Mapping the functional domains of elongation factor-2 kinase

Regulating eEF2 and eEF2K in skeletal muscle by exercise

Skeletal muscle is a flexible and adaptable tissue that strongly responds to exercise training. The skeletal muscle responds to exercise by increasing muscle protein synthesis (MPS) when energy is available. One of protein synthesis's major rate-limiting and critical regulatory steps is the translation elongation pathway. The process of translation elongation in skeletal muscle is highly regulated. It requires elongation factors that are intensely affected by various physiological stimuli such as exercise and the total available energy of cells. Studies have shown that exercise involves the elongation pathway by numerous signalling pathways. Since the elongation pathway, has been far less studied than the other translation steps, its comprehensive prospect and quantitative understanding remain in the dark. This study highlights the current understanding of the effect of exercise training on the translation elongation pathway focussing on the molecular factors affecting the pathway, including Ca2+, AMPK, PKA, mTORC1/P70S6K, MAPKs, and myostatin. We further discussed the mode and volume of exercise training intervention on the translation elongation pathway.What is the topic of this review? This review summarises the impacts of exercise training on the translation elongation pathway in skeletal muscle focussing on eEF2 and eEF2K.What advances does it highlight? This review highlights mechanisms and factors that profoundly influence the translation elongation pathway and argues that exercise might modulate the response. This review also combines the experimental observations focussing on the regulation of translation elongation during and after exercise. The findings widen our horizon to the notion of mechanisms involved in muscle protein synthesis (MPS) through translation elongation response to exercise training.

eEF2K Inhibitor Design: The Progression of Exemplary Structure-Based Drug Design

The α-kinase, eEF2K, phosphorylates the threonine 56 residue of eEF2 to inhibit global peptide elongation (protein translation). As a master regulator of protein synthesis, in combination with its unique atypical kinase active site, investigations into the targeting of eEF2K represents a case of intense structure-based drug design that includes the use of modern computational techniques. The role of eEF2K is incredibly diverse and has been scrutinized in several different diseases including cancer and neurological disorders-with numerous studies inhibiting eEF2K as a potential treatment option, as described in this paper. Using available crystal structures of related α-kinases, particularly MHCKA, we report how homology modeling has been used to improve inhibitor design and efficacy. This review presents an overview of eEF2K related drug discovery efforts predating from the 1990's, to more recent in vivo studies in rat models. We also provide the reader with a basic introduction to several approaches and software programs used to undertake such drug discovery campaigns. With the recent exciting publication of an eEF2K crystal structure, we present our view regarding the future of eEF2K drug discovery.

Structure prediction of eukaryotic elongation factor-2 kinase and identification of the binding mechanisms of its inhibitors: homology modeling, molecular docking, and molecular dynamics simulation

Protein kinases emerged as one of the most successful families of drug targets due to their increased activity and involvement in mediating critical signal transduction pathways in cancer cells. Recent evidence suggests that eukaryotic elongation factor 2 kinase (eEF-2K) is a potential therapeutic target for treating some highly aggressive solid cancers, including lung, pancreatic and triple-negative breast cancers. Thus, several compounds have been developed for the inhibition of the enzyme activity, but they are not sufficiently specific and potent. Besides, the crystal structure of this kinase remains unknown. Hence, the functional organization and regulation of eEF-2K remain poorly characterized. For this purpose, we constructed a homology model of eEF-2K and then used docking methodology to better understanding the binding mechanism of eEF-2K with 58 compounds that have been proposed as existing inhibitors. The results of this analysis were compared with the experimental results and the compounds effective against eEF-2K were determined against eEF-2K as a result of both studies. And finally, molecular dynamics (MD) simulations were performed for the stability of eEF-2K with these compounds. According to these study defined that the binding mechanism of eEF-2K with inhibitors at the molecular level and elucidated the residues of eEF-2K that play an important role in enzyme selectivity and ligand affinity. This information may lead to new selective and potential drug molecules to be for inhibition of eEF-2K.Communicated by Ramaswamy H. Sarma.

Solution Structure of the Carboxy-Terminal Tandem Repeat Domain of Eukaryotic Elongation Factor 2 Kinase and Its Role in Substrate Recognition

Eukaryotic elongation factor 2 kinase (eEF-2K), an atypical calmodulin-activated protein kinase, regulates translational elongation by phosphorylating its substrate, eukaryotic elongation factor 2 (eEF-2), thereby reducing its affinity for the ribosome. The activation and activity of eEF-2K are critical for survival under energy-deprived conditions and is implicated in a variety of essential physiological processes. Previous biochemical experiments have indicated that the binding site for the substrate eEF-2 is located in the C-terminal domain of eEF-2K, a region predicted to harbor several α-helical repeats. Here, using NMR methodology, we have determined the solution structure of a C-terminal fragment of eEF-2K, eEF-2K562-725 that encodes two α-helical repeats. The structure of eEF-2K562-725 shows signatures characteristic of TPR domains and of their SEL1-like sub-family. Furthermore, using the analyses of NMR spectral perturbations and ITC measurements, we have localized the eEF-2 binding site on eEF-2K562-725. We find that eEF-2K562-725 engages eEF-2 with an affinity comparable to that of the full-length enzyme. Furthermore, eEF-2K562-725 is able to inhibit the phosphorylation of eEF-2 by full-length eEF-2K in trans. Our present studies establish that eEF-2K562-725 encodes the major elements necessary to enable the eEF-2K/eEF-2 interactions.

Structural Dynamics of the Activation of Elongation Factor 2 Kinase by Ca2+-Calmodulin

Eukaryotic elongation factor 2 kinase (eEF-2K), the only known calmodulin (CaM)-activated α-kinase, phosphorylates eukaryotic elongation factor 2 (eEF-2) on a specific threonine (Thr-56) diminishing its affinity for the ribosome and reducing the rate of nascent chain elongation during translation. Despite its critical cellular role, the precise mechanisms underlying the CaM-mediated activation of eEF-2K remain poorly defined. Here, employing a minimal eEF-2K construct (TR) that exhibits activity comparable to the wild-type enzyme and is fully activated by CaM in vitro and in cells, and using a variety of complimentary biophysical techniques in combination with computational modeling, we provide a structural mechanism by which CaM activates eEF-2K. Native mass analysis reveals that CaM, with two bound Ca²⁺ ions, forms a stoichiometric 1:1 complex with TR. Chemical crosslinking mass spectrometry and small-angle X-ray scattering measurements localize CaM near the N-lobe of the TR kinase domain and the spatially proximal C-terminal helical repeat. Hydrogen/deuterium exchange mass spectrometry and methyl NMR indicate that the conformational changes induced on TR by the engagement of CaM are not localized but are transmitted to remote regions that include the catalytic site and the functionally important phosphate binding pocket. The structural insights obtained from the present analyses, together with our previously published kinetics data, suggest that TR, and by inference, wild-type eEF-2K, upon engaging CaM undergoes a conformational transition resulting in a state that is primed to efficiently auto-phosphorylate on the primary activating T348 en route to full activation.

Eukaryotic Elongation Factor 2 Kinase (eEF2K) in Cancer

Eukaryotic elongation factor 2 kinase (eEF2K) is a highly unusual protein kinase that negatively regulates the elongation step of protein synthesis. This step uses the vast majority of the large amount of energy and amino acids required for protein synthesis. eEF2K activity is controlled by an array of regulatory inputs, including inhibition by signalling through mammalian target of rapamycin complex 1 (mTORC1). eEF2K is activated under conditions of stress, such as energy depletion or nutrient deprivation, which can arise in poorly-vascularised tumours. In many such stress conditions, eEF2K exerts cytoprotective effects. A growing body of data indicates eEF2K aids the growth of solid tumours in vivo. Since eEF2K is not essential (in mice) under ‘normal’ conditions, eEF2K may be a useful target in the treatment of solid tumours. However, some reports suggest that eEF2K may actually impair tumorigenesis in some situations. Such a dual role of eEF2K in cancer would be analogous to the situation for other pathways involved in cell metabolism, such as autophagy and mTORC1. Further studies are needed to define the role of eEF2K in different tumour types and at differing stages in tumorigenesis, and to assess its utility as a therapeutic target in oncology.

Direct and indirect activation of eukaryotic elongation factor 2 kinase by AMP-activated protein kinase

BACKGROUND

Eukaryotic elongation factor 2 (eEF2) kinase (eEF2K) is a key regulator of protein synthesis in mammalian cells. It phosphorylates and inhibits eEF2, the translation factor necessary for peptide translocation during the elongation phase of protein synthesis. When cellular energy demand outweighs energy supply, AMP-activated protein kinase (AMPK) and eEF2K become activated, leading to eEF2 phosphorylation, which reduces the rate of protein synthesis, a process that consumes a large proportion of cellular energy under optimal conditions.

AIM

The goal of the present study was to elucidate the mechanisms by which AMPK activation leads to increased eEF2 phosphorylation to decrease protein synthesis.

METHODS

Using genetically modified mouse embryo fibroblasts (MEFs), effects of treatments with commonly used AMPK activators to increase eEF2 phosphorylation were compared with that of the novel compound 991. Bacterially expressed recombinant eEF2K was phosphorylated in vitro by recombinant activated AMPK for phosphorylation site-identification by mass spectrometry followed by site-directed mutagenesis of the identified sites to alanine residues to study effects on the kinetic properties of eEF2K. Wild-type eEF2K and a Ser491/Ser492 mutant were retrovirally re-introduced in eEF2K-deficient MEFs and effects of 991 treatment on eEF2 phosphorylation and protein synthesis rates were studied in these cells.

RESULTS & CONCLUSIONS

AMPK activation leads to increased eEF2 phosphorylation in MEFs mainly by direct activation of eEF2K and partly by inhibition of mammalian target of rapamycin complex 1 (mTORC1) signaling. Treatment of MEFs with AMPK activators can also lead to eEF2K activation independently of AMPK probably via a rise in intracellular Ca²⁺. AMPK activates eEF2K by multi-site phosphorylation and the newly identified Ser491/Ser492 is important for activation, leading to mTOR-independent inhibition of protein synthesis. Our study provides new insights into the control of eEF2K by AMPK, with implications for linking metabolic stress to decreased protein synthesis to conserve energy reserves, a pathway that is of major importance in cancer cell survival.

A decrease in eukaryotic elongation factor 2 phosphorylation is required for local translation of sensorin and long-term facilitation in Aplysia

Mechanistic target of rapamycin complex 1 (mTORC1)-dependent protein synthesis is required for many forms of synaptic plasticity and memory, but the downstream pathways important for synaptic plasticity are poorly understood. Long-term facilitation (LTF) in Aplysia is a form of synaptic plasticity that is closely linked to behavioral memory and an attractive model system for examining the important downstream targets for mTORC1 in regulating synaptic plasticity. Although mTORC1-regulated protein synthesis has been strongly linked to translation initiation, translation elongation is also regulated by mTORC1 and LTF leads to an mTORC1-dependent decrease in eukaryotic elongation factor 2 (eEF2) phosphorylation. The purpose of this study is to test the hypothesis that the decrease in eEF2 phosphorylation is required for mTORC1-dependent translation and plasticity. We show that the LTF-induced decrease in eEF2 phosphorylation is blocked by expression of an eEF2 kinase (eEF2K) modified to be resistant to mTORC1 regulation. We found that expression of this modified kinase blocked LTF. LTF requires local protein synthesis of the neuropeptide sensorin and importantly, local sensorin synthesis can be measured using a dendra fluorescent protein containing the 5' and 3' untranslated regions (UTRs) of sensorin. Using this construct, we show that blocking eEF2 dephosphorylation also blocks the increase in local sensorin synthesis. These results identify decreases in eEF2 phosphorylation as a critical downstream effector of mTOR required for long-term plasticity and identify an important translational target regulated by decreases in eEF2 phosphorylation.

Signal Integration at Elongation Factor 2 Kinase: THE ROLES OF CALCIUM, CALMODULIN, AND SER-500 PHOSPHORYLATION

Eukaryotic elongation factor 2 kinase (eEF-2K), the only calmodulin (CaM)-dependent member of the unique α-kinase family, impedes protein synthesis by phosphorylating eEF-2. We recently identified Thr-348 and Ser-500 as two key autophosphorylation sites within eEF-2K that regulate its activity. eEF-2K is regulated by Ca²⁺ ions and multiple upstream signaling pathways, but how it integrates these signals into a coherent output, i.e. phosphorylation of eEF-2, is unclear. This study focuses on understanding how the post-translational phosphorylation of Ser-500 integrates with Ca²⁺ and CaM to regulate eEF-2K. CaM is shown to be absolutely necessary for efficient activity of eEF-2K, and Ca²⁺ is shown to enhance the affinity of CaM toward eEF-2K. Ser-500 is found to undergo autophosphorylation in cells treated with ionomycin and is likely also targeted by PKA. In vitro, autophosphorylation of Ser-500 is found to require Ca²⁺ and CaM and is inhibited by mutations that compromise binding of phosphorylated Thr-348 to an allosteric binding pocket on the kinase domain. A phosphomimetic Ser-500 to aspartic acid mutation (eEF-2K S500D) enhances the rate of activation (Thr-348 autophosphorylation) by 6-fold and lowers the EC₅₀ for Ca²⁺/CaM binding to activated eEF-2K (Thr-348 phosphorylated) by 20-fold. This is predicted to result in an elevation of the cellular fraction of active eEF-2K. In support of this mechanism, eEF-2K knock-out MCF10A cells reconstituted with eEF-2K S500D display relatively high levels of phospho-eEF-2 under basal conditions. This study reports how phosphorylation of a regulatory site (Ser-500) integrates with Ca²⁺ and CaM to influence eEF-2K activity.

A high-throughput screening assay for eukaryotic elongation factor 2 kinase inhibitors

Eukaryotic elongation factor 2 kinase (eEF2K) inhibitors may aid in the development of new therapeutic agents to combat cancer. Purified human eEF2K was obtained from an Escherichia coli expression system and a luminescence-based high-throughput screening (HTS) assay was developed using MH-1 peptide as the substrate. The luminescent readouts correlated with the amount of adenosine triphosphate remaining in the kinase reaction. This method was applied to a large-scale screening campaign against a diverse compound library and subsequent confirmation studies. Nine initial hits showing inhibitory activities on eEF2K were identified from 56,000 synthetic compounds during the HTS campaign, of which, five were chosen to test their effects in cancer cell lines.

Discovery of new substrates of the elongation factor-2 kinase suggests a broader role in the cellular nutrient response

Elongation Factor-2 Kinase (eEF2K) in an unusual mammalian enzyme that has one known substrate, elongation factor-2. It belongs to a class of kinases, called alpha kinases, that has little sequence identity to the >500 conventional protein kinases, but performs the same reaction and has similar catalytic residues. The phosphorylation of eEF2 blocks translation elongation, which is thought to be critical to regulating cellular energy usage. Here we report a system for discovering new substrates of alpha kinases and identify the first new substrates of eEF2K including AMPK and alpha4, and determine a sequence motif for the kinase that shows a requirement for threonine residues as the target of phosphorylation. These new substrates suggest that eEF2K has a more diverse role in regulating cellular energy usage that involves multiple pathways and regulatory feedback.

Structure of the C-Terminal Helical Repeat Domain of Eukaryotic Elongation Factor 2 Kinase

Eukaryotic elongation factor 2 kinase (eEF-2K) phosphorylates its only known physiological substrate, elongation factor 2 (eEF-2), which reduces the affinity of eEF-2 for the ribosome and results in an overall reduction in protein translation rates. The C-terminal region of eEF-2K, which is predicted to contain several SEL-1-like helical repeats (SLRs), is required for the phosphorylation of eEF-2. Using solution nuclear magnetic resonance methodology, we have determined the structure of a 99-residue fragment from the extreme C-terminus of eEF-2K (eEF-2K627-725) that encompasses a region previously suggested to be essential for eEF-2 phosphorylation. eEF-2K627-725 contains four helices, of which the first (αI) is flexible, and does not pack stably against the ordered helical core formed by the last three helices (αII-αIV). The helical core is structurally similar to members of the tetratricopeptide repeat (TPR) family that includes SLRs. The two penultimate helices, αII and αIII, comprise the TPR, and the last helix, αIV, appears to have a capping function. The eEF-2K627-725 structure illustrates that the C-terminal deletion that was shown to abolish eEF-2 phosphorylation does so by destabilizing αIV and, therefore, the helical core. Indeed, mutation of two conserved C-terminal tyrosines (Y712A/Y713A) in eEF-2K previously shown to abolish eEF-2 phosphorylation leads to the unfolding of eEF-2K627-725. Preliminary functional analyses indicate that neither a peptide encoding a region deemed crucial for eEF-2 binding nor isolated eEF-2K627-725 inhibits eEF-2 phosphorylation by full-length eEF-2K. Taken together, our data suggest that the extreme C-terminal region of eEF-2K, in isolation, does not provide a primary docking site for eEF-2.

Eukaryotic elongation factor-2 kinase (eEF2K): a potential therapeutic target in cancer

Eukaryotic elongation factor-2 kinase (eEF2K), encoded by the EEF2K gene, is well-known to be a Ca(2+)/calmodulin (CaM)-dependent kinase which can negatively modulate protein synthesis. It is highly conserved among eukaryotes from mammals to invertebrates, of which human and mouse may have 99 % overall amino acid identity. This kinase can phosphorylate eukaryotic elongation factor-2 (eEF2) or undergo the process of autophosphorylation at multiple sites to inhibit its function in translation elongation. Due to the fact that regulation of eEF2 by eEF2K is an evolutionarily conserved mechanism, eEF2K activity may confer tumor cell adaption to metabolic stress under acute nutrient depletion, and the high expressed level of eEF2K has been found in several types of malignancies. eEF2K may modulate the expression of some apoptotic proteins such as XIAP, c-FLIPL, Bcl-XL, PI3KCI and p70(S6K) to inhibit apoptotic process in cancer. On the other hand, it plays a regulatory role in autophagy involved in mTORC1, AMPK and Atg8, thereby promoting cancer cell survival. Additionally, eEF2K may play a crucial role in the crosstalk between apoptosis and autophagy in cancer. Collectively, these findings have led to the conclusions that eEF2K may contribute to carcinogenesis, and thus being utilized as a potential target for future cancer therapy.

eEF-2 Phosphorylation Down-Regulates P-Glycoprotein Over-Expression in Rat Brain Microvessel Endothelial Cells

Objective

We investigated whether glutamate, NMDA receptors, and eukaryote elongation factor-2 kinase (eEF-2K)/eEF-2 regulate P-glycoprotein expression, and the effects of the eEF-2K inhibitor NH125 on the expression of P-glycoprotein in rat brain microvessel endothelial cells (RBMECs).

Methods

Cortex was obtained from newborn Wistar rat brains. After surface vessels and meninges were removed, the pellet containing microvessels was resuspended and incubated at 37°C in culture medium. Cell viability was assessed by the MTT assay. RBMECs were identified by immunohistochemistry with anti-vWF. P-glycoprotein, phospho-eEF-2, and eEF-2 expression were determined by western blot analysis. Mdr1a gene expression was analyzed by RT-PCR.

Results

Mdr1a mRNA, P-glycoprotein and phospho-eEF-2 expression increased in L-glutamate stimulated RBMECs. P-glycoprotein and phospho-eEF-2 expression were down-regulated after NH125 treatment in L-glutamate stimulated RBMECs.

Conclusions

eEF-2K/eEF-2 should have played an important role in the regulation of P-glycoprotein expression in RBMECs. eEF-2K inhibitor NH125 could serve as an efficacious anti-multidrug resistant agent.

Molecular Mechanism for the Control of Eukaryotic Elongation Factor 2 Kinase by pH: Role in Cancer Cell Survival

Acidification of the extracellular and/or intracellular environment is involved in many aspects of cell physiology and pathology. Eukaryotic elongation factor 2 kinase (eEF2K) is a Ca(2+)/calmodulin-dependent kinase that regulates translation elongation by phosphorylating and inhibiting eEF2. Here we show that extracellular acidosis elicits activation of eEF2K in vivo, leading to enhanced phosphorylation of eEF2. We identify five histidine residues in eEF2K that are crucial for the activation of eEF2K during acidosis. Three of them (H80, H87, and H94) are in its calmodulin-binding site, and their protonation appears to enhance the ability of calmodulin to activate eEF2K. The other two histidines (H227 and H230) lie in the catalytic domain of eEF2K. We also identify His108 in calmodulin as essential for activation of eEF2K. Acidification of cancer cell microenvironments is a hallmark of malignant solid tumors. Knocking down eEF2K in cancer cells attenuated the decrease in global protein synthesis when cells were cultured at acidic pH. Importantly, activation of eEF2K is linked to cancer cell survival under acidic conditions. Inhibition of eEF2K promotes cancer cell death under acidosis.

Eukaryotic elongation factor 2 kinase regulates the cold stress response by slowing translation elongation

Cells respond to external stress conditions by controlling gene expression, a process which occurs rapidly via post-transcriptional regulation at the level of protein synthesis. Global control of translation is mediated by modification of translation factors to allow reprogramming of the translatome and synthesis of specific proteins that are required for stress protection or initiation of apoptosis. In the present study, we have investigated how global protein synthesis rates are regulated upon mild cooling. We demonstrate that although there are changes to the factors that control initiation, including phosphorylation of eukaryotic translation initiation factor 2 (eIF2) on the α-subunit, the reduction in the global translation rate is mediated by regulation of elongation via phosphorylation of eukaryotic elongation factor 2 (eEF2) by its specific kinase, eEF2K (eukaryotic elongation factor 2 kinase). The AMP/ATP ratio increases following cooling, consistent with a reduction in metabolic rates, giving rise to activation of AMPK (5'-AMP-activated protein kinase), which is upstream of eEF2K. However, our data show that the major trigger for activation of eEF2K upon mild cooling is the release of Ca2+ ions from the endoplasmic reticulum (ER) and, importantly, that it is possible to restore protein synthesis rates in cooled cells by inhibition of this pathway at multiple points. As cooling has both therapeutic and industrial applications, our data provide important new insights into how the cellular responses to this stress are regulated, opening up new possibilities to modulate these responses for medical or industrial use at physiological or cooler temperatures.

Elongation Factor 2 Kinase Is Regulated by Proline Hydroxylation and Protects Cells during Hypoxia

Protein synthesis, especially translation elongation, requires large amounts of energy, which is often generated by oxidative metabolism. Elongation is controlled by phosphorylation of eukaryotic elongation factor 2 (eEF2), which inhibits its activity and is catalyzed by eEF2 kinase (eEF2K), a calcium/calmodulin-dependent α-kinase. Hypoxia causes the activation of eEF2K and induces eEF2 phosphorylation independently of previously known inputs into eEF2K. Here, we show that eEF2K is subject to hydroxylation on proline-98. Proline hydroxylation is catalyzed by proline hydroxylases, oxygen-dependent enzymes which are inactivated during hypoxia. Pharmacological inhibition of proline hydroxylases also stimulates eEF2 phosphorylation. Pro98 lies in a universally conserved linker between the calmodulin-binding and catalytic domains of eEF2K. Its hydroxylation partially impairs the binding of calmodulin to eEF2K and markedly limits the calmodulin-stimulated activity of eEF2K. Neuronal cells depend on oxygen, and eEF2K helps to protect them from hypoxia. eEF2K is the first example of a protein directly involved in a major energy-consuming process to be regulated by proline hydroxylation. Since eEF2K is cytoprotective during hypoxia and other conditions of nutrient insufficiency, it may be a valuable target for therapy of poorly vascularized solid tumors.

Eukaryotic elongation factor 2 kinase activity is controlled by multiple inputs from oncogenic signaling

Eukaryotic elongation factor 2 kinase (eEF2K), an atypical calmodulin-dependent protein kinase, phosphorylates and inhibits eEF2, slowing down translation elongation. eEF2K contains an N-terminal catalytic domain, a C-terminal α-helical region and a linker containing several regulatory phosphorylation sites. eEF2K is expressed at high levels in certain cancers, where it may act to help cell survival, e.g., during nutrient starvation. However, it is a negative regulator of protein synthesis and thus cell growth, suggesting that cancer cells may possess mechanisms to inhibit eEF2K under good growth conditions, to allow protein synthesis to proceed. We show here that the mTORC1 pathway and the oncogenic Ras/Raf/MEK/extracellular signal-regulated kinase (ERK) pathway cooperate to restrict eEF2K activity. We identify multiple sites in eEF2K whose phosphorylation is regulated by mTORC1 and/or ERK, including new ones in the linker region. We demonstrate that certain sites are phosphorylated directly by mTOR or ERK. Our data reveal that glycogen synthase kinase 3 signaling also regulates eEF2 phosphorylation. In addition, we show that phosphorylation sites remote from the N-terminal calmodulin-binding motif regulate the phosphorylation of N-terminal sites that control CaM binding. Mutations in the former sites, which occur in cancer cells, cause the activation of eEF2K. eEF2K is thus regulated by a network of oncogenic signaling pathways.

The molecular mechanism of eukaryotic elongation factor 2 kinase activation

Calmodulin (CaM)-dependent eukaryotic elongation factor 2 kinase (eEF-2K) impedes protein synthesis through phosphorylation of eukaryotic elongation factor 2 (eEF-2). It is subject to complex regulation by multiple upstream signaling pathways, through poorly described mechanisms. Precise integration of these signals is critical for eEF-2K to appropriately regulate protein translation rates. Here, an allosteric mechanism comprising two sequential conformations is described for eEF-2K activation. First, Ca(2+)/CaM binds eEF-2K with high affinity (Kd(CaM)(app) = 24 ± 5 nm) to enhance its ability to autophosphorylate Thr-348 in the regulatory loop (R-loop) by > 10(4)-fold (k(auto) = 2.6 ± 0.3 s(-1)). Subsequent binding of phospho-Thr-348 to a conserved basic pocket in the kinase domain potentially drives a conformational transition of the R-loop, which is essential for efficient substrate phosphorylation. Ca(2+)/CaM binding activates autophosphorylated eEF-2K by allosterically enhancing k(cat)(app) for peptide substrate phosphorylation by 10(3)-fold. Thr-348 autophosphorylation results in a 25-fold increase in the specificity constant (k(cat)(app)/K(m)(Pep-S) (app)), with equal contributions from k(cat)(app) and K(m)(Pep-S)(app), suggesting that peptide substrate binding is partly impeded in the unphosphorylated enzyme. In cells, Thr-348 autophosphorylation appears to control the catalytic output of active eEF-2K, contributing more than 5-fold to its ability to promote eEF-2 phosphorylation. Fundamentally, eEF-2K activation appears to be analogous to an amplifier, where output volume may be controlled by either toggling the power switch (switching on the kinase) or altering the volume control (modulating stability of the active R-loop conformation). Because upstream signaling events have the potential to modulate either allosteric step, this mechanism allows for exquisite control of eEF-2K output.

Eukaryotic elongation factor 2 kinase, an unusual enzyme with multiple roles

Eukaryotic elongation factor 2 kinase (eEF2K) is a member of the small group of atypical 'α-kinases'. It phosphorylates and inhibits eukaryotic elongation factor 2, to slow down the elongation stage of protein synthesis, which normally consumes a great deal of energy and amino acids. The activity of eEF2K is normally dependent on calcium ions and calmodulin. eEF2K is also regulated by a plethora of other inputs, including inhibition by signalling downstream of anabolic signalling pathways such as the mammalian target of rapamycin complex 1. Recent data show that eEF2K helps to protect cancer cells against nutrient starvation and is also cytoprotective in other settings, including hypoxia. Growing evidence points to roles for eEF2K in neurological processes such as learning and memory and perhaps in depression.

A conserved loop in the catalytic domain of eukaryotic elongation factor 2 kinase plays a key role in its substrate specificity

Eukaryotic elongation factor 2 kinase (eEF2K) is the best-characterized member of the α-kinase family. Within this group, only eEF2K and myosin heavy chain kinases (MHCKs) have known substrates. Here we have studied the roles of specific residues, selected on the basis of structural data for MHCK A and TRPM7, in the function of eEF2K. Our data provide the first information regarding the basis of the substrate specificity of α-kinases, in particular the roles of residues in the so-called N/D loop, which appears to occupy a position in the structure of α-kinases similar to that of the activation loop in other kinases. Several mutations in the EEF2K gene occur in tumors, one of which (Arg303Cys) is at a highly conserved residue in the N/D loop. This mutation greatly enhances eEF2K activity and may be cytoprotective. Our data support the concept that the major autophosphorylation site (Thr348 in eEF2K) docks into a binding pocket to help create the kinase-competent conformation. This is similar to the situation for MHCK A and is consistent with this being a common feature of α-kinases.

High-Throughput Screens for eEF-2 Kinase

eEF-2 kinase is a potential therapeutic target for breast cancer, gliomas, and depression. No potent inhibitors of eEF-2K have been reported, and thus development of high-throughput assay systems may expedite the process. Two high-throughput assays are described for eEF-2K using recombinant, tag-free enzyme purified from bacteria. The first is a fluorescence-based assay that uses the phosphorylation of a Sox-based peptide substrate by eEF-2K, which results in a 5-fold increase in fluorescence emission, allowing for continuous monitoring of the kinase activity. The second is a luminescence-based assay that produces a luminescence signal, which correlates with the amount of adenosine triphosphate remaining in the kinase reaction. Both assays have been optimized and miniaturized for a 384-well plate format and validated in screens. In conclusion, we demonstrated that a traditional radiolabeled assay can be readily transferred to universal spectroscopic assays that are robust and will facilitate high-throughput screening of larger size libraries for the identification of small-molecule inhibitors and significantly contribute to the development of therapies for targeting eEF2K.

Eukaryotic elongation factor 2 kinase regulates the development of hypertension through oxidative stress-dependent vascular inflammation

Eukaryotic elongation factor 2 kinase (eEF2K) is a Ca2+/calmodulin-dependent protein kinase. We recently demonstrated that eEF2K protein increases in mesenteric artery from spontaneously hypertensive rats (SHR). Pathogenesis of hypertension is regulated in part by vascular inflammation. We tested the hypothesis whether eEF2K mediates vascular inflammatory responses and development of hypertension. In vascular endothelial cells, small interfering RNA (siRNA) against eEF2K inhibited induction of VCAM-1 and endothelial-selectin as well as monocyte adhesion by TNF-α (10 ng/ml). eEF2K siRNA inhibited phosphorylation of JNK and NF-κB p65 as well as reactive oxygen species (ROS) production by TNF-α. In vascular smooth muscle cells, eEF2K siRNA also inhibited VCAM-1 induction and phosphorylation of JNK and NF-κB by TNF-α. In vivo, increased blood pressure in SHR and ROS production, induction of inflammatory molecules, and hypertrophy in SHR superior mesenteric artery were reduced by an eEF2K inhibitor NH125 (500 μg·kg(-1)·day(-1)). In SHR superior mesenteric artery, impairment of ACh-induced relaxation was normalized by NH125. The present results for the first time demonstrate that eEF2K mediates TNF-α-induced vascular inflammation via ROS-dependent mechanism, which is at least partly responsible for the development of hypertension in SHR.

Phosphorylation of eukaryotic elongation factor 2 (eEF2) by cyclin A-cyclin-dependent kinase 2 regulates its inhibition by eEF2 kinase

Protein synthesis is highly regulated via both initiation and elongation. One mechanism that inhibits elongation is phosphorylation of eukaryotic elongation factor 2 (eEF2) on threonine 56 (T56) by eEF2 kinase (eEF2K). T56 phosphorylation inactivates eEF2 and is the only known normal eEF2 functional modification. In contrast, eEF2K undergoes extensive regulatory phosphorylations that allow diverse pathways to impact elongation. We describe a new mode of eEF2 regulation and show that its phosphorylation by cyclin A-cyclin-dependent kinase 2 (CDK2) on a novel site, serine 595 (S595), directly regulates T56 phosphorylation by eEF2K. S595 phosphorylation varies during the cell cycle and is required for efficient T56 phosphorylation in vivo. Importantly, S595 phosphorylation by cyclin A-CDK2 directly stimulates eEF2 T56 phosphorylation by eEF2K in vitro, and we suggest that S595 phosphorylation facilitates T56 phosphorylation by recruiting eEF2K to eEF2. S595 phosphorylation is thus the first known eEF2 modification that regulates its inhibition by eEF2K and provides a novel mechanism linking the cell cycle machinery to translational control. Because all known eEF2 regulation is exerted via eEF2K, S595 phosphorylation may globally couple the cell cycle machinery to regulatory pathways that impact eEF2K activity.

Phosphorylation of elongation factor-2 kinase differentially regulates the enzyme's stability under stress conditions

Eukaryotic elongation factor-2 kinase (eEF-2K) is a Ca(2+)/calmodulin-dependent enzyme that negatively regulates protein synthesis. eEF-2K has been shown to be up-regulated in cancer, and to play an important role in cell survival through inhibition of protein synthesis. Post-translational modification of protein synthesis machinery is important for its regulation and could be critical for survival of cancer cells encountering stress. The purpose of our study was to examine the regulation of eEF-2K during stress with a focus on the roles of phosphorylation in determining the stability of eEF-2K. We found that stress conditions (nutrient deprivation and hypoxia) increase eEF-2K protein. mRNA levels are only transiently increased and shortly return to normal, while eEF-2K protein levels continue to increase after further exposure to stress. A seemingly paradoxical decrease in eEF-2K stability was found when glioma cells were subjected to stress despite increased protein expression. We further demonstrated that phosphorylation of eEF-2K differentially affects the enzyme's turnover under both normal and stress conditions, as evidenced by the different half-lives of phosphorylation-defective mutants of eEF-2K. We further found that the eEF-2K site (Ser398) phosphorylated by AMPK is pivotal to the protein's stability, as the half-life of S398A mutant increases to greater than 24h under both normal and stress conditions. These data indicate that eEF-2K is regulated at multiple levels with phosphorylation playing a critical role in the enzyme's turnover under stressful conditions. The complexity of eEF-2K phosphorylation highlights the intricacies of protein synthesis control during cellular stress.

Identification of autophosphorylation sites in eukaryotic elongation factor-2 kinase

eEF2K [eEF2 (eukaryotic elongation factor 2) kinase] phosphorylates and inactivates the translation elongation factor eEF2. eEF2K is not a member of the main eukaryotic protein kinase superfamily, but instead belongs to a small group of so-called α-kinases. The activity of eEF2K is normally dependent upon Ca²⁺ and calmodulin. eEF2K has previously been shown to undergo autophosphorylation, the stoichiometry of which suggested the existence of multiple sites. In the present study we have identified several autophosphorylation sites, including Thr³⁴⁸, Thr³⁵³, Ser³⁶⁶ and Ser⁴⁴⁵, all of which are highly conserved among vertebrate eEF2Ks. We also identified a number of other sites, including Ser⁷⁸, a known site of phosphorylation, and others, some of which are less well conserved. None of the sites lies in the catalytic domain, but three affect eEF2K activity. Mutation of Ser⁷⁸, Thr³⁴⁸ and Ser³⁶⁶ to a non-phosphorylatable alanine residue decreased eEF2K activity. Phosphorylation of Thr³⁴⁸ was detected by immunoblotting after transfecting wild-type eEF2K into HEK (human embryonic kidney)-293 cells, but not after transfection with a kinase-inactive construct, confirming that this is indeed a site of autophosphorylation. Thr³⁴⁸ appears to be constitutively autophosphorylated in vitro. Interestingly, other recent data suggest that the corresponding residue in other α-kinases is also autophosphorylated and contributes to the activation of these enzymes [Crawley, Gharaei, Ye, Yang, Raveh, London, Schueler-Furman, Jia and Cote (2011) J. Biol. Chem. 286, 2607–2616]. Ser³⁶⁶ phosphorylation was also detected in intact cells, but was still observed in the kinase-inactive construct, demonstrating that this site is phosphorylated not only autocatalytically but also in trans by other kinases.

Insights into the regulation of eukaryotic elongation factor 2 kinase and the interplay between its domains

eEF2K (eukaryotic elongation factor 2 kinase) is a Ca²⁺/CaM (calmodulin)-dependent protein kinase which regulates the translation elongation machinery. eEF2K belongs to the small group of so-called ‘α-kinases’ which are distinct from the main eukaryotic protein kinase superfamily. In addition to the α-kinase catalytic domain, other domains have been identified in eEF2K: a CaM-binding region, N-terminal to the kinase domain; a C-terminal region containing several predicted α-helices (resembling SEL1 domains); and a probably rather unstructured ‘linker’ region connecting them. In the present paper, we demonstrate: (i) that several highly conserved residues, implicated in binding ATP or metal ions, are critical for eEF2K activity; (ii) that Ca²⁺/CaM enhance the ability of eEF2K to bind to ATP, providing the first insight into the allosteric control of eEF2K; (iii) that the CaM-binding/α-kinase domain of eEF2K itself possesses autokinase activity, but is unable to phosphorylate substrates in trans; (iv) that phosphorylation of these substrates requires the SEL1-like domains of eEF2K; and (v) that highly conserved residues in the C-terminal tip of eEF2K are essential for the phosphorylation of eEF2, but not a peptide substrate. On the basis of these findings, we propose a model for the functional organization and control of eEF2K.

Investigating the kinetic mechanism of inhibition of elongation factor 2 kinase by NH125: evidence of a common in vitro artifact

Evidence that elongation factor 2 kinase (eEF-2K) has potential as a target for anticancer therapy and possibly for the treatment of depression is emerging. Here the steady-state kinetic mechanism of eEF-2K is presented using a peptide substrate and is shown to conform to an ordered sequential mechanism with ATP binding first. Substrate inhibition by the peptide was observed and revealed to be competitive with ATP, explaining the observed ordered mechanism. Several small molecules are reported to inhibit eEF-2K activity with the most notable being the histidine kinase inhibitor NH125, which has been used in a number of studies to characterize eEF-2K activity in cells. While NH125 was previously reported to inhibit eEF-2K in vitro with an IC(50) of 60 nM, its mechanism of action was not established. Using the same kinetic assay, the ability of an authentic sample of NH125 to inhibit eEF-2K was assessed over a range of substrate and inhibitor concentrations. A typical dose-response curve for the inhibition of eEF-2K by NH125 is best fit to an IC(50) of 18 ± 0.25 μM and a Hill coefficient of 3.7 ± 0.14, suggesting that NH125 is a weak inhibitor of eEF-2K under the experimental conditions of a standard in vitro kinase assay. To test the possibility that NH125 is a potent inhibitor of eEF2 phosphorylation, we assessed its ability to inhibit the phosphorylation of eEF2. Under standard kinase assay conditions, NH125 exhibits a similar weak ability to inhibit the phosphorylation of eEF2 by eEF-2K. Notably, the activity of NH125 is severely abrogated by the addition of 0.1% Triton to the kinase assay through a process that can be reversed upon dilution. These studies suggest that NH125 is a nonspecific colloidal aggregator in vitro, a notion further supported by the observation that NH125 inhibits other protein kinases, such as ERK2 and TRPM7 in a manner similar to that of eEF-2K. As NH125 is reported to inhibit eEF-2K in a cellular environment, its ability to inhibit eEF2 phosphorylation was assessed in MDA-MB-231 breast cancer, A549 lung cancer, and HEK-293T cell lines using a Western blot approach. No sign of a decrease in the level of eEF2 phosphorylation was observed up to 12 h following addition of NH125 to the media. Furthermore, contrary to the previously reported literatures, NH125 induced the phosphorylation of eEF-2.

The role of molecular regulation and targeting in regulating calcium/calmodulin stimulated protein kinases

Calcium/calmodulin-stimulated protein kinases can be classified as one of two types - restricted or multifunctional. This family of kinases contains several structural similarities: all possess a calmodulin binding motif and an autoinhibitory region. In addition, all of the calcium/calmodulin-stimulated protein kinases examined in this chapter are regulated by phosphorylation, which either activates or inhibits their kinase activity. However, as the multifunctional calcium/calmodulin-stimulated protein kinases are ubiquitously expressed, yet regulate a broad range of cellular functions, additional levels of regulation that control these cell-specific functions must exist. These additional layers of control include gene expression, signaling pathways, and expression of binding proteins and molecular targeting. All of the multifunctional calcium/calmodulin-stimulated protein kinases examined in this chapter appear to be regulated by these additional layers of control, however, this does not appear to be the case for the restricted kinases.

1-Benzyl-3-cetyl-2-methylimidazolium iodide (NH125) induces phosphorylation of eukaryotic elongation factor-2 (eEF2): a cautionary note on the anticancer mechanism of an eEF2 kinase inhibitor

Eukaryotic elongation factor-2 kinase (eEF2K) relays growth and stress signals to protein synthesis through phosphorylation and inactivation of eukaryotic elongation factor 2 (eEF2). 1-Benzyl-3-cetyl-2-methylimidazolium iodide (NH125) is a widely accepted inhibitor of mammalian eEF2K and an efficacious anti-proliferation agent against different cancer cells. It implied that eEF2K could be an efficacious anticancer target. However, eEF2K siRNA was ineffective against cancer cells including those sensitive to NH125. To test if pharmacological intervention differs from siRNA interference, we identified a highly selective small molecule eEF2K inhibitor A-484954. Like siRNA, A-484954 had little effect on cancer cell growth. We carefully examined the effect of NH125 and A-484954 on phosphorylation of eEF2, the known cellular substrate of eEF2K. Surprisingly, NH125 increased eEF2 phosphorylation, whereas A-484954 inhibited the phosphorylation as expected for an eEF2K inhibitor. Both A-484954 and eEF2K siRNA inhibited eEF2K and reduced eEF2 phosphorylation with little effect on cancer cell growth. These data demonstrated clearly that the anticancer activity of NH125 was more correlated with induction of eEF2 phosphorylation than inhibition of eEF2K. Actually, induction of eEF2 phosphorylation was reported to correlate with inhibition of cancer cell growth. We compared several known inducers of eEF2 phosphorylation including AMPK activators and an mTOR inhibitor. Interestingly, stronger induction of eEF2 phosphorylation correlated with more effective growth inhibition. We also explored signal transduction pathways leading to NH125-induced eEF2 phosphorylation. Preliminary data suggested that NH125-induced eEF2 phosphorylation was likely mediated through multiple pathways. These observations identified an opportunity for a new multipathway approach to anticancer therapies.

Purification and characterization of tagless recombinant human elongation factor 2 kinase (eEF-2K) expressed in Escherichia coli

The eukaryotic elongation factor 2 kinase (eEF-2K) modulates the rate of protein synthesis by impeding the elongation phase of translation by inactivating the eukaryotic elongation factor 2 (eEF-2) via phosphorylation. eEF-2K is known to be activated by calcium and calmodulin, whereas the mTOR and MAPK pathways are suggested to negatively regulate kinase activity. Despite its pivotal role in translation regulation and potential role in tumor survival, the structure, function, and regulation of eEF-2K have not been described in detail. This deficiency may result from the difficulty of obtaining the recombinant kinase in a form suitable for biochemical analysis. Here we report the purification and characterization of recombinant human eEF-2K expressed in the Escherichia coli strain Rosetta-gami 2(DE3). Successive chromatography steps utilizing Ni-NTA affinity, anion-exchange, and gel filtration columns accomplished purification. Cleavage of the thioredoxin-His(6)-tag from the N-terminus of the expressed kinase with TEV protease yielded 9 mg of recombinant (G-D-I)-eEF-2K per liter of culture. Light scattering shows that eEF-2K is a monomer of ∼85 kDa. In vitro kinetic analysis confirmed that recombinant human eEF-2K is able to phosphorylate wheat germ eEF-2 with kinetic parameters comparable to the mammalian enzyme.

Eukaryotic elongation factor-2 (eEF2): its regulation and peptide chain elongation

Regulation at the level of translation in eukaryotes is feasible because of the longer lifetime of eukaryotic mRNAs in the cell. The elongation stage of mRNA translation requires a substantial amount of energy and also eukaryotic elongation factors (eEFs). The important component of eEFs, i.e. eEF2 promotes the GTP-dependent translocation of the nascent protein chain from the A-site to the P-site of the ribosome. Mostly the eEF2 is regulated by phosphorylation and dephosphorylation by a specific kinase known as eEF2 kinase, which itself is up-regulated by various mechanisms in the eukaryotic cell. The activity of this kinase is dependent on calcium ions and calmodulin. Recently it has been shown that the activity of eEF2 kinase is regulated by MAP kinase signalling and mTOR signalling pathway. There are also various stimuli that control the peptide chain elongation in eukaryotic cell; some stimuli inhibit and some activate eEF2. These reports provide the mechanisms by which cells likely serve to slow down protein synthesis and conserve energy under nutrient deprived conditions via regulation of eEF2. The regulation via eEF2 has also been seen in mammary tissue of lactating cows, suggesting that eEF2 may be a limiting factor in milk protein synthesis. Regulation at this level provides the molecular understanding about the control of protein translocation reactions in eukaryotes, which is critical for numerous biological phenomenons. Further the elongation factors could be potential targets for regulation of protein synthesis like milk protein synthesis and hence probably its foreseeable application to synthetic biology.

Identification of eukaryotic elongation factor-2 as a novel cellular target of lithium and glycogen synthase kinase-3

Inhibition of glycogen synthase kinase-3 (GSK-3) is thought to be a major consequence of the biological and clinical activity of the mood stabilizer lithium, however, lithium and GSK-3 may activate distinct cellular pathways. We employed a proteomic method to uncover new downstream targets of lithium, and then examined how these proteins are related to GSK-3. Proteomic analysis identified eukaryotic elongation factor-2 (eEF-2) as a cellular target of lithium. This was verified in SH-SY5Y cells and animal models. In cells, lithium decreased eEF-2 phosphorylation at its key inhibitory site, threonine 56, and blocked the enhancement of eEF-2 phosphorylation normally coupled with stress conditions such as nutrient and serum deprivation. Unexpectedly, inhibition of GSK-3 enhanced eEF-2 phosphorylation, and overexpression of GSK-3α or GSK-3β resulted in a strong reduction in eEF-2 phosphorylation. Chronic administration of lithium reduced the hippocampal fraction of phospho-eEF-2 (phospho-eEF-2/total eEF-2) twofold in two different mouse strains. In summary, unexpectedly eEF-2 is activated by both lithium and GSK-3, whereas, lithium treatment and inhibition of GSK-3 have opposing effects on eEF-2.

The alpha-kinase family: an exceptional branch on the protein kinase tree

The alpha-kinase family represents a class of atypical protein kinases that display little sequence similarity to conventional protein kinases. Early studies on myosin heavy chain kinases in Dictyostelium discoideum revealed their unusual propensity to phosphorylate serine and threonine residues in the context of an alpha-helix. Although recent studies show that some members of this family can also phosphorylate residues in non-helical regions, the name alpha-kinase has remained. During evolution, the alpha-kinase domains combined with many different functional subdomains such as von Willebrand factor-like motifs (vWKa) and even cation channels (TRPM6 and TRPM7). As a result, these kinases are implicated in a large variety of cellular processes such as protein translation, Mg(2+) homeostasis, intracellular transport, cell migration, adhesion, and proliferation. Here, we review the current state of knowledge on different members of this kinase family and discuss the potential use of alpha-kinases as drug targets in diseases such as cancer.

Identification of dimer interactions required for the catalytic activity of the TRPM7 alpha-kinase domain

TRPM7 (transient receptor potential melastatin) combines an ion channel domain with a C-terminal protein kinase domain that belongs to the atypical alpha-kinase family. The TRPM7 alpha-kinase domain assembles into a dimer through the exchange of an N-terminal segment that extends from residue 1551 to residue 1577 [Yamaguchi, Matsushita, Nairn and Kuriyan (2001) Mol. Cell 7, 1047-1057]. Here, we show, by analysis of truncation mutants, that residues 1553-1562 of the N-terminus are essential for kinase activity but not dimer formation. Within this 'activation sequence', site-directed mutagenesis identified Tyr-1553 and Arg-1558 as residues critical for activity. Examination of the TRPM7 kinase domain structure suggests that the activation sequence interacts with the other subunit to help position a catalytic loop that contains the invariant Asp-1765 residue. Residues 1563-1570 of the N-terminal segment are critical for dimer assembly. Mutation of Leu-1564, Ile-1568 or Phe-1570 to alanine abolished both kinase activity and dimer formation. The activity of a monomeric TRPM7 kinase domain lacking the entire N-terminal segment was rescued by a GST (glutathione transferase) fusion protein containing residues 1548-1576 of TRPM7, showing that all interactions essential for activity are provided by the N-terminal segment. Activity was also restored by GST fused to the N-terminal segment of TRPM6 (residues 1711-1740), demonstrating the feasibility of forming functional TRPM6-TRPM7 alpha-kinase domain heterodimers. It is proposed that covalent modifications or binding interactions that alter the conformation of the N-terminal exchanged segment may provide a means to regulate TRPM7 kinase activity.

Massive autophosphorylation of the Ser/Thr-rich domain controls protein kinase activity of TRPM6 and TRPM7

TRPM6 and TRPM7 are bifunctional proteins expressing a TRP channel fused to an atypical α-kinase domain. While the gating properties of TRPM6 and TRPM7 channels have been studied in detail, little is known about the mechanisms regulating kinase activity. Recently, we found that TRPM7 associates with its substrate myosin II via a kinase-dependent mechanism suggesting a role for autophosphorylation in substrate recognition. Here, we demonstrate that the cytosolic C-terminus of TRPM7 undergoes massive autophosphorylation (32±4 mol/mol), which strongly increases the rate of substrate phosphorylation. Phosphomapping by mass spectrometry indicates that the majority of autophosphorylation sites (37 out of 46) map to a Ser/Thr-rich region immediately N-terminal of the catalytic domain. Deletion of this region prevents substrate phosphorylation without affecting intrinsic catalytic activity suggesting that the Ser/Thr-rich domain contributes to substrate recognition. Surprisingly, the TRPM6-kinase is regulated by an analogous mechanism despite a lack of sequence conservation with the TRPM7 Ser/Thr-rich domain. In conclusion, our findings support a model where massive autophosphorylation outside the catalytic domain of TRPM6 and TRPM7 may facilitate kinase-substrate interactions leading to enhanced phosphorylation of those substrates.

Alpha-kinase 1, a new component in apical protein transport

A key aspect in the structure of epithelial cells is the maintenance of a polarized organization based on a highly specific sorting machinery for cargo destined for the apical or the basolateral membrane domain at the exit site of the trans-Golgi network. We could recently identify two distinct post-trans-Golgi network vesicle populations that travel along separate routes to the plasma membrane, a lipid raft-dependent and a lipid raft-independent pathway. A new component of raft-carrying apical vesicles is alpha-kinase 1 (ALPK1), which was identified in immunoisolated vesicles carrying raft-associated sucrase-isomaltase (SI). This kinase was absent from vesicles carrying raft-non-associated lactase-phlorizin hydrolase. The expression of ALPK1 increases by the time of epithelial cell differentiation, whereas the intracellular localization of ALPK1 on apical transport vesicles was confirmed by confocal analysis. A phosphorylation assay on isolated SI-carrying vesicles revealed the phosphorylation of a protein band of about 105 kDa, which could be identified as the motor protein myosin I. Finally, a specific reduction of ALPK1-expression by RNA interference results in a significant decrease in the apical delivery of SI. Taken together, our data suggest that the phosphorylation of myosin I by ALPK1 is an essential process in the apical trafficking of raft-associated SI.

Channel Function Is Dissociated from the Intrinsic Kinase Activity and Autophosphorylation of TRPM7/ChaK1

TRPM7/ChaK1 is a unique channel/kinase that contains a TRPM channel domain with 6 transmembrane segments fused to a novel serine-threonine kinase domain at its C terminus. The goal of this study was to investigate a possible role of kinase activity and autophosphorylation in regulation of channel activity of TRPM7/ChaK1. Residues essential for kinase activity were identified by site-directed mutagenesis. Two major sites of autophosphorylation were identified in vitro by mass spectrometry at Ser(1511) and Ser(1567), and these sites were found to be phosphorylated in intact cells. TRPM7/ChaK1 is a cation-selective channel that exhibits strong outward rectification and inhibition by millimolar levels of internal [Mg(2+)]. Mutation of the two autophosphorylation sites or of a key catalytic site that abolished kinase activity did not alter channel activity measured by whole-cell recording or Ca(2+) influx. Inhibition by internal Mg(2+) was also unaffected in the autophosphorylation site or "kinase-dead" mutants. Moreover, kinase activity was enhanced by Mg(2+), was decreased by Zn(2+), and was unaffected by Ca(2+). In contrast, channel activity was inhibited by all three of these divalent cations. However, deletion of much of C-terminal kinase domain resulted in expression of an apparently inactive channel. We conclude that neither current activity nor regulation by internal Mg(2+) is affected by kinase activity or autophosphorylation but that the kinase domain may play a structural role in channel assembly or subcellular localization.

Identification and characterization of a novel alpha-kinase with a von Willebrand factor A-like motif localized to the contractile vacuole and Golgi complex in Dictyostelium discoideum

We have identified a new protein kinase in Dictyostelium discoideum that carries the same conserved class of "alpha-kinase" catalytic domain as reported previously in myosin heavy chain kinases (MHCKs) in this amoeba but that has a completely novel domain organization. The protein contains an N-terminal von Willebrand factor A (vWFA)-like motif and is therefore named VwkA. Manipulation of VwkA expression level via high copy number plasmids (VwkA++ cells) or gene disruption (vwkA null cells) results in an array of cellular defects, including impaired growth and multinucleation in suspension culture, impaired development, and alterations in myosin II abundance and assembly. Despite sequence similarity to MHCKs, the purified protein failed to phosphorylate myosin II in vitro. Autophosphorylation activity, however, was enhanced by calcium/calmodulin, and the enzyme can be precipitated from cellular lysates with calmodulin-agarose, suggesting that VwkA may directly bind calmodulin. VwkA is cytosolic in distribution but enriched on the membranes of the contractile vacuole and Golgi-like structures in the cell. We propose that VwkA likely acts indirectly to influence myosin II abundance and assembly behavior and possibly has broader roles than previously characterized alpha kinases in this organism, which all seem to be MHCKs.

Alpha-kinases: analysis of the family and comparison with conventional protein kinases

Alpha-kinases are a recently discovered family of protein kinases that have no detectable sequence homology to conventional protein kinases (CPKs). They include elongation factor 2 kinase, Dictyostelium myosin heavy chain kinases and many other protein kinases from diverse organisms, as revealed by various genome sequencing projects. Mammals have six alpha-kinases, including two channel-kinases-novel signaling molecules that contain an alpha-kinase domain fused to an ion-channel. Analysis of all known alpha-kinase sequences reveals the presence of several highly conserved motifs. Despite the fact that alpha-kinases have no detectable sequence identity with CPKs, the recently determined three-dimensional structure of the channel-kinase TRPM7/ChaK1 kinase domain reveals that alpha-kinases have a fold very similar to CPKs. Using the structural alignment of channel-kinase TRPM7/ChaK1 with cyclic-AMP dependent kinase, the consensus motifs of alpha-kinases and CPKs were aligned and compared. Remarkably, the majority of structural elements, sequence motifs, and the position of key amino acid residues important for catalysis appear to be very similar in alpha-kinases and CPKs. Differences between alpha-kinases and CPKs, and their possible impact on substrate recognition are discussed.

A novel mTOR-regulated phosphorylation site in elongation factor 2 kinase modulates the activity of the kinase and its binding to calmodulin

Eukaryotic elongation factor 2 (eEF2) kinase is an unusual calcium- and calmodulin-dependent protein kinase that is regulated by insulin through the rapamycin-sensitive mTOR pathway. Here we show that insulin decreases the ability of eEF2 kinase to bind calmodulin in a rapamycin-sensitive manner. We identify a novel phosphorylation site in eEF2 kinase (Ser78) that is located immediately next to its calmodulin-binding motif. Phosphorylation of this site is increased by insulin in a rapamycin-sensitive fashion. Regulation of the phosphorylation of Ser78 also requires amino acids and the protein kinase phosphoinositide-dependent kinase 1. Mutation of this site to alanine strongly attenuates the effects of insulin and rapamycin both on the binding of calmodulin to eEF2 kinase and on eEF2 kinase activity. Phosphorylation of Ser78 is thus likely to link insulin and mTOR signaling to the control of eEF2 phosphorylation and chain elongation. This site is not a target for known kinases in the mTOR pathway, e.g., the S6 kinases, implying that it is phosphorylated by a novel mTOR-linked protein kinase that serves to couple hormones and amino acids to the control of translation elongation. eEF2 kinase is thus a target for mTOR signaling independently of previously known downstream components of the pathway.

Stimulation of the AMP-activated Protein Kinase Leads to Activation of Eukaryotic Elongation Factor 2 Kinase and to Its Phosphorylation at a Novel Site, Serine 398

Protein synthesis consumes a high proportion of the metabolic energy of mammalian cells, and most of this is used by peptide chain elongation. An important regulator of energy supply and demand in eukaryotic cells is the AMP-activated protein kinase (AMPK). The rate of peptide chain elongation can be modulated through the phosphorylation of eukaryotic elongation factor (eEF) 2, which inhibits its activity and is catalyzed by a specific calcium/calmodulin-dependent protein kinase termed eEF2 kinase. Here we show that AMPK directly phosphorylates eEF2 kinase, and we identify the major site of phosphorylation as Ser-398 in a regulatory domain of eEF2 kinase. AMPK also phosphorylates two other sites (Ser-78 and Ser-366) in eEF2 kinase in vitro. We develop appropriate phosphospecific antisera and show that phosphorylation of Ser-398 in eEF2 kinase is enhanced in intact cells under a range of conditions that activate AMPK and increase the phosphorylation of eEF2. Ser-78 and Ser-366 do not appear to be phosphorylated by AMPK within cells. Although cardiomyocytes appear to contain a distinct isoform of eEF2 kinase, it also contains a site corresponding to Ser-398 that is phosphorylated by AMPK in vitro. Stimuli that activate AMPK and increase eEF2 phosphorylation within cells increase the activity of eEF2 kinase. Thus, AMPK and eEF2 kinase may provide a key link between cellular energy status and the inhibition of protein synthesis, a major consumer of metabolic energy.

Signaling pathways regulating Dictyostelium myosin II

Dictyostelium myosin II is a conventional, two-headed myosin that consists of two copies each of a myosin heavy chain (MHC), an essential light chain (ELC) and a regulatory light chain (RLC). The MHC is comprised of an amino-terminal motor domain, a neck region that binds the RLC and ELC and a carboxyl-terminal alpha-helical coiled-coil tail. Electrostatic interactions between the tail domains mediate the self-assembly of myosin II into bipolar filaments that are capable of interacting with actin filaments to generate a contractile force. In this review we discuss the regulation of Dictyostelium myosin II by a myosin light chain kinase (MLCK-A) that phosphorylates the RLC and increases motor activity and by MHC kinases (MHCKs) that phosphorylate the tail and prevent filament assembly. Dictyostelium may express as many as four MHCKs (MHCK A-D) consisting of an atypical alpha-kinase catalytic domain and a carboxyl-terminal WD repeat domain that targets myosin II filaments. A previously reported MHCK, termed MHC-PKC, now seems more likely to be a diacylglycerol kinase (DgkA). The relationship of the MHCKs to the larger family of alpha-kinases is discussed and key features of the structure of the alpha-kinase catalytic domain are reviewed. Potential upstream regulators of myosin II are described, including DgkA, cGMP, cAMP and PAKa, a target for Rac GTPases. Recent results point to a complex network of signaling pathways responsible for controling the activity and localization of myosin II in the cell.

Characterization of the protein kinase activity of TRPM7/ChaK1, a protein kinase fused to the transient receptor potential ion channel

Channel-kinase TRPM7/ChaK1 is a member of a recently discovered family of protein kinases called alpha-kinases that display no sequence homology to conventional protein kinases. It is an unusual bifunctional protein that contains an alpha-kinase domain fused to an ion channel. The TRPM7/ChaK1 channel has been characterized using electrophysiological techniques, and recent evidence suggests that it may play a key role in the regulation of magnesium homeostasis. However, little is known about its protein kinase activity. To characterize the kinase activity of TRPM7/ChaK1, we expressed the kinase catalytic domain in bacteria. ChaK1-cat is able to undergo autophosphorylation and to phosphorylate myelin basic protein and histone H3 on serine and threonine residues. The kinase is specific for ATP and cannot use GTP as a substrate. ChaK1-cat is insensitive to staurosporine (up to 0.1 mM) but can be inhibited by rottlerin. Because the kinase domain is physically linked to an ion channel, we investigated the effect of ions on ChaK1-cat activity. The kinase requires Mg(2+) (optimum at 4-10 mM) or Mn(2+) (optimum at 3-5 mM), with activity in the presence of Mn(2+) being 2 orders of magnitude higher than in the presence of Mg(2+). Zn(2+) and Co(2+) inhibited ChaK1-cat kinase activity. Ca(2+) at concentrations up to 1 mM did not affect kinase activity. Considering intracellular ion concentrations, our results suggest that, among divalent metal ions, only Mg(2+) can directly modulate TRPM7/ChaK1 kinase activity in vivo.

Myosin heavy chain kinase B participates in the regulation of myosin assembly into the cytoskeleton

Myosin II plays critical roles in events such as cytokinesis, chemotactic migration, and morphological changes during multicellular development. The amoeba Dictyostelium discoideum provides a simple system for the study of this contractile protein. In this system, myosin II filament assembly is regulated by myosin heavy chain (MHC) phosphorylation in the tail region of the molecule. Earlier studies identified an alpha-kinase, MHC kinase A (MHCK A), which phosphorylates three mapped threonine residues in the myosin tail, driving myosin disassembly. Using molecular and genomic approaches, we have identified a series of related kinases in Dictyostelium. The enzyme MHCK B shares with MHCK A a domain organization that includes a highly novel catalytic domain coupled to a carboxyl-terminal WD repeat domain. We have engineered, expressed, and purified a FLAG-tagged version of the novel kinase. In the present study, we report detailed biochemical and cellular studies documenting that MHCK B plays a physiological role in the control of Dictyostelium myosin II assembly and disassembly during the vegetative life of Dictyostelium amoebae. The presented data supports a model of multiple related MHCKs in this system, with different regulatory mechanisms and pathways controlling each enzyme.

Histidine kinases and histidine phosphorylated proteins in mammalian cell biology, signal transduction and cancer

Intensive investigation of protein tyrosine, serine and threonine phosphorylation has lead to advances in signal transduction research and cancer treatment. This feature summarizes research on mammalian proteins exhibiting histidine phosphorylation. Histidine kinases are well known in prokaryotic and lower eukaryotic systems where they form the 'two-component' signal transduction system. The relative invisibility of histidine phosphorylation in mammalian cells may result from technical obstacles such as its acid lability, which precludes detection in electrophoretic systems, amino acid sequencing, etc. Emerging data have identified mammalian histidine kinases for the kinase suppressor of ras, a scaffold molecule for the Map kinase pathway, as well as histone H4, aldolase C and the beta-subunit of heterotrimeric G proteins. Additional mammalian proteins of interest to signal transduction and cancer research exhibit histidine phosphorylation, including P-selectin, annexin I and the 20S proteasome. Other candidate histidine phosphorylated proteins are identified. These data suggest the existence of another series of phosphorylation patterns in signal transduction.

Regulation of peptide-chain elongation in mammalian cells

The elongation phase of mRNA translation is the stage at which the polypeptide is assembled and requires a substantial amount of metabolic energy. Translation elongation in mammals requires a set of nonribosomal proteins called eukaryotic elongation actors or eEFs. Several of these proteins are subject to phosphorylation in mammalian cells, including the factors eEF1A and eEF1B that are involved in recruitment of amino acyl-tRNAs to the ribosome. eEF2, which mediates ribosomal translocation, is also phosphorylated and this inhibits its activity. The kinase acting on eEF2 is an unusual and specific one, whose activity is dependent on calcium ions and calmodulin. Recent work has shown that the activity of eEF2 kinase is regulated by MAP kinase signalling and by the nutrient-sensitive mTOR signalling pathway, which serve to activate eEF2 in response to mitogenic or hormonal stimuli. Conversely, eEF2 is inactivated by phosphorylation in response to stimuli that increase energy demand or reduce its supply. This likely serves to slow down protein synthesis and thus conserve energy under such circumstances.