Catalytic domain of the Serine/Threonine kinases, Mitogen-activated protein kinase-activated protein kinases
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. This subfamily is composed of the MAPK-activated protein kinases MK2, MK3, MK5 (also called PRAK for p38-regulated/activated protein kinase), and related proteins. These proteins contain a catalytic kinase domain followed by a C-terminal autoinhibitory region that contains nuclear localization (NLS) and nuclear export (NES) signals with a p38 MAPK docking motif that overlaps the NLS. In addition, MK2 and MK3 contain an N-terminal proline-rich region that can bind to SH3 domains. MK2 and MK3 are bonafide substrates for the MAPK p38, while MK5 plays a functional role in the p38 MAPK pathway although their direct interaction has been difficult to detect. MK2 and MK3 are closely related and show, thus far, indistinguishable substrate specificity, while MK5 shows a distinct spectrum of substrates. MK2 and MK3 are mainly involved in the regulation of gene expression and they participate in diverse cellular processes such as endocytosis, cytokine production, cytoskeletal reorganization, cell migration, cell cycle control and chromatin remodeling. They are implicated in inflammation and cance and their substrates include mRNA-AU-rich-element (ARE)-binding proteins (TTP and hnRNP A0), Hsp proteins (Hsp27 and Hsp25) and RSK, among others. MK2/3 are both expressed ubiquitously but MK2 is expressed at significantly higher levels. MK5 is a ubiquitous protein that is implicated in neuronal morphogenesis, cell migration, and tumor angiogenesis. It interacts with PKA, which induces cytoplasmic translocation of MK5. Its substrates includes p53, ERK3/4, Hsp27, and cytosolic phospholipase A2 (cPLA2). The MAPKAPK subfamily is part of a larger superfamily that includes the catalytic domains of other protein STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.
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