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.