Death-associated protein kinase phosphorylates mammalian ribosomal protein S6 and reduces protein synthesis

Ribosomal Protein S6: A Potential Therapeutic Target against Cancer?

Ribosomal protein S6 (RPS6) is a component of the 40S small ribosomal subunit and participates in the control of mRNA translation. Additionally, phospho (p)-RPS6 has been recognized as a surrogate marker for the activated PI3K/AKT/mTORC1 pathway, which occurs in many cancer types. However, downstream mechanisms regulated by RPS6 or p-RPS remains elusive, and the therapeutic implication of RPS6 is underappreciated despite an approximately half a century history of research on this protein. In addition, substantial evidence from RPS6 knockdown experiments suggests the potential role of RPS6 in maintaining cancer cell proliferation. This motivates us to investigate the current knowledge of RPS6 functions in cancer. In this review article, we reviewed the current information about the transcriptional regulation, upstream regulators, and extra-ribosomal roles of RPS6, with a focus on its involvement in cancer. We also discussed the therapeutic potential of RPS6 in cancer.

Aberrant modulation of ribosomal protein S6 phosphorylation confers acquired resistance to MAPK pathway inhibitors in BRAF-mutant melanoma

BRAF and MEK inhibitors have shown remarkable clinical efficacy in BRAF-mutant melanoma; however, most patients develop resistance, which limits the clinical benefit of these agents. In this study, we found that the human melanoma cell clones, A375-DR and A375-TR, with acquired resistance to BRAF inhibitor dabrafenib and MEK inhibitor trametinib, were cross resistant to other MAPK pathway inhibitors. In these resistant cells, phosphorylation of ribosomal protein S6 (rpS6) but not phosphorylation of ERK or p90 ribosomal S6 kinase (RSK) were unable to be inhibited by MAPK pathway inhibitors. Notably, knockdown of rpS6 in these cells effectively downregulated G₁ phase-related proteins, including RB, cyclin D1, and CDK6, induced cell cycle arrest, and inhibited proliferation, suggesting that aberrant modulation of rpS6 phosphorylation contributed to the acquired resistance. Interestingly, RSK inhibitor had little effect on rpS6 phosphorylation and cell proliferation in resistant cells, whereas P70S6K inhibitor showed stronger inhibitory effects on rpS6 phosphorylation and cell proliferation in resistant cells than in parental cells. Thus regulation of rpS6 phosphorylation, which is predominantly mediated by BRAF/MEK/ERK/RSK signaling in parental cells, was switched to mTOR/P70S6K signaling in resistant cells. Furthermore, mTOR inhibitors alone overcame acquired resistance and rescued the sensitivity of the resistant cells when combined with BRAF/MEK inhibitors. Taken together, our findings indicate that RSK-independent phosphorylation of rpS6 confers resistance to MAPK pathway inhibitors in BRAF-mutant melanoma, and that mTOR inhibitor-based regimens may provide alternative strategies to overcome this acquired resistance.

Ribosomal Protein S6 Phosphorylation in the Nervous System: From Regulation to Function

Since the discovery of the phosphorylation of the 40S ribosomal protein S6 (rpS6) about four decades ago, much effort has been made to uncover the molecular mechanisms underlying the regulation of this post-translational modification. In the field of neuroscience, rpS6 phosphorylation is commonly used as a readout of the mammalian target of rapamycin complex 1 signaling activation or as a marker for neuronal activity. Nevertheless, its biological role in neurons still remains puzzling. Here we review the pharmacological and physiological stimuli regulating this modification in the nervous system as well as the pathways that transduce these signals into rpS6 phosphorylation. Altered rpS6 phosphorylation observed in various genetic and pathophysiological mouse models is also discussed. Finally, we examine the current state of knowledge on the physiological role of this post-translational modification and highlight the questions that remain to be addressed.

Ribosomal Protein S6 Phosphorylation: Four Decades of Research

The phosphorylation of ribosomal protein S6 (rpS6) has been described for the first time about four decades ago. Since then, numerous studies have shown that this modification occurs in response to a wide variety of stimuli on five evolutionarily conserved serine residues. However, despite a large body of information on the respective kinases and the signal transduction pathways, the physiological role of rpS6 phosphorylation remained obscure until genetic manipulations were applied in both yeast and mammals in an attempt to block this modification. Thus, studies based on both mice and cultured cells subjected to disruption of the genes encoding rpS6 and the respective kinases, as well as the substitution of the phosphorylatable serine residues in rpS6, have laid the ground for the elucidation of the multiple roles of this protein and its posttranslational modification. This review focuses primarily on newly identified kinases that phosphorylate rpS6, pathways that transduce various signals into rpS6 phosphorylation, and the recently established physiological functions of this modification. It should be noted, however, that despite the significant progress made in the last decade, the molecular mechanism(s) underlying the diverse effects of rpS6 phosphorylation on cellular and organismal physiology are still poorly understood.

Death-associated protein kinase: A molecule with functional antagonistic duality and a potential role in inflammatory bowel disease (Review)

The cytoskeleton-associated serine/threonine kinase death-associated protein kinase (DAPK) has been described as a cancer gene chameleon with functional antagonistic duality in a cell type and context specific manner. The broad range of interaction partners and substrates link DAPK to inflammatory processes especially in the gut. Herein we summarize our knowledge on the role of DAPK in different cell types that play a role under inflammatory conditions in the gut. Besides some promising experimental data suggesting DAPK as an interesting drug target in inflammatory bowel disease there are many open questions regarding direct evidence for a role of DAPK in intestinal inflammation.

DAPK3 suppresses acini morphogenesis and is required for mouse development

Death-associated protein kinase (DAPK3) is a serine/threonine kinase involved in various signaling pathways important to tissue homeostasis and mammalian biology. Considered to be a putative tumor suppressor, the molecular mechanism by which DAPK3 exerts its suppressive function is not fully understood and the field lacks an appropriate mouse model. To address these gaps, an in vitro three-dimensional tumorigenesis model was used and a constitutive DAPK3-knockout mouse was generated. In the 3D morphogenesis model, loss of DAPK3 through lentiviral-mediated knockdown enlarged acinar size by accelerated acini proliferation and apoptosis while maintaining acini polarity. Depletion of DAPK3 enhanced growth factor-dependent mTOR activation and, furthermore, enlarged DAPK3 acini structures were uniquely sensitive to low doses of rapamycin. Simultaneous knockdown of RAPTOR, a key mTORC1 component, reversed the augmented acinar size in DAPK3-depleted structures indicating an epistatic interaction. Using a validated gene trap strategy to generate a constitutive DAPK3-knockout mouse, it was demonstrated that DAPK3 is vital for early mouse development. The Dapk3 promoter exhibits spatiotemporal activity in developing mice and is actively expressed in normal breast epithelia of adult mice. Importantly, reduction of DAPK3 expression correlates with the development of ductal carcinoma in situ (DCIS) and more aggressive breast cancer as observed in the Oncomine database of clinical breast cancer specimens.

IMPLICATIONS

Novel cellular and mouse modeling studies of DAPK3 shed light on its tumor-suppressive mechanisms and provide direct evidence that DAPK3 has relevance in early development.

The DAP-kinase interactome

DAP-kinase (DAPK) is a Ca(2+)/calmodulin regulated Ser/Thr kinase that activates a diverse range of cellular activities. It is subject to multiple layers of regulation involving both intramolecular signaling, and interactions with additional proteins, including other kinases and phosphatases. Its protein stability is modulated by at least three distinct ubiquitin-dependent systems. Like many kinases, DAPK participates in several signaling cascades, by phosphorylating additional kinases such as ZIP-kinase and protein kinase D (PKD), or Pin1, a phospho-directed peptidyl-prolyl isomerase that regulates the function of many phosphorylated proteins. Other substrate targets have more direct cellular effects; for example, phosphorylation of the myosin II regulatory chain and tropomyosin mediate some of DAPK's cytoskeletal functions, including membrane blebbing during cell death and cell motility. DAPK induces distinct death pathways of apoptosis, autophagy and programmed necrosis. Among the substrates implicated in these processes, phosphorylation of PKD, Beclin 1, and the NMDA receptor has been reported. Interestingly, not all cellular effects are mediated by DAPK's catalytic activity. For example, by virtue of protein-protein interactions alone, DAPK activates pyruvate kinase isoform M2, the microtubule affinity regulating kinases and inflammasome protein NLRP3, to promote glycolysis, influence microtubule dynamics, and enhance interleukin-1β production, respectively. In addition, a number of other substrates and interacting proteins have been identified, the physiological significance of which has not yet been established. All of these substrates, effectors and regulators together comprise the DAPK interactome. By presenting the components of the interactome network, this review will clarify both the mechanisms by which DAPK function is regulated, and by which it mediates its various cellular effects.

Enzyme dimension of the ribosomal protein S4 across plant and animal kingdoms

BACKGROUND

The protein S4 of the smaller ribosomal subunit is centrally important for its anchorage role in ribosome assembly and rRNA binding. Eubacterial S4 also facilitates synthesis of rRNA, and restrains translation of ribosomal proteins of the same polycistronic mRNA. Eukaryotic S4 has no homolog in eubacterial kingdom, nor are such extraribosomal functions of S4 known in plants and animals even as genetic evidence suggests that deficiency of S4X isoform in 46,XX human females may produce Turner syndrome (45,XO).

METHODS

Recombinant human S4X and rice S4 were used to determine their enzymatic action in the cleavage of synthetic peptide substrates and natural proteins. We also studied autoproteolysis of the recombinant S4 proteins, and examined the growth and proliferation of S4-transfected human embryonic kidney cells.

RESULTS

Extraribosomal enzyme nature of eukaryotic S4 is described. Both human S4X and rice S4 are cysteine proteases capable of hydrolyzing a wide spectrum of peptides and natural proteins of diverse origin. Whereas rice S4 also cleaves the -XXXD↓- consensus sequence assumed to be specific for caspase-9 and granzyme B, human S4 does not. Curiously, both human and rice S4 show multiple-site autoproteolysis leading to self-annihilation. Overexpression of human S4 blocks the growth and proliferation of transfected embryonic kidney cells, presumably due to the extraribosomal enzyme trait reported.

CONCLUSIONS

The S4 proteins of humans and rice, prototypes of eukaryota, are non-specific cysteine proteases in the extraribosomal milieu.

GENERAL SIGNIFICANCE

The enzyme nature of S4 is relevant toward understanding not only the origin of ribosomal proteins, but also processes in cell biology and diseases.

Arrested cell proliferation through cysteine protease activity of eukaryotic ribosomal protein S4

S4 is an integral protein of the smaller subunit of cytosolic ribosome. In prokaryotes, it regulates the synthesis of ribosomal proteins by feedback inhibition of the α-operon gene expression, and it facilitates ribosomal RNA synthesis by direct binding to RNA polymerase. However, functional roles of S4 in eukaryotes are poorly understood, although its deficiency in humans is thought to produce Turner syndrome. We report here that wheat S4 is a cysteine protease capable of abrogating total protein synthesis in an actively translating cell-free system of rabbit reticulocytes. The translation-blocked medium, imaged by atomic force microscopy, scanning electron microscopy, and transmission electron microscopy, shows dispersed polysomes, and the disbanded polyribosome elements aggregate to form larger bodies. We also show that human embryonic kidney cells transfected with recombinant wheat S4 are unable to grow and proliferate. The mutant S4 protein, where the putative active site residue Cys 41 is replaced by a phenylalanine, can neither suppress protein synthesis nor arrest cell proliferation, suggesting that the observed phenomenon arises from the cysteine protease attribute of S4. The results also inspire many questions concerning in vivo significance of extraribosomal roles of eukaryotic S4 performed through its protease activity.

Cysteine protease attribute of eukaryotic ribosomal protein S4

BACKGROUND

Ribosomal proteins often carry out extraribosomal functions. The protein S4 from the smaller subunit of Escherichia coli, for instance, regulates self synthesis and acts as a transcription factor. In humans, S4 might be involved in Turner syndrome. Recent studies also associate many ribosomal proteins with malignancy, and cell death and survival. The list of extraribosomal functions of ribosomal proteins thus continues to grow.

METHODS

Enzymatic action of recombinant wheat S4 on fluorogenic peptide substrates Ac-XEXD↓-AFC (N-acetyl-residue-Glu-residue-Asp-7-amino-4-trifluoromethylcoumarin) and Z-FR↓-AMC (N-CBZ-Phe-Arg-aminomethylcoumarin) as well as proteins has been examined under a variety of solution conditions.

RESULTS

Eukaryotic ribosomal protein S4 is an endoprotease exhibiting all characteristics of cysteine proteases. The K(m) value for the cleavage of Z-FR↓-AMC by a cysteine mutant (C41F) is about 70-fold higher relative to that for the wild-type protein under identical conditions, implying that S4 is indeed a cysteine protease. Interestingly, activity responses of the S4 protein and caspases toward environmental parameters, including pH, temperature, ionic strength, and Mg(2+) and Zn(2+) concentrations, are quite similar. Respective kinetic constants for their cleavage action on Ac-LEHD↓-AFC are also similar. However, S4 cannot be a caspase, because unlike the latter it also hydrolyzes the cathepsin substrate Z-FR↓-AMC.

GENERAL SIGNIFICANCE

The eukaryotic S4 is a generic cysteine protease capable of hydrolyzing a broad spectrum of synthetic substrates and proteins. The enzyme attribute of eukaryotic ribosomal protein S4 is a new phenomenon. Its possible involvement in cell growth and proliferations are presented in the light of known extraribosomal roles of ribosomal proteins.

Cloning, Escherichia coli expression, purification, characterization, and enzyme assay of the ribosomal protein S4 from wheat seedlings (Triticum vulgare)

S4 is a paradigm of ribosomal proteins involved in multifarious activities both within and outside the ribosome. For a detailed biochemical and structural investigations of eukaryotic S4, the wheat S4 gene has been cloned and expressed in Escherichia coli, and the protein purified to a high degree of homogeneity. The 285-residue recombinant protein containing an N-terminal His(6) tag along with fourteen additional residues derived from the cloning vector is characterized by a molecular mass of 31981.24 Da. The actual sequence of 265 amino acids having a molecular mass of 29931 Da completely defines the primary structure of wheat S4. Homology modeling shows a bi-lobed protein topology arising from folding of the polypeptide into two domains, consistent with the fold topology of prokaryotic S4. The purified protein is stable and folded since it can be reversibly unfolded in guanidinium hydrochloride, and is capable of hydrolyzing cysteine protease-specific peptide-based fluorescence substrates, including Ac-DEVD-AFC (N-acetyl-Asp-Glu-Val-Asp-7-amino-4-trifluoromethylcoumarin) and Z-FR-AMC (N-CBZ-Phe-Arg-aminomethylcoumarin).

Tumor suppressor death-associated protein kinase is required for full IL-1β production

Interleukin-1β (IL-1β) is critical for inflammation and control of infection. The production of IL-1β depends on expression of pro-IL-1β and inflammasome component induced by inflammatory stimuli, followed by assembly of inflammasome to generate caspase-1 for cleavage of pro-IL-1β. Here we show that tumor suppressor death-associated protein kinase (DAPK) deficiency impaired IL-1β production in macrophages. Generation of tumor necrosis factor-α in macrophages, in contrast, was not affected by DAPK knockout. Two tiers of defects in IL-1β generation were found in DAPK-deficient macrophages: decreased pro-IL-1β induction by some stimuli and reduced caspase-1 activation by all inflammatory stimuli examined. With a normal NLRP3 induction in DAPK-deficient macrophages, the diminished caspase-1 generation is attributed to impaired inflammasome assembly. There is a direct binding of DAPK to NLRP3, suggesting an involvement of DAPK in inflammasome formation. We further illustrated that the formation of NLRP3 inflammasome in situ induced by inflammatory signals was impaired by DAPK deficiency. Taken together, our results identify DAPK as a molecule required for full production of IL-1β and functional assembly of the NLRP3 inflammasome. In addition, DAPK knockout reduced uric acid crystal-triggered peritonitis, suggesting that DAPK may serve as a target in the treatment of IL-1β-associated autoinflammatory diseases.

The tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder that results from mutations in the TSC1 or TSC2 genes and is associated with hamartoma formation in multiple organ systems. The neurological manifestations of TSC are particularly challenging and include infantile spasms, intractable epilepsy, cognitive disabilities, and autism. Progress over the past 15 years has demonstrated that the TSC1 or TSC2 encoded proteins modulate cell function via the mTOR signaling cascade and serve as keystones in regulating cell growth and proliferation. The mTOR pathway provides an intersection for an intricate network of protein cascades that respond to cellular nutrition, energy levels, and growth-factor stimulation. In the brain, TSC1 and TSC2 have been implicated in cell body size, dendritic arborization, axonal outgrowth and targeting, neuronal migration, cortical lamination, and spine formation. Antagonism of the mTOR pathway with rapamycin and related compounds may provide new therapeutic options for TSC patients.

Lethal weapons: DAP-kinase, autophagy and cell death: DAP-kinase regulates autophagy

Recently, DAP-kinase was identified as one of the essential regulators of autophagy, activated by signals such as cytokines and ER stress. DAP-kinase is a tumor suppressor that mediates several cell death pathways, such as apoptosis and programmed necrosis. Likewise, functional studies suggest that DAP-kinase may direct autophagy specifically towards autophagic cell death. Several recent studies have mapped DAP-kinase function to distinct stages in autophagy signaling. These include the Beclin-1/phosphatidylinositol 3-kinase (PI(3)K) complex, which is necessary for autophagosome formation, and an interaction with the LC3 binding protein, MAP1B, which may regulate vesicle trafficking. This review will summarize the functional and mechanistic studies that have linked DAP-kinase to the regulation of autophagy in general, and autophagic cell death, in particular.

Targeting protein kinases in central nervous system disorders

Protein kinases are a growing drug target class in disorders in peripheral tissues, but the development of kinase-targeted therapies for central nervous system (CNS) diseases remains a challenge, largely owing to issues associated specifically with CNS drug discovery. However, several candidate therapeutics that target CNS protein kinases are now in various stages of preclinical and clinical development. We review candidate compounds and discuss selected CNS protein kinases that are emerging as important therapeutic targets. In addition, we analyse trends in small-molecule properties that correlate with key challenges in CNS drug discovery, such as blood-brain barrier penetrance and cytochrome P450-mediated metabolism, and discuss the potential of future approaches that will integrate molecular-fragment expansion with pharmacoinformatics to address these challenges.

RETRACTED: Identification of phosphorylated p38 as a novel DAPK-interacting partner during TNFalpha-induced apoptosis in colorectal tumor cells

Death-associated protein kinase (DAPK) is a serine/threonine kinase that contributes to pro-apoptotic signaling on cytokine exposure. The role of DAPK in macrophage-associated tumor cell death is currently unknown. Recently, we suggested a new function for DAPK in the induction of apoptosis during the interaction between colorectal tumor cells and tumor-associated macrophages. Using a cell-culture model with conditioned supernatants of differentiated/activated macrophages (U937) and human HCT116 colorectal tumor cells, we replicated DAPK-associated tumor cell death; this model likely reflects the in vivo tumor setting. In this study, we show that tumor necrosis factor-alpha exposure under conditions of macrophage activation induced DAPK-dependent apoptosis in the colorectal tumor cell line HCT116. Simultaneously, early phosphorylation of p38 mitogen-activated protein kinase (phospho-p38) was observed. We identified the phospho-p38 mitogen-activated protein kinase as a novel interacting protein of DAPK in tumor necrosis factor-alpha-induced apoptosis. The general relevance of this interaction was verified in two colorectal cell lines without functional p53 (ie, HCT116 p53(-/-) and HT29 mutant) and in human colon cancer and ulcerative colitis tissues. Supernatants of freshly isolated human macrophages were also able to induce DAPK and phospho-p38. Our findings highlight the mechanisms that underlie DAPK regulation in tumor cell death evoked by immune cells.

Mice deficient in ribosomal protein S6 phosphorylation suffer from muscle weakness that reflects a growth defect and energy deficit

Background

Mice, whose ribosomal protein S6 cannot be phosphorylated due to replacement of all five phosphorylatable serine residues by alanines (rpS6^P−/−), are viable and fertile. However, phenotypic characterization of these mice and embryo fibroblasts derived from them, has established the role of these modifications in the regulation of the size of several cell types, as well as pancreatic β-cell function and glucose homeostasis. A relatively passive behavior of these mice has raised the possibility that they suffer from muscle weakness, which has, indeed, been confirmed by a variety of physical performance tests.

Methodology/Principal Findings

A large variety of experimental methodologies, including morphometric measurements of histological preparations, high throughput proteomic analysis, positron emission tomography (PET) and numerous biochemical assays, were used in an attempt to establish the mechanism underlying the relative weakness of rpS6^P−/− muscles. Collectively, these experiments have demonstrated that the physical inferiority appears to result from two defects: a) a decrease in total muscle mass that reflects impaired growth, rather than aberrant differentiation of myofibers, as well as a diminished abundance of contractile proteins; and b) a reduced content of ATP and phosphocreatine, two readily available energy sources. The abundance of three mitochondrial proteins has been shown to diminish in the knockin mouse. However, the apparent energy deficiency in this genotype does not result from a lower mitochondrial mass or compromised activity of enzymes of the oxidative phosphorylation, nor does it reflect a decline in insulin-dependent glucose uptake, or diminution in storage of glycogen or triacylglycerol (TG) in the muscle.

Conclusions/Significance

This study establishes rpS6 phosphorylation as a determinant of muscle strength through its role in regulation of myofiber growth and energy content. Interestingly, a similar role has been assigned for ribosomal protein S6 kinase 1, even though it regulates myoblast growth in an rpS6 phosphorylation-independent fashion.

Structural insight into nucleotide recognition by human death-associated protein kinase

The crystal structures of DAPK–ADP–Mg²⁺ and DAPK–AMP-PNP–Mg²⁺ complexes were determined at 1.85 and 2.00 Å resolution, respectively. Comparison of the two nucleotide-bound states with apo DAPK revealed localized changes in the glycine-rich loop region that were indicative of a transition from a more open state to a more closed state on binding of the nucleotide substrate and to an intermediate state with the bound nucleotide product.

Death-associated protein kinase (DAPK) is a member of the Ca²⁺/calmodulin-regulated family of serine/threonine protein kinases. The role of the kinase activity of DAPK in eukaryotic cell apoptosis and the ability of bioavailable DAPK inhibitors to rescue neuronal death after brain injury have made it a drug-discovery target for neurodegenerative disorders. In order to understand the recognition of nucleotides by DAPK and to gain insight into DAPK catalysis, the crystal structure of human DAPK was solved in complex with ADP and Mg²⁺ at 1.85 Å resolution. ADP is a product of the kinase reaction and product release is considered to be the rate-limiting step of protein kinase catalytic cycles. The structure of DAPK–ADP–Mg²⁺ was compared with a newly determined DAPK–AMP-PNP–Mg²⁺ structure and the previously determined apo DAPK structure (PDB code 1jks). The comparison shows that nucleotide-induced changes are localized to the glycine-rich loop region of DAPK.

Peptide combinatorial libraries identify TSC2 as a death-associated protein kinase (DAPK) death domain-binding protein and reveal a stimulatory role for DAPK in mTORC1 signaling

Death-associated protein kinase (DAPK) is a multidomain enzyme that plays a central role in autophagic and apoptotic signaling, although the protein-protein interactions regulating DAPK functions are not well defined. Peptide aptamer libraries were used to identify the tumor suppressor protein tuberin (TSC2) as a novel DAPK death domain-binding protein, and we evaluated whether DAPK is a positive or negative effector of the TSC2-regulated mammalian target of rapamycin (mTORC1) signaling pathway. Binding studies using death domain miniproteins in vitro and deletion analysis in vivo determined that the death domain of DAPK is the major site for the interaction with TSC2. Recombinant DAPK phosphorylates TSC2 in vitro, and DAPK kinase activity is stimulated by growth factor signaling. Transfection of DAPK promotes phosphorylation of TSC2 in vivo, whereas short interfering RNA-mediated attenuation of DAPK reduces growth factor-stimulated phosphorylation of TSC2. DAPK-dependent phosphorylation leads to TSC1-TSC2 complex dissociation, and consequently manipulation of DAPK by transfection or short interfering RNA demonstrated that DAPK is a positive regulator of mTORC1 in response to growth factor activation. Epistatic studies suggest that DAPK functions downstream from the RAS-MEK-ERK and phosphatidylinositol 3-kinase-AKT growth factor signaling pathways. DAPK(+/-) mouse embryo fibroblasts have attenuated mTORC1 signaling compared with DAPK+/+ counterparts, and overexpression of DAPK in DAPK(+/-) MEFs stimulates mTORC1 activity. These data uncover a novel interaction between DAPK and TSC2 proteins that has revealed a positive link between growth factor stimulation of DAPK and mTORC1 signaling that may ultimately affect autophagy, cell survival, or apoptosis.

A high throughput proteomics screen identifies novel substrates of death-associated protein kinase

Death-associated protein kinase (DAPk) is a Ser/Thr kinase whose activity is necessary for different cell death phenotypes. Although its contribution to cell death is well established, only a handful of direct substrates have been identified; these do not fully account for the multiple cellular effects of DAPk. To identify such substrates on a large scale, we developed an in vitro, unbiased, proteomics-based assay to search for novel DAPk substrates. Biochemical fractionation and mass spectrometric analysis were used to purify and identify several potential substrates from HeLa cell lysate. Here we report the identification of two such candidate substrates, the ribosomal protein L5 and MCM3, a replication licensing factor. Although L5 proved to be a weak substrate, MCM3 was efficiently and specifically phosphorylated by DAPk on a unique site, Ser160. Significantly DAPk phosphorylated this site in vivo upon overexpression in 293T cells. Activation of endogenous DAPk by increasing intracellular Ca2+ also led to increased phosphorylation of MCM3. Importantly short hairpin RNA-mediated knockdown of endogenous DAPk blocked both basal phosphorylation and Ca2+-induced phosphorylation, indicating that DAPk is both necessary and sufficient for MCM3 Ser160 phosphorylation in vivo. Identification of MCM3 as an in vivo DAPk substrate indicates the usefulness of this approach for identification of physiologically relevant substrates that may shed light on novel functions of the kinase.

Physiological roles of ribosomal protein S6: one of its kind

The phosphorylation of ribosomal protein S6 (rpS6), which occurs in response to a wide variety of stimuli on five evolutionarily conserved serine residues, has attracted much attention since its discovery more than three decades ago. However, despite a large body of information on the respective kinases and the signal transduction pathways, the role of this phosphorylation remained obscure. It is only recent that targeting the genes encoding rpS6, the phosphorylatable serine residues or the respective kinases that the unique role of rpS6 and its posttranslational modification have started to be elucidated. This review focuses primarily on the critical role of rpS6 for mouse development, the pathways that transduce various signals into rpS6 phosphorylation, and the physiological functions of this modification. The mechanism(s) underlying the diverse effects of rpS6 phosphorylation on cellular and organismal physiology has yet to be determined. However, a model emerging from the currently available data suggests that rpS6 phosphorylation operates, at least partly, by counteracting positive signals simultaneously induced by rpS6 kinase, and thus might be involved in fine-tuning of the cellular response to these signals.