Difference between revisions of "Kinase Subfamily ERK7"
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Latest revision as of 20:01, 2 May 2014
ERK7 (Extracellular signal Regulated Kinase) is an ancient but poorly studied member of the MAPK family, found in almost all eukaryotes. While it has the usual TxY activation loop motif, it appears to be autoactivated rather than induced by a MEK kinase. It has reported functions in cell cycle, DNA damage and transcriptional regulation.
Erk7 is found in almost all eukaryotes other than fungi and plants. In several protist lineages it is the only MAPK other than Erk1, suggesting ancient and conserved functions. Most animals have a single copy of Erk7. Mammalian Erk7 is also known as ERK8 (in human) and MAPK15.
Control of activity
Erk7 members have a conserved T[DE]Y motif in their activation loops, similar to most other MAPK. Both residues are seen to be phosphorylated in the human protein (Phosphosite). These residues in most MAPK are phosphorylated by a MEK (MAP2K) kinase, but no such upstream MEK has been described for Erk7, and it is likely to be controlled by autophosphorylation. Human Erk7 expressed in E. coli was dual-phosphorylated, while catalytically-dead mutants of Erk7 were not phosphorylated in human cells under conditions where the wild type was phosphorylated and active and an Erk7 inhibitor (Ro 318220) also blocked Erk7 phosphorylation. . However, the initial cloning of human Erk7 (aka ERK8)  reported low catalytic activity in protein produced from E. coli or COS cells.
Reports differ as to whether Erk7 can be activated by external stimuli, perhaps due to its high basal level of phosphorylation. Low or moderate levels of activation by serum have been reported, and activation through a mutant Ret and Abl has also been explored . Src-dependent Erk7 activation has also been reported .
Truncation of the C-terminus, just after the Erk7-CKD domain  leads to loss of activation loop phosphorylation, suggesting that the C-terminus acts in cis as an activation region. The truncated form also failed to localize to the nucleus.
All Erk7 have an N-terminal kinase domain, followed immediately by a unique conserved region of about 45 AA (Erk7-CKD) and then usually an extended and relatively poorly conserved C-terminal tail (192 AA in human, 511 AA in Drosophila, 116 AA in C. elegans, though missing in some protists, such as Dictyostelium and Giardia). A 12 AA PIP (PCNA-interacting Protein) motif covers the last 8 AA of the kinase domain and the first four AA of the CKD domain . The human protein has a number of PxxP SH2-interacting sites that have been implicated in Src activation, though the C-terminus is generally Proline-rich. The mammalian form has several PxxxP motifs (8 in human, with 6 clustered soon after the Erk7-CKD), and a mutation to two of these eliminates chromatin association and increases activity, suggesting that it is an autoinhibitory region . However, most of these motifs are poorly conserved even within mammals (e.g. rat only has three of them).
Cancer and Cell Cycle
Manipulation of Erk7 levels in a colon carcinoma cell line correlated with transformation and c-Jun phosphorylation, and Erk7 was able to phosphorylate c-Jun in an in vitro assay . By contrast, Erk7 expression is negatively correlated with breast cancer progression, where it appears to induce destruction of Estrogen Receptor Alpha (ERa) by enhancing its ubiquitination  (below). Transfection of human Erk7 to HEK203T cells also caused a reduction in growth .
Nuclear Hormone Signaling
Human Erk7 associates with Hic-5 (TGFB1I1), a nuclear hormone receptor coactivator. Erk7 could block androgen receptor (AR) and glucocorticoid receptor (GR) transcriptional activity, suggesting a direct interaction between Erk7 and GRalpha . This interaction was retained when the C-terminal region was deleted and in a catalytically inactive mutant.
Erk7 binds Estrogen-Related Receptor alpha (ERRa) through two LxxLL boxes towards the end of the kinase domain (not the C-terminal region, as originally reported) . These motifs are well conserved in vertebrate Erk7 and moderately conserved in invertebrates, but are not found in other MAPK subfamilies. This binding causes translocation of ERRa from the nucleus to the cytoplasm.
Human Erk7 also phosphorylates the related Estrogen Receptor alpha (ERa), and induces its ubiquitination and destruction . A kinase dead mutant protected ERa from turnover. Human ERa-positive breast tumors had decreased Erk7 expression, suggesting that it acts as a tumor suppressor.
Erk7 protein itself is expressed at a low level and is rapidly degraded by ubiquitination, mediated both by the N-terminus of the kinase domain and the C-terminal region . This is likely mediated by the SCF complex, and may be an important control given the apparent constitutive activity of Erk7. The DNA alkylating agent MMS also causes proteasome-dependent destruction of Erk7 .
DNA damage and chromatin interactions
Erk7 is largely localized to the nucleus. It contains a PCNA-interaction region at the end of the kinase domain (PIP box, see above), and a separate chromatin-interaction region in the C-terminus. Erk7 binding to PCNA can prevent PCNA degradation by interfering with its binding to the E3 ligase HDM2, also mediated through a PIP box . In this study knockdown of Erk7 caused an increase in DNA damage.
Erk7 has been reported to be activated by hydrogen peroxide [1, 12] and other agents that cause single-stranded DNA breaks  (but see  for a caution). Inhibitors show that this activation is not dependent on either ATM or ATR activity. Kinase activity is required for Erk7 association with chromatin, suggesting that the increase in activity on peroxide treatment may be designed to stabilize PCNA levels.
Rat Erk7 could phosphorylate Fos, Myc and MBP in vitro, but did not phosphorylate Elk1, PHAS1, Jun, or ATF2 in one study . However, human Erk7 was shown not to phosphorylate Fos under identical conditions , and Jun has elsewhere been implicated as an Erk7 substrate .
The C-terminal region of human or rat Erk7 interacts via Y2H with CLIC3, a poorly-studied intracellular chloride channel that is localized mostly to the nucleus .
Human Erk7 was found in an RNAi screen for telomerase activity inhibition, along with a number of other MAPK and MEK kinases.  This effect was shown to be dependent on Erk7 kinase activity, and sensitive to semi-selectiv Erk7 inhibitors.
Src binding to Erk7 was shown in vitro and in immunoprecipitates, possibly through PxxP motifs in the Proline-rich C-terminus . This study also showed Src-dependent activation of Erk7. Another study  showed co-immunoprecipitation of Erk7 and Abl, and activation of Erk7 by Abl, but showed that this was independent of the PxxP motifs in Erk7 or the SH3 domain of Abl.
Functions of Other Homologs
In bee, Erk7 was one of three genes implicated by QTL mapping in the the time of onset of foraging behavior , and also shows strong expression correlation with foraging behavior.
A small-scale RNAi screen in Trypansoma brucei showed Erk7 (Erk8) to be required for normal proliferation. . A pair of RNAi screens in Drosophila S2 cells also reported that CG32703 is required for cell cycle progression [17, 18].
The Dictyostelium homolog, erkB (erk2) is activated in response either to cAMP (induces chemotaxis and development) or folate (induces chemotaxis and feeding).  The cAMP signal is thought to be mediated by a direct interaction between a G-alpha protein and Erk2 , though this is may not be absolutely required , and erkB is required to produce further cAMP in response to the cAMP signal. erkB may act through docking and phosphorylating REGA, a cAMP-specific phosphodiesterase.
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