can inactivate, in intact cells, the MAPKs extracellular signal-regulated kinase 1/2 (ERK1/2) and p38α, respectively (3 ,4 ). Furthermore, a MAPK dual-specific phosphatase family has been characterized, whose members differentially dephosphorylate and inactivate distinct MAPKs (5 ,6 ). Finally, the related protein tyrosine phosphatases (PTPs) PTP-SL and HePTP/LC-PTP inactivate the MAPKs by specific dephosphorylation of the phosphotyrosine regulatory residue (7 ). In particular, PTP-SL dephosphorylates the MAPKs ERK1/2 and p38α (but not c-Jun N-terminal kinase [JNK]) after association through a 16 amino acid kinase interaction motif, located in the cytosolic regulatory domain of this PTP (8 –10 ). Here, protocols are described to assess the effects of PTPs on the tyrosine dephosphorylation of the MAPKs regulatory sites and on their kinase activities, both in intact cells and in vitro.