MAP/ERK kinase

MAPK cascades are composed of three cytoplasmic kinases, the MAPKKK, MAPKK, and MAPK, that are regulated by phosphorylation (Fig. 1) [1, 2]. The MAPKKK, also called MEKK for MEK kinase, is a serine/threonine kinase. Selective activation of MAPKKKs by upstream cellular stimuli results in the phosphorylation of MAPKK, also called MEK for MAP/ERK kinase by the MAPKKK. MAPKKK members are structurally diverse and are differentially regulated by specific upstream stimuli. The MAPKK is phosphorylated by the MAPKKK on two specific serine/ threonine residues in its activation loop. The MAPKK family members are dual specificity kinases capable of phosphorylating critical threonine and tyrosine residues in the activation loop of the MAPKs. MAPKKs have the fewest members in the MAPK signaling module. MAPKs are a family of serine/threonine kinases that upon activation by their respective MAPKKs, are capable of phosphorylating cytoplasmic substrates as well as... 

The MAPK/ERK proteins are at the lower end of signal transduction within a MAPK module and are generally preceded by two other protein kinases (Fig. 10.2). The MAPK/ERK proteins receive the signal in the form of an activating phosphorylation by a preceding protein kinase known as MAP/ERK kinase (MEK) or also MAP kinase kinase (MAPKK). 

Fig. 10.2. Components and activation of the ERK pathway. Ordering and specificity of protein kinases in the ERK pathway. ExtraceUular signals are registered via receptor tyrosine kinases and passed on to the Ras protein. Ras GTP activates protein kinases belonging to the group of MAPKK kinases (Raf kinases and MEEKs). The MAPKK kinases phosphorylate the downstream group of protein kinases, the MAPKKs at two Ser residues. The MAPKKs phosphorylate the MAPKs (ERKl and ERK2) at a Tyr and a Thr residue, and thus are classified as dual specificity kinases. MAPK mitogenic activated protein kinase ERK extracellularly regulated kinase MEK MAP/ERK kinase MAPKK MAPK kinase MAPKKK MAPKK kinase MEKK MEK kinase.

The events following the activation of Ras ultimately led to the activation of MAP kinase, followed by activation of expression of immediate-early response genes. Activation of MAP kinase requires two intermediate steps, both of which involve a phosphorylation (Figure 8.5). The immediate activator of MAP kinase is MAP-kinase-kinase (also called MAP kinase-ERK kinase, or MEK), a most unusual enzyme that phosphorylates MAP kinase on both a threonine (T) and a tyrosine (Y) residue. These are in the target-sequence seven residues (LTEYVATRWYRAPE) (Table 8.1)... 

CDK, cyclin-dependent kinase ERK, extracellular signal-regulated kinase GRK, G protein receptor kinase JNK, Jun kinase MAP kinase, mitogen activated protein kinase MEK, MAP kinase and ERK kinases RSK, ribosomal S6 kinase, GSK, glycogen synthase kinase SAPK, stress-activated protein kinase SEK, SAPK kinase. 

MAPK/ERK kinase, activates a MAP kinase or ERK through phosphorylation... 

Another approach is the inhibition of the TNF-converting enzyme (TACE), which converts proTNF into its mature, proinflammatory form. Also, numerous inhibitors of P38 and Erk-MAP kinases have been synthesized and some have reached the clinical trial stage. MAP-p38 kinase occupies a central role in the signaling network responsible for the up-regulation of proinflammatory cytokines like interleukin 1 and TNFa. The pathway of Erk-MAP kinase is often up-regulated in human tumors and as such represents an attractive target for the development of anticancer drugs. ... 

The final link in the kinase cascade activated by Ras-GTP emerged from studies in which scientists fractionated extracts of cultured cells searching for a kinase activity that could phosphorylate MAP kinase and that was present only in cells stimulated with growth factors, not quiescent cells. This work led to Identification of MEK, a kinase that specifically phosphorylates one threonine and one tyrosine residue on MAP kinase, thereby activating its catalytic activity. (The acronym MEK comes from MAP and ERK kinase.) Later studies showed that MEK binds to the C-terminal catalytic domain of Raf and is phosphorylated by the Raf serine/ threonine kinase this phosphorylation activates the catalytic activity of MEK. Hence, activation of Ras induces a kinase cascade that Includes Raf, MEK, and MAP kinase activated RTK Ras Raf MEK MAP kinase. 

To complement the phospho-ERK results above, assessment of endogenous ERK kinase activity was carried out using a nonradioactive p44/42 MAP kinase assay kit (Cell Signaling, 9800). Briefly, the p44/42 MAP kinase is immunoprecipitated from lysates and incubated with the Elk-1... 

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