ERK7的结构性活化与功能

ERK7是细胞外信号调节激酶家族中的新成员.尽管ERK7激酶的活化环上含有ERK家族成员共有的TEY基序,但是在活化上与其它ERK家族成员截然不同,ERK7无需典型活化ERK的细胞外刺激或JNK和p38激酶活化物而发生自磷酸化并足以使其在缺乏上游激酶时发生活化,且发现ERK7中非激酶结构域的C端区域调节其结构性活化、核定位、生长抑制.此外,N端的20个氨基酸作为ERK7降解的首要决定因素调节ERK7的表达.新近研究表明ERK7与乳腺肿瘤、神经元分化、胚胎发生密切相关.本文就ERK7的结构特性活化、调控及功能等作了综述.     

Extracellular Signal-Regulated Kinase 7 (ERK7), a Novel ERK with a C-Terminal Domain That Regulates Its Activity, Its Cellular Localization, and Cell Growth

Mitogen-activated protein (MAP) kinases play distinct roles in a variety of cellular signaling pathways and are regulated through multiple mechanisms. In this study, a novel 61-kDa member of the MAP kinase family, termed extracellular signal-regulated kinase 7 (ERK7), has been cloned and characterized. Although it has the signature TEY activation motif of ERK1 and ERK2, ERK7 is not activated by extracellular stimuli that typically activate ERK1 and ERK2 or by common activators of c-Jun N-terminal kinase (JNK) and p38 kinase. Instead, ERK7 has appreciable constitutive activity in serum-starved cells that is dependent on the presence of its C-terminal domain. Interestingly, the C-terminal tail, not the kinase domain, of ERK7 regulates its nuclear localization and inhibition of growth. Taken together, these results elucidate a novel type of MAP kinase whereby interactions via its C-terminal tail, rather than extracellular signal-mediated activation cascades, regulate its activity, localization, and function.     

Noncanonical function of MEKK2 and MEK5 PB1 domains for coordinated extracellular signal-regulated kinase 5 and c-Jun N-terminal kinase signaling

MEKK2 and MEK5 encode Phox/Bem1p (PB1) domains that heterodimerize with one another. MEKK2, MEK5, and extracellular signal-related kinase 5 (ERK5) form a ternary complex through interactions involving the MEKK2 and MEK5 PB1 domains and a 34-amino-acid C-terminal extension of the MEK5 PB1 domain. This C-terminal extension encodes an ERK5 docking site required for MEK5 activation of ERK5. The PB1 domains bind in a front-to-back arrangement, with a cluster of basic amino acids in the front of the MEKK2 PB1 domain binding to the back-end acidic clusters of the MEK5 PB1 domain. The C-terminal moiety, including the acidic cluster of the MEKK2 PB1 domain, is not required for MEK5 binding and binds MKK7. Quiescent MEKK2 preferentially binds MEK5, and MEKK2 activation results in ERK5 activation. Activated MEKK2 binds and activates MKK7, leading to JNK activation. The findings define how the MEKK2 and MEK5 PB1 domains are uniquely used for differential binding of two mitogen-activated protein kinase kinases, MEK5 and MKK7, for the coordinated control of ERK5 and c-Jun N-terminal kinase activation.     

ERK7 is an autoactivated member of the MAPK family

Extracellular signal-regulated kinase 7 (ERK7) shares significant sequence homology with other members of the ERK family of signal transduction proteins, including the signature TEY activation motif. However, ERK7 has several distinguishing characteristics. Unlike other ERKs, ERK7 has been shown to have significant constitutive activity in serum-starved cells, which is not increased further by extracellular stimuli that typically activate other members of the mitogen-activated protein kinase (MAPK) family. On the other hand, ERK7's activation state and kinase activity appear to be regulated by its ability to utilize ATP and the presence of its extended C-terminal region. In this study, we investigated the mechanism of ERK7 activation. The results suggest that 1) MAPK kinase (MEK) inhibitors do not suppress ERK7 kinase activity; 2) intramolecular autophosphorylation is sufficient for activation of ERK7 in the absence of an upstream MEK; and 3) multiple regions of the C-terminal domain of ERK7 regulate its kinase activity. Taken together, these results indicate that autophosphorylation is sufficient for ERK7 activation and that the C-terminal domain regulates its kinase activity through multiple interactions.     

Human JIK, a novel member of the STE20 kinase family that inhibits JNK and is negatively regulated by epidermal growth factor

Mammalian members related to Saccharomyces cerevisiae serine/threonine kinase STE20 can be divided into two subfamilies based on their structure and function. The PAK subfamily is characterized by an N-terminal p21-binding domain (also known as CRIB domain), a C-terminal kinase domain, and is regulated by the small GTP-binding proteins Rac1 and Cdc42Hs. The second group is represented by the GCK-like members, which contain an N-terminal catalytic domain and lack the p21-binding domain. Some of them have been demonstrated to induce c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) cascade, while others have been shown to be activated by a subset of stress conditions or apoptotic agents, although little is known about their specific function. Here, we have identified a novel human STE20-related serine/threonine kinase, belonging to the GCK-like subfamily. This kinase does not induce the JNK/SAPK pathway, but, instead, inhibits the basal activity of JNK/SAPK, and diminishes its activation in response to human epidermal growth factor (EGF). Therefore, we designated this molecule JIK for JNK/SAPK-inhibitory kinase. The inhibition of JNK/SAPK signaling pathway by JIK was found to occur between the EGF receptor and the small GTP-binding proteins Rac1 and Cdc42Hs. In contrast, JIK does not activate nor does it inhibit ERK2, ERK6, p38, or ERK5. Furthermore, JIK kinase activity is not modulated by any exogenous stimuli, but, interestingly, it is dramatically decreased upon EGF receptor activation. Thus, JIK might represent the first member of the STE20 kinase family whose activity can be negatively regulated by tyrosine kinase receptors, and whose downstream targets inhibit, rather than enhance, JNK/SAPK activation.     

Different domains of the mitogen-activated protein kinases ERK3 and ERK2 direct subcellular localization and upstream specificity in vivo.

Extracellular signal-regulated kinase 3 (ERK3) is a member of the mitogen-activated protein (MAP) kinase family. ERK3 is most similar in its kinase catalytic domain to ERK2, yet it displays many unique properties. Among these, unlike ERK2, which translocates to the nucleus following activation, ERK3 is constitutively localized to the nucleus, despite the lack of a defined nuclear localization sequence. We created two chimeras between ERK2 and the catalytic domain of ERK3 (ERK3DeltaC), and some mutants of these chimeras, to examine the basis for the different behaviors of these two MAP kinase family members. We find the following: 1) the N-terminal folding domain of ERK3 functions in phosphoryl transfer reactions with the C-terminal folding domain of ERK2; 2) the C-terminal halves of ERK2 and ERK3DeltaC are primarily responsible for their subcellular localization in resting cells; and 3) the N-terminal folding domain of ERK2 is required for its activation in cells, its interaction with MEK1, and its accumulation in the nucleus.     

90-kDa ribosomal S6 kinase is phosphorylated and activated by 3-phosphoinositide-dependent protein kinase-1.

90-kDa ribosomal S6 kinase-2 (RSK2) belongs to a family of growth factor-activated serine/threonine kinases composed of two kinase domains connected by a regulatory linker region. The N-terminal kinase of RSK2 is involved in substrate phosphorylation. Its activation requires phosphorylation of the linker region at Ser(369), catalyzed by extracellular signal-regulated kinase (ERK), and at Ser(386), catalyzed by the C-terminal kinase, after its activation by ERK. In addition, the N-terminal kinase must be phosphorylated at Ser(227) in the activation loop by an as yet unidentified kinase. Here, we show that the isolated N-terminal kinase of RSK2 (amino acids 1-360) is phosphorylated at Ser(227) by PDK1, a constitutively active kinase, leading to 100-fold stimulation of kinase activity. In COS7 cells, ectopic PDK1 induced the phosphorylation of full-length RSK2 at Ser(227) and Ser(386), without involvement of ERK, leading to partial activation of RSK2. Similarly, two other members of the RSK family, RSK1 and RSK3, were partially activated by PDK1 in COS7 cells. Finally, our data indicate that full activation of RSK2 by growth factor requires the cooperation of ERK and PDK1 through phosphorylation of Ser(227), Ser(369), and Ser(386). Our study extend recent findings which implicate PDK1 in the activation of protein kinases B and C and p70(S6K), suggesting that PDK1 controls several major growth factor-activated signal transduction pathways.     

Molecular cloning and characterization of a novel dual specificity phosphatase, MKP-5.

A group of dual specificity protein phosphatases negatively regulates members of the mitogen-activated protein kinase (MAPK) superfamily, which consists of three major subfamilies, MAPK/extracellular signal-regulated kinase (ERK), stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK), and p38. Nine members of this group of dual specificity phosphatases have previously been cloned. They show distinct substrate specificities for MAPKs, different tissue distribution and subcellular localization, and different modes of inducibility of their expression by extracellular stimuli. Here we have cloned and characterized a novel dual specificity phosphatase, which we have designated MKP-5. MKP-5 is a protein of 482 amino acids with a calculated molecular mass of 52.6 kDa and consists of 150 N-terminal amino acids of unknown function, two Cdc25 homology 2 regions in the middle, and a C-terminal catalytic domain. MKP-5 binds to p38 and SAPK/JNK, but not to MAPK/ERK, and inactivates p38 and SAPK/JNK, but not MAPK/ERK. p38 is a preferred substrate. The subcellular localization of MKP-5 is unique; it is present evenly in both the cytoplasm and the nucleus. MKP-5 mRNA is widely expressed in various tissues and organs, and its expression in cultured cells is elevated by stress stimuli. These results suggest that MKP-5 is a novel type of dual specificity phosphatase specific for p38 and SAPK/JNK.     

Mapping the functional domains of the Golgi stacking factor GRASP65

The Golgi reassembly stacking protein (GRASP) family has been implicated in the stacking of Golgi cisternae and the regulation of Golgi disassembly/reassembly during mitosis in mammalian cells. GRASP65 is a dimer that can directly link adjacent surfaces through trans-oligomerization in a mitotically regulated manner. Here we show that the N-terminal GRASP domain (amino acids 1-201) is both necessary and sufficient for dimerization and trans-oligomerization but is not mitotically regulated. The C-terminal serine/proline-rich domain (amino acids 202-446) cannot dimerize nor can it link adjacent surfaces. It does, however, confer mitotic regulation on the GRASP domain through multiple sites phosphorylated by the mitotic kinases, cdc2/B1, and the polo-like kinase. Transient expression corroborated these results by showing that the GRASP domain alone inhibited mitotic fragmentation of the Golgi apparatus.     

A novel ERK-like, CRK-like protein kinase that modulates growth in Trypanosoma brucei via an autoregulatory C-terminal extension

The protozoan parasite Trypanosoma brucei undergoes a complex developmental cycle coordinated with cell cycle control. These processes in eukaryotes are frequently regulated through mitogen-activated protein kinases (MAPKs) and cyclin-dependent protein kinases (CDKs), respectively. We have discovered a novel protein kinase which shares features of both ERK-type MAPKs and CDKs (T. brucei ERK-like, CDK-like protein kinase). This molecule, named TbECK1, is similar to the unusual mammalian KKIAMRE protein kinase family. Moreover, TbECK1 possesses a long C-terminal extension reminiscent of those found in mammalian ERK5, ERK7 and ERK8. Expression analyses demonstrate that TbECK1 is constitutively expressed during the trypanosome life cycle at both RNA and protein level. In transgenic parasites we demonstrate that expression of a mutant of TbECK1 that lacks the C-terminal extension produces a slow growth phenotype, associated with the appearance of cells with aberrant karyotypes. Using this as an assay we further demonstrate that the phenotype is dependent upon the potential for catalytic activity of TbECK1 and on the integrity of at least one of the phosphorylable amino acids in its phosphorylation lip. C-terminal extensions are a common feature of kinetoplastid protein kinases. Our results demonstrate for the first time that this domain has a regulatory function.     

Regulation of the inducible nuclear dual-specificity phosphatase DUSP5 by ERK MAPK

DUSP5 is an inducible, nuclear, dual-specificity phosphatase, which specifically interacts with and inactivates the ERK1/2 MAP kinases in mammalian cells. In addition, expression of DUSP5 causes nuclear translocation of ERK2 indicating that it may act as a nuclear anchor for the inactive kinase. Here we show that induction of DUSP5 mRNA and protein in response to growth factors is dependent on ERK1/2 activation and that the accumulation of DUSP5 protein is regulated by rapid proteasomal degradation. DUSP5 is phosphorylated by ERK1/2 both in vitro and in vivo on three sites (Thr321, Ser346 and Ser376) within its C-terminal domain. DUSP5 phosphorylation is absolutely dependent on the conserved kinase interaction motif (KIM) within the amino-terminal domain of DUSP5, indicating that the same protein–protein contacts are required for both the inactivation of ERK2 by DUSP5 and for DUSP5 to act as a substrate for this MAPK. Using a combination of pharmacological inhibitors and phospho-site mutants we can find no evidence that phosphorylation of DUSP5 by ERK2 significantly affects either the half-life of the DUSP5 protein or its ability to bind to, inactivate or anchor ERK2 in the nucleus. However, co-expression of ERK2 results in significant stabilisation of DUSP5, which is accompanied by reduced levels of DUSP5 ubiquitination. These changes are independent of ERK2 kinase activity but absolutely depend on the ability of ERK2 to bind to DUSP5. We conclude that DUSP5 is stabilised by complex formation with its physiological substrate and that this may reinforce its activity as both a phosphatase and nuclear anchor for ERK2.     

Endogenous expression and protein kinase A-dependent phosphorylation of the guanine nucleotide exchange factor Ras-GRF1 in human embryonic kidney 293 cells

We have previously reported the Ras-dependent activation of the mitogen-activated protein kinases p44 and p42, also termed extracellular signal-regulated kinases (ERK)1 and 2 (ERK1/2), mediated through Gs-coupled serotonin receptors transiently expressed in human embryonic kidney (HEK) 293 cells. Whereas Gi- and Gq-coupled receptors have been shown to activate Ras through the guanine nucleotide exchange factor (GEF) called Ras-GRF1 (CDC25Mm) by binding of Ca2+/calmodulin to its N-terminal IQ domain, the mechanism of Ras activation through Gs-coupled receptors is not fully understood. We report the endogenous expression of Ras-GRF1 in HEK293 cells. Serotonin stimulation of HEK293 cells transiently expressing Gs-coupled 5-HT7 receptors induced protein kinase A-dependent phosphorylation of the endogenous human Ras-GRF1 on Ser927 and of transfected mouse Ras-GRF1 on Ser916. Ras-GRF1 overexpression increased basal and serotonin-stimulated ERK1/2 phosphorylation. Mutations of Ser916 inhibiting (Ser916Ala) or mimicking (Ser916Asp/Glu) phosphorylation did not alter these effects. However, the deletion of amino acids 1-225, including the Ca2+/calmodulin-binding IQ domain, from Ras-GRF1 reduced both basal and serotonin-stimulated ERK1/2 phosphorylation. Furthermore, serotonin treatment of HEK293 cells stably expressing 5-HT7 receptors increased [Ca2+]i, and the serotonin-induced ERK1/2 phosphorylation was Ca2+-dependent. Therefore, both cAMP and Ca2+ may contribute to the Ras-dependent ERK1/2 activation after 5-HT7 receptor stimulation, through activation of a guanine nucleotide exchange factor with activity towards Ras.     

The MAP kinase ERK5 binds to and phosphorylates p90 RSK

We showed previously that p90 RSK was activated in cells expressing an activated mutant of MEK5, the activator of the MAP kinase ERK5. Based on the following evidence, we suggest that ERK5 can directly activate RSK in cells. ERK5 binds to RSK in vitro and co-immunoprecipitates from cell extracts; activation of ERK5 weakens its binding to RSK, suggesting that RSK is released upon activation. Phosphorylation of RSK by ERK5 in vitro causes its activation, indicating that RSK is a substrate of ERK5. In cells activation of ERK5 but not p38 or the c-Jun N-terminal kinase is associated with RSK activation. The large C-terminal domain of ERK5 is not required for binding or activation of RSK by ERK5; however, the common docking or CD domain of ERK5 and the docking or D domain of RSK are important for their association.     

Phosphorylation-dependent activation of TAK1 mitogen-activated protein kinase kinase kinase by TAB1

TAK1 is a mitogen-activated protein kinase kinase kinase (MAP3K) that is involved in the c-Jun N-terminal kinase/p38 MAPKs and NF-kappaB signaling pathways. Here, we characterized the molecular mechanisms of TAK1 activation by its specific activator TAB1. Autophosphorylation of two threonine residues in the activation loop of TAK1 was necessary for TAK1 activation. Association with TAK1 and induction of TAK1 autophosphorylation required the C-terminal 24 amino acids of TAB1, but full TAK1 activation required additional C-terminal Ser/Thr rich sequences. These results demonstrated that the association between the kinase domain of TAK1 and the C-terminal TAB1 triggered the phosphorylation-dependent TAK1 activation mechanism.     

Tyrosine 763 of the murine granulocyte colony-stimulating factor receptor mediates Ras-dependent activation of the JNK/SAPK mitogen-activated protein kinase pathway.

The receptor for granulocyte colony-stimulating factor (G-CSF) can mediate differentiation and proliferation of hemopoietic cells. A proliferative signal is associated with activation of the ERK mitogen-activated protein kinase (MAPK) pathway. To determine whether other MAPK pathways are activated by G-CSF signalling, we have investigated activation of JNK/SAPK in cells proliferating in response to G-CSF. Here we show that G-CSF and interleukin-3 activate JNK/SAPK in two hemopoietic cell lines. The region of the G-CSF receptor required for G-CSF-induced JNK/SAPK activation is located within the C-terminal 68 amino acids of the cytoplasmic domain, which contains Tyr 763. Mutation of Tyr 763 to Phe completely blocks JNK/SAPK activation. However, the C-terminal 68 amino acids are not required for ERK2 activation. We show that activation of JNK/SAPK, like that of ERK2, is dependent on Ras but that higher levels of Ras-GTP are associated with activation of JNK/SAPK than with activation of ERK2. Two separate functional regions of the G-CSF receptor contribute to activation of Ras. The Y763F mutation reduces G-CSF-induced Ras activation from 30 to 35% Ras-GTP to 10 to 13% Ras-GTP. Low levels of Ras activation (10 to 13% Ras-GTP), which are sufficient for ERK2 activation, require only the 100 membrane-proximal amino acids. High levels of Ras-GTP provided by expression of oncogenic Ras are not sufficient to activate JNK/SAPK. An additional signal, also mediated by Tyr 763, is required for activation of JNK/SAPK.