Regulation and catalytic mechanism of p38 signaling. The pathways involving MAPKs were first delineated by the discovery of ERK and were found to be heavily regulated three-tiered kinase pathways. MAPKs have a conserved Thr-x-Tyr activating motif in the activation loop of the kinase that is specific to each family member. The MAPKs are activated by reversible Tyr and Thr phosphorylation by dual specific MAPK kinases (MKKs). These MKKs are themselves activated via Ser/Thr phosphorylation by MAPK kinases kinases (MKKKs or MAP3Ks). Once activated, MAPKs directed to their substrates by proline residues, though specific MAPK docking sites on the substrates allow for specificity. Inactivation of MAPKs is mediated by phosphatases, proteins that dephosphorylate both Thr and Tyr residues of all the three major MAPKs. The most common of these phosphatases include pp1, pp2C or the MKP family of MAPK-specific phosphatases. The p38-specific activation sequence is Thr-Gly-Tyr (TGY) corresponding to Thr180 and Tyr182, which is primarily phosphorylated by MKK3/6 in response to cytokines, heat shock, and mitogenic activating stimuli (74). Upstream activation of MKK3/6 often occurs through the apoptosis-related kinase-1 (ASK-1), MEKKs, and Tak1. Signaling components to the MAPK pathways are often have more than one biological function and regulation (Fig 1.5). This is the case with the p38 and JNK pathways which, in many cases, run parallel to each other yet are at times independent and differentially activated. p38 and JNK typically share MKKKs based on experiments where overexpression of particular MKKKs and even some specific stimuli leads to co- activation JNK and p38 (75). Under physiological conditions, p38 MAPK activation, like the activation of the other MAPKs, is transient. p38 is dephosphorylated by all the major MAPK phosphatases though the MKP family of phosphatases acts primarily on the alpha and beta isoforms, with the other isoforms being resistant to inactivation by MKP. Published reports also suggest a feedback mechanism in which phosphorylated p38 may also activate dual-specific phosphatases, threonine phosphatases, and tyrosine phosphatases in order to terminate p38 signaling. As all other kinases, p38 uses ATP to transfer a phosphate group to a specific targeted substrate. The ordered sequential kinetic mechanism of p38 involves the initial substrate binding which is then required for ATP to bind strongly (76). This was based on the use of ATF as the p38 MAPK substrate yet it is possible that specific subs...
