MAP kinase activation (Homo sapiens)
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Description
The mitogen activated protein kinase (MAPK) cascade, one of the most ancient and evolutionarily conserved signaling pathways, is involved in many processes of immune responses. The MAP kinases cascade transduces signals from the cell membrane to the nucleus in response to a wide range of stimuli (Chang and Karin, 2001; Johnson et al, 2002).
There are three major groups of MAP kinases
- the extracellular signal-regulated protein kinases ERK1/2,
- the p38 MAP kinase
- and the c-Jun NH-terminal kinases JNK.
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Nuclear events mediated by MAP
kinasesThe major cytosolic target of activated ERKs are RSKs (90 kDa Ribosomal protein S6 Kinase). Active RSKs translocates to the nucleus and phosphorylates such factors as c-Fos(on Ser362), SRF (Serum Response Factor) at Ser103, and CREB (Cyclic AMP Response Element-Binding protein) at Ser133. In the nucleus activated ERKs phosphorylate many other targets such as MSKs (Mitogen- and Stress-activated protein kinases), MNK (MAP interacting kinase) and Elk1 (on Serine383 and Serine389). ERK can directly phosphorylate CREB and also AP-1 components c-Jun and c-Fos. Another important target of ERK is NF-KappaB. Recent studies reveals that nuclear pore proteins are direct substrates for ERK (Kosako H et al, 2009). Other ERK nuclear targets include c-Myc, HSF1 (Heat-Shock Factor-1), STAT1/3 (Signal Transducer and Activator of Transcription-1/3), and many more transcription factors.
Activated p38 MAPK is able to phosphorylate a variety of substrates, including transcription factors STAT1, p53, ATF2 (Activating transcription factor 2), MEF2 (Myocyte enhancer factor-2), protein kinases MSK1, MNK, MAPKAPK2/3, death/survival molecules (Bcl2, caspases), and cell cycle control factors (cyclin D1).
JNK, once activated, phosphorylates a range of nuclear substrates, including transcription factors Jun, ATF, Elk1, p53, STAT1/3 and many other factors. JNK has also been shown to directly phosphorylate many nuclear hormone receptors. For example, peroxisome proliferator-activated receptor 1 (PPAR-1) and retinoic acid receptors RXR and RAR are substrates for JNK. Other JNK targets are heterogeneous nuclear ribonucleoprotein K (hnRNP-K) and the Pol I-specific transcription factor TIF-IA, which regulates ribosome synthesis. Other adaptor and scaffold proteins have also been characterized as nonnuclear substrates of JNK.
In response to stimuli the cell surface receptors transmit signals inducing MAP3 kinases, e.g., TPL2, MEKK1, which in turn phosphorylate MAP2Ks (MEK1/2). MAP2K then phosphorylate and activate the MAPK1/3 (ERK1 and ERK2 MAPKs). Activated MAPK1/3 phosphorylate and regulate the activities of an ever growing pool of substrates that are estimated to comprise over 160 proteins (Yoon and Seger 2006). The majority of ERK substrates are nuclear proteins, but others are found in the cytoplasm and other organelles. Activated MAPK1/3 can translocate to the nucleus, where they phosphorylate and regulate various transcription factors, such as Ets family transcription factors (e.g., ELK1), ultimately leading to changes in gene expression (Zuber J et al. 2000).
MAPK:
p-S272,T222,T334-MAPKAPK2, p-S,2T-MAPKAPK3Annotated Interactions
JNK is differentially regulated by MKK4 and MKK7 depending on the stimulus. MKK7 is the primary activator of JNK in TNF, LPS, and PGN responses. However, TLR3 cascade requires both MKK4 and MKK7. Some studies reported that in three JNK isoforms tested MKK4 shows a striking preference for the tyrosine residue (Tyr-185), and MKK7 a striking preference for the threonine residue (Thr-183).
Upon activation, p38 MAPK alpha activates MK2 by phosphorylating Thr-222, Ser-272, and Thr-334 (Ben-Levy et al. 1995).
The phosphorylation of MK2 at Thr-334 attenuates the affinity of the binary complex MK2:p38 alpha by an order of magnitude and leads to a large conformational change of an autoinhibitory domain in MK2. This conformational change unmasks not only the MK2 substrate-binding site but also the MK2 nuclear export signal (NES) thus leading to the MK2:p38 alpha translocation from the nucleus to the cytoplasm. Cytoplasmic active MK2 then phosphorylates downstream targets such as the heat-shock protein HSP27 and tristetraprolin (TTP) (Meng et al. 2002, Lukas et al. 2004, White et al. 2007).
MAPKAPK (MAPK-activated protein) kinase 3 (MK3, also known as 3pK) has been identified as the second p38 MAPK-activated kinase that is stimulated by different stresses (McLaughlin et al. 1996; Sithanandam et al. 1996; reviewed in Gaestel 2006). MK3 shows 75% sequence identity to MK2 and, like MK2, is activated by p38 MAPK alpha and p38 MAPK beta. MK3 phosphorylates peptide substrates with kinetic constants similar to MK2 and phosphorylates the same serine residues in HSP27 at the same relative rates as MK2 (Clifton et al. 1996) indicating an identical phosphorylation-site consensus sequence. Hence, it is assumed that its substrate spectrum is either identical to or at least overlapping with MK2.
p38 MAPK alpha phosphorylates MK2 at Thr222, Ser272, and Thr334. The phosphorylation of Thr334 but not the kinase activity of MK2 has been demonstrated to be critical for the nuclear export of the p38 alpha - MK2 complex. Phosphorylation of Thr334 is believed to induce a conformational change in the complex exposing NES prior to interaction with the leptomycin B-sensitive nuclear export receptor.
Residues involved in activation of these protein kinases correspond to human Ser271, Thr275 in MKK7 and Ser257, Thr261 in MKK4.
Cell lines lacking MKK4 exhibit defective activation of JNK and AP-1 dependent transcription activity in response to some cellular stresses; JNK and p38 MAPK activities were decreased by around 80% and 20%, respectively, following deletion of the mkk4 gene.
Activation of MKK3 and MKK6 occurs through phosphorylation of serine and threonine residues at the typical Ser-Xaa-Ala-Xaa-Thr motif in their activation loop. Residues involved into these protein kinases activation correspond to human sites Ser189 and Thr193 for MKK3 and Ser207 and Thr211 for MKK6 .
A20-binding inhibitor of NFkappaB2 (ABIN-2 ot TNIP2) interacts with Tpl2 and p105 but preferentially forms a ternary complex with both proteins. As ABIN2 is a polyubiquitin binding protein, it has been suggested that it may facilitate recruitment of the p105/Tpl2 complex to the activated IKK complex, allowing IKK2 induced p105 phosphorylation and consequent Tpl2 activation.
Positions of phosphorylations represented here are inferred from general experimental data (Zheng & Guan, 1994).
The catalytic subunit of MAP3K8 (TPL2) was reported to undergo phosphorylation at Thr290 in human embryonic kidney 293 (HEK293) cells transfected with MAP3K8 (Luciano BS et al. 2004; Cho J et al. 2005; Stafford MJ et al. 2006). Mutation of this residue to alanine prevented the LPS-stimulated activation of MAP3K8 in mouse macrophages (Cho J et al. 2005). Experiments with a small-molecule inhibitor of MAP3K8 have suggested that Thr290 is autophosphosphorylated after IL-1 beta stimulation of IL-1R-expressing HEK293T cells (Handoyo H et al. 2009). However, a catalytically inactive mutant of MAP3K8 (Tpl2-K167M) was reported to become phosphorylated at Thr290 in transfected HEK-293 cells, suggesting that Thr290 phosphorylation did not occur as a result of autophosphorylation (Cho J et al. 2005) In addition, the phosphorylation at Thr290 was also reported to be catalysed by IKBKB, based on small interfering RNA(siRNA)-knockdown studies and the use of high concentrations of the IKBKB inhibitor PS1145 (Cho J et al. 2005). However, the other work showed that lower concentrations of PS1145, but nevertheless sufficient to completely inhibit IKBKB, did not affect the IL-1-stimulated phosphorylation of transfected MAP3K8 at Thr290, suggesting that the IL-1 beta stimulated phosphorylation of Thr290 is catalysed by a protein kinase distinct from IKBKB. (Stafford MJ et al. 2006). Thus, phosphorylation at Thr290 is required for the physiological activation of MAP3K8 by external signals, although the mode of the modification remains to be clarified.
Activation of MAP3K8 may also occur trough phosphorylation on Ser62 and Ser400 (Stafford MJ et al. 2006; Roget K et al. 2012).
MAPK:
p-S272,T222,T334-MAPKAPK2, p-S,2T-MAPKAPK3