Overall Rate Of Protein Synthesis Research Articles

The Key Parameters that Govern Translation Efficiency.

Translation of mRNA into protein is a fundamental yet complex biological process with multiple factors that can potentially affect its efficiency. Here, we study a stochastic model describing the traffic flow of ribosomes along the mRNA and identify the key parameters that govern the overall rate of protein synthesis, sensitivity to initiation rate changes, and efficiency of ribosome usage. By analyzing a continuum limit of the model, we obtain closed-form expressions for stationary currents and ribosomal densities, which agree well with Monte Carlo simulations. Furthermore, we completely characterize the phase transitions in the system, and by applying our theoretical results, we formulate design principles that detail how to tune the key parameters we identified to optimize translation efficiency. Using ribosome profiling data from S.cerevisiae, we show that its translation system is generally consistent with these principles. Our theoretical results have implications for evolutionary biology, as well as for synthetic biology.

Involvement of released sphingosine 1-phosphate/sphingosine 1-phosphate receptor axis in skeletal muscle atrophy

Skeletal muscle (SkM) atrophy is caused by several and heterogeneous conditions, such as cancer, neuromuscular disorders and aging. In most types of SkM atrophy overall rates of protein synthesis are suppressed, protein degradation is consistently elevated and atrogenes, such as the ubiquitin ligase Atrogin-1/MAFbx, are up-regulated. The molecular regulators of SkM waste are multiple and only in part known.Sphingolipids represent a class of bioactive molecules capable of modulating the destiny of many cell types, including SkM cells. In particular, we and others have shown that sphingosine 1phosphate (S1P), formed by sphingosine kinase (SphK), is able to act as trophic and morphogenic factor in myoblasts.Here, we report the first evidence that the atrophic phenotype observed in both muscle obtained from mice bearing the C26 adenocarcinoma and C2C12 myotubes treated with dexamethasone was characterized by reduced levels of active phospho-SphK1. The importance of SphK1 activity is also confirmed by the specific pharmacological inhibition of SphK1 able to increase Atrogin-1/MAFbx expression and reduce myotube size and myonuclei number. Furthermore, we found that SkM atrophy was accomplished by significant increase of S1P transporter Spns2 and in changes in the pattern of S1P receptor (S1PRs) subtype expression paralleled by increased Atrogin-1/MAFbx expression, suggesting a role for the released S1P and of specific S1PR-mediated signaling pathways in the control of the ubiquitin ligase. Altogether, these findings provide the first evidence that SphK1/released S1P/S1PR axis acts as a molecular regulator of SkM atrophy, thereby representing a new possible target for therapy in many patho-physiological conditions.

Translational Control in the Brain in Health and Disease.

Translational control in neurons is crucially required for long-lasting changes in synaptic function and memory storage. The importance of protein synthesis control to brain processes is underscored by the large number of neurological disorders in which translation rates are perturbed, such as autism and neurodegenerative disorders. Here we review the general principles of neuronal translation, focusing on the particular relevance of several key regulators of nervous system translation, including eukaryotic initiation factor 2α (eIF2α), the mechanistic (or mammalian) target of rapamycin complex 1 (mTORC1), and the eukaryotic elongation factor 2 (eEF2). These pathways regulate the overall rate of protein synthesis in neurons and have selective effects on the translation of specific messenger RNAs (mRNAs). The importance of these general and specific translational control mechanisms is considered in the normal functioning of the nervous system, particularly during synaptic plasticity underlying memory, and in the context of neurological disorders.

TRNA tracking for direct measurements of protein synthesis kinetics in live cells.

Our ability to directly relate results from test tube biochemical experiments to the kinetics in living cells is very limited. Here we present experimental and analytical tools to directly study the kinetics of fast biochemical reactions in live cells. Dye-labeled molecules are electroporated into bacterial cells and tracked using super-resolved single-molecule microscopy. Trajectories are analyzed by machine-learning algorithms to directly monitor transitions between bound and free states. In particular, we measure the dwell-time of tRNAs on ribosomes, and hence achieve direct measurements of translation rates inside living cells at codon resolution. We find elongation rates with tRNAPhe in perfect agreement with previous indirect estimates, and that once fMet-tRNAfMet has bound to the 30S ribosomal subunit, initiation of translation is surprisingly fast and does not limit the overall rate of protein synthesis. The experimental and analytical tools for direct kinetics measurements in live cells have applications far beyond bacterial protein synthesis.

Molecular Mechanisms of Signal Transduction in Heme-Containing Sensor Proteins

Heme (the iron protoporphyrin IX complex) is one of the most important and extensively characterized cofactors required for proper functioning of many proteins and enzymes. Heme is involved in O2 storage (myoglobin), O2 transfer (hemoglobin), oxygenation (hydroxylation) reactions (cytochromes P450, NO synthase), electron transfer (cytochrome c) and many other functions. In prototype heme-containing proteins, heme itself forms the catalytic or functional center. These proteins are usually composed of one compact structure (one domain). However, recently it was found that heme itself can regulate many physiological processes in many organisms including humans. The novel role of heme is mediated by a group of hemoproteins called heme-containing sensor proteins. Generally, it is known that the heme can act as (i) a preliminary signal of heme sensor proteins and (ii) a gas sensing site of heme-based gas sensor proteins. Conceptually, these proteins are always composed of at least two domains: one is a sensor domain and the other is a functional domain. However, the structure-function relationship and mechanisms of communication between these domains have not been fully understood. Therefore, we selected model systems from each subfamily of heme-containing sensor proteins - for example, a heme regulated inhibitor, HRI (subfamily i) and a globin-coupled oxygen sensor histidine kinase, AfGcHK (subfamily ii) - in order to study the signal transduction in the novel group of hemoproteins (i.e. heme-containing sensor proteins). The AfGcHK is a part of the two-component signal transduction system from the soil bacterium Anaeromyxobacter sp. Fw109-5. Once the oxygen molecule (as the first signal) binds to the heme iron complex in the sensor domain of AfGcHK, the functional domain is stimulated, leading to autophosphorylation at a conserved His residue in the functional domain. The phosphate group of phosphorylated AfGcHK is then transferred to the cognate response regulator. The HRI seems to be one of the most important existing heme sensor proteins for eukaryote survival in response to cell emergency states involved in clinical conditions important for human health. The functional domain of HRI exhibits the kinase activity towards its substrate - eukaryotic initiation factor 2α (eIF2α) depending on the heme availability. The enzyme is active in the absence of heme. The heme binding to the HRI protein induces a global structural change, leading to the heme-induced catalytic inhibition. This sensor protein is important for regulation of eukaryotic proteosynthesis. Eukaryotic cells decrease their overall rates of protein synthesis for survival in response to a variety of stress conditions, such as shortage of amino acids, UV light illumination, viral infection, accumulation of denatured proteins, and shortage of heme. The decrease (inhibition) in protein synthesis is caused by phosphorylation of eIF2α at Ser-51 by eIF2α kinases that respond specifically to stress. HRI is a member of the eIF2α kinase family that controls globin synthesis in response to the heme concentration in reticulocytes to balance the molar ratio of heme and globin. In addition to globin, HRI controls the synthesis of tryptophan 2,3-dioxygenase and cytochrome P450 2B in liver upon acute porphyria. Several biochemical approaches were utilized in order to study the signal transduction between the sensor and function domains of the model systems: (i) enzyme kinetic study, (ii) hydrogen/deuterium exchange experiments associated with mass spectrometry, (iii) X-ray crystallography and others. Overall our results indicated that the coordination and oxidation state of the sensor domain heme iron affect the enzyme’s catalytic activity of the function domain. The possible contact area between sensor and function domains was also reveled. All these data together will be discussed in order to illustrate the mechanism of signal transduction in the heme-containing sensor proteins.

The sensitivity of the yeast, Saccharomyces cerevisiae, to acetic acid is influenced by DOM34 and RPL36A.

The presence of acetic acid during industrial alcohol fermentation reduces the yield of fermentation by imposing additional stress on the yeast cells. The biology of cellular responses to stress has been a subject of vigorous investigations. Although much has been learned, details of some of these responses remain poorly understood. Members of heat shock chaperone HSP proteins have been linked to acetic acid and heat shock stress responses in yeast. Both acetic acid and heat shock have been identified to trigger different cellular responses including reduction of global protein synthesis and induction of programmed cell death. Yeast HSC82 and HSP82 code for two important heat shock proteins that together account for 1–2% of total cellular proteins. Both proteins have been linked to responses to acetic acid and heat shock. In contrast to the overall rate of protein synthesis which is reduced, the expression of HSC82 and HSP82 is induced in response to acetic acid stress. In the current study we identified two yeast genes DOM34 and RPL36A that are linked to acetic acid and heat shock sensitivity. We investigated the influence of these genes on the expression of HSP proteins. Our observations suggest that Dom34 and RPL36A influence translation in a CAP-independent manner.

A ribosomal strategy for magnesium deficiency

Under low-magnesium stress, bacteria suppress ribosome synthesis.

Gene Regulation in Developing Chloroplasts Disentangled.

Gene Regulation in Developing Chloroplasts Disentangled.

Phosphorylation of eIF2α on Threonine 169 is not required for Trypanosoma brucei cell cycle arrest during differentiation

The trypanosome life cycle consists of a series of developmental forms each adapted to an environment in the relevant insect and/or mammalian host. The differentiation process from the mammalian bloodstream form to the insect-midgut procyclic form in Trypanosoma brucei occurs in two steps in vivo. First proliferating ‘slender' bloodstream forms differentiate to non-dividing ‘stumpy' forms arrested in G1. Second, in response to environmental cues, stumpy bloodstream forms re-enter the cell cycle and start to proliferate as procyclic forms after a lag during which both cell morphology and gene expression are modified. Nearly all arrested cells have lower rates of protein synthesis when compared to the proliferating equivalent. In eukaryotes, one mechanism used to regulate the overall rate of protein synthesis involves phosphorylation of the alpha subunit of initiation factor eIF2 (eIF2α). The effect of eIF2α phosphorylation is to prevent the action of eIF2B, the guanine nucleotide exchange factor that activates eIF2 for the next rounds of initiation. To investigate the role of the phosphorylation of eIF2α in the life cycle of T. brucei, a cell line was made with a single eIF2α gene that contained the phosphorylation site, threonine 169, mutated to alanine. These cells were capable of differentiating from proliferating bloodstream form cells into arrested stumpy forms in mice and into procyclic forms in vitro and in tsetse flies. These results indicate that translation attenuation mediated by the phosphorylation of eIF2α on threonine 169 is not necessary for the cell cycle arrest associated with these differentiation processes.

An unbiased proteomics approach to identify human cytomegalovirus RNA-associated proteins

Post-transcriptional events regulate herpesvirus gene expression, yet few herpesvirus RNA-binding proteins have been identified. We used an unbiased approach coupling oligo(dT) affinity capture with proteomics to identify viral RNA-associated proteins during infection. Using this approach, we identified and confirmed changes in the abundance or activity of two host RNA-associated proteins, DHX9 and DDX3, in cells infected with human cytomegalovirus (HCMV). We also identified and confirmed previously unreported activities for the HCMV US22 and pp71 proteins as RNA-associated viral proteins and confirmed that a known viral RNA-binding protein, pTRS1, associates with RNA in infected cells. Further, we found that HCMV pp71 co-sedimented with polysomes, associated with host and viral RNAs, and stimulated the overall rate of protein synthesis. These results demonstrate that oligo(dT) affinity capture coupled with proteomics provides a rapid and straightforward means to identify RNA-associated viral proteins during infection that may participate in the post-transcriptional control of gene expression.

Mechanisms regulating skeletal muscle growth and atrophy

Skeletal muscle mass increases during postnatal development through a process of hypertrophy, i.e. enlargement of individual muscle fibers, and a similar process may be induced in adult skeletal muscle in response to contractile activity, such as strength exercise, and specific hormones, such as androgens and β-adrenergic agonists. Muscle hypertrophy occurs when the overall rates of protein synthesis exceed the rates of protein degradation. Two major signaling pathways control protein synthesis, the IGF1-Akt-mTOR pathway, acting as a positive regulator, and the myostatin-Smad2/3 pathway, acting as a negative regulator, and additional pathways have recently been identified. Proliferation and fusion of satellite cells, leading to an increase in the number of myonuclei, may also contribute to muscle growth during early but not late stages of postnatal development and in some forms of muscle hypertrophy in the adult. Muscle atrophy occurs when protein degradation rates exceed protein synthesis, and may be induced in adult skeletal muscle in a variety of conditions, including starvation, denervation, cancer cachexia, heart failure and aging. Two major protein degradation pathways, the proteasomal and the autophagic-lysosomal pathways, are activated during muscle atrophy and variably contribute to the loss of muscle mass. These pathways involve a variety of atrophy-related genes or atrogenes, which are controlled by specific transcription factors, such as FoxO3, which is negatively regulated by Akt, and NF-κB, which is activated by inflammatory cytokines.

Afferent regulation of chicken auditory brainstem neurons: Rapid changes in phosphorylation of elongation factor 2

The relationships between protein synthesis and neuronal survival are poorly understood. In chicken nucleus magnocellularis (NM), significant alterations in overall protein synthesis precede neuronal death induced by deprivation of excitatory afferent activity. Previously we demonstrated an initial reduction in the overall rate of protein synthesis in all deprived NM neurons, followed by quick recovery (starting at 6 hours) in some, but not all, neurons. Neurons with recovered protein synthesis ultimately survive, whereas others become "ghost" cells (no detectable Nissl substance) at 12-24 hours and die within 48 hours. To explore the mechanisms underlying this differential influence of afferent input on protein synthesis and cell survival, the current study investigates the involvement of eukaryotic translation elongation factor 2 (eEF2), the phosphorylation of which reduces overall protein synthesis. Using immunocytochemistry for either total or phosphorylated eEF2 (p-eEF2), we found significant reductions in the level of phosphorylated, but not total, eEF2 in NM neurons as early as 0.5-1 hour following cochlea removal. Unexpectedly, neurons with low levels of p-eEF2 show reduced protein synthesis at 6 hours, indicated by a marker for active ribosomes. At 12 hours, all "ghost" cells exhibited little or no p-eEF2 staining, although not every neuron with a comparable low level of p-eEF2 was a "ghost" cell. These observations demonstrate that a reduced level of p-eEF2 is not responsible for immediate responses (including reduced overall protein synthesis) of a neuron to compromised afferent input but may impair the neuron's ability to initiate recovery signaling for survival and make the neuron more vulnerable to death.

Functional Proteomic Analysis of Long-term Growth Factor Stimulation and Receptor Tyrosine Kinase Coactivation in Swiss 3T3 Fibroblasts

In Swiss 3T3 fibroblasts, long-term stimulation with PDGF, but not insulin-like growth factor 1 (IGF-1) or EGF, results in the establishment of an elongated migratory phenotype, characterized by the formation of retractile dendritic protrusions and absence of actin stress fibers and focal adhesion complexes. To identify receptor tyrosine kinase-specific reorganization of the Swiss 3T3 proteome during phenotypic differentiation, we compared changes in the pattern of protein synthesis and phosphorylation during long-term exposure to PDGF, IGF-1, EGF, and their combinations using 2DE-based proteomics after (35)S- and (33)P-metabolic labeling. One hundred and five differentially regulated proteins were identified by mass spectrometry and some of these extensively validated. PDGF stimulation produced the highest overall rate of protein synthesis at any given time and induced the most sustained phospho-signaling. Simultaneous activation with two or three of the growth factors revealed both synergistic and antagonistic effects on protein synthesis and expression levels with PDGF showing dominance over both IGF-1 and EGF in generating distinct proteome compositions. Using signaling pathway inhibitors, PI3K was identified as an early site for signal diversification, with sustained activity of the PI3K/AKT pathway critical for regulating late protein synthesis and phosphorylation of target proteins and required for maintaining the PDGF-dependent motile phenotype. Several proteins were identified with novel PI3K/Akt-dependent synthesis and phosphorylations including eEF2, PRS7, RACK-1, acidic calponin, NAP1L1, Hsp73, and fascin. The data also reveal induction/suppression of key F-actin and actomyosin regulators and chaperonins that enable PDGFR to direct the assembly of a motile cytoskeleton, despite simultaneous antagonistic signaling activities. Together, the study demonstrates that long-term exposure to different growth factors results in receptor tyrosine kinase-specific regulation of relatively small subproteomes, and implies that the strength and longevity of receptor tyrosine kinase-specific signals are critical in defining the composition and functional activity of the resulting proteome.

Translation Regulation as a Therapeutic Target in Cancer

Protein synthesis is a vital cellular process that regulates growth and metabolism. It is controlled via signaling networks in response to environmental changes, including the presence of nutrients, mitogens, or starvation. The phosphorylation state of proteins involved in translation initiation is a limiting factor that regulates the formation or activity of translational complexes. In cancer cells, hyperactivated signaling pathways influence translation, allowing uncontrolled growth and survival. In addition, several components of translation initiation have been found to be mutated, posttranslationally modified, or differentially expressed, and some act as oncogenes in cancer cells. Translational alterations can increase the overall rate of protein synthesis as well as activate regulatory mechanisms leading to the translation of specific messenger RNAs for proteins that promote cancer progression and survival. Many recent studies investigating such mechanisms have produced ideas for therapeutic intervention. This review describes altered mechanisms of protein synthesis in human cancers and discusses therapeutic approaches based on the targeting of translation.

Atrogin1/MAFbx

See related article, pages 161–171 Proteins in cardiac and skeletal muscle cells, as in other cells, are continually being synthesized and degraded back to their constituent amino acids. Protein turnover in cardiac myocytes utilizes the same proteolytic systems as other eukaryotic cells: the ubiquitin-proteasome pathway, which catalyzes the rapid degradation of misfolded and regulatory proteins, and the lysosomal-autophagic system, which degrades organelles and aggregated proteins. These systems are of major importance in determining cardiac size and functional capacity. The overall rates of proteolysis in a cell and the degradation of individual components are precisely regulated. For example, cardiac hypertrophy occurs when overall rates of protein synthesis exceed overall rates of protein degradation; conversely cells decrease in mass when degradation rates exceed synthesis, as occurs in skeletal muscle with disuse, fasting, and many systemic diseases, including cardiac failure. In addition, the levels of individual proteins, whether they are enzymes, transcription factors, or components of the sarcomere, are determined in large part by their rates of ubiquitin-mediated degradation. In this issue of Circulation Research , Usui et al1 demonstrate how a single ubiquitination enzyme can have major effects on cardiac growth and function. In this pathway, proteins are targeted for degradation by the 26S proteasome by covalent attachment of a chain of ubiquitin molecules. This multistep pathway first involves the activation of the small protein ubiquitin by an enzyme, E1, which then transfers the highly reactive ubiquitin to one of the cell's many ubiquitin-conjugating enzymes, E2s. A ubiquitin protein ligase, E3, then binds the protein substrate and the ubiquitin-E2 and catalyzes the formation of a chain of ubiquitins on the protein. Different E2-E3 pairs function in the degradation of different proteins, and the specificity of the E3s for specific groups of proteins provides exquisite selectivity to this degradation process. The content …

Noncytotoxic Inhibition of Viral Infection through eIF4F-Independent Suppression of Translation by 4EGi-1

The eukaryotic initiation factor eIF4F recruits ribosomes to capped mRNAs while eIF2 mediates start codon recognition to initiate protein synthesis. Increasing interest in targeting translation to suppress tumor growth has led to the development of new classes of inhibitors, including 4EGi-1, which disrupts eIF4F complexes. However, the full effects of this inhibitor and its potential uses in the treatment of other disease states remain unclear. Here, we show that overall rates of protein synthesis in primary human cells were affected only modestly by eIF4F disruption using the mTOR inhibitor Torin1, yet were highly sensitive to 4EGi-1. Translational suppression occurred even at concentrations of 4EGi-1 that were below those required to significantly alter eIF4F levels but were instead found to increase the association of ribosomal complexes containing inactive eIF2α. Although highly stable in culture, the effects of 4EGi-1 on both cellular protein synthesis and ribosome association were readily reversible upon inhibitor removal. In addition, despite potently inhibiting translation, prolonged exposure to 4EGi-1 had only modest effects on cell morphology and protein abundance without affecting viability or stress tolerance to any significant degree, although differential effects on heat shock protein (hsp) expression highlighted distinct 4EGi-1-sensitive modes of hsp induction. In contrast, 4EGi-1 potently suppressed poxvirus replication as well as both reactivation and lytic phases of herpesvirus infection. These findings identify a novel way in which 4EGi-1 affects the host cell's protein synthesis machinery and demonstrate its potential as a noncytotoxic inhibitor of diverse forms of viral infection.

JunB transcription factor maintains skeletal muscle mass and promotes hypertrophy

The size of skeletal muscle cells is precisely regulated by intracellular signaling networks that determine the balance between overall rates of protein synthesis and degradation. Myofiber growth and protein synthesis are stimulated by the IGF-1/Akt/mammalian target of rapamycin (mTOR) pathway. In this study, we show that the transcription factor JunB is also a major determinant of whether adult muscles grow or atrophy. We found that in atrophying myotubes, JunB is excluded from the nucleus and that decreasing JunB expression by RNA interference in adult muscles causes atrophy. Furthermore, JunB overexpression induces hypertrophy without affecting satellite cell proliferation and stimulated protein synthesis independently of the Akt/mTOR pathway. When JunB is transfected into denervated muscles, fiber atrophy is prevented. JunB blocks FoxO3 binding to atrogin-1 and MuRF-1 promoters and thus reduces protein breakdown. Therefore, JunB is important not only in dividing populations but also in adult muscle, where it is required for the maintenance of muscle size and can induce rapid hypertrophy and block atrophy.

A Novel Glycerophosphodiester Phosphodiesterase, GDE5, Controls Skeletal Muscle Development via a Non-enzymatic Mechanism

Mammalian glycerophosphodiester phosphodiesterases (GP-PDEs) have been identified recently and shown to be implicated in several physiological functions. This study isolated a novel GP-PDE, GDE5, and showed that GDE5 selectively hydrolyzes glycerophosphocholine (GroPCho) and controls skeletal muscle development. We show that GDE5 expression was reduced in atrophied skeletal muscles in mice and that decreasing GDE5 abundance promoted myoblastic differentiation, suggesting that decreased GDE5 expression has a counter-regulatory effect on the progression of skeletal muscle atrophy. Forced expression of full-length GDE5 in cultured myoblasts suppressed myogenic differentiation. Unexpectedly, a truncated GDE5 construct (GDE5DeltaC471), which contained a GP-PDE sequence identified in other GP-PDEs but lacked GroPCho phosphodiesterase activity, showed a similar inhibitory effect. Furthermore, transgenic mice specifically expressing GDE5DeltaC471 in skeletal muscle showed less skeletal muscle mass, especially type II fiber-rich muscle. These results indicate that GDE5 negatively regulates skeletal muscle development even without GroPCho phosphodiesterase activity, providing novel insight into the biological significance of mammalian GP-PDE function in a non-enzymatic mechanism.

Methylglyoxal activates Gcn2 to phosphorylate eIF2α independently of the TOR pathway in Saccharomyces cerevisiae

Methylglyoxal is a ubiquitous 2-oxoaldehyde derived from glycolysis. Previously, we have reported that methylglyoxal attenuates the rate of overall protein synthesis in Saccharomyces cerevisiae through phosphorylation of the alpha subunit of translation initiation factor 2 (eIF2alpha) in a Gcn2-dependent manner. Phosphorylation of eIF2alpha impedes the formation of a translation initiation complex, and subsequently, overall protein synthesis is reduced. Uncharged tRNA plays an important role in the activation of Gcn2, although we found that MG treatment did not elevate the levels of uncharged tRNA. Rapamycin, a potent inhibitor of TOR kinase, is known to induce phosphorylation of eIF2alpha without affecting the levels of uncharged tRNA. We determined the correlation between methylglyoxal and TOR kinase activity and found that phosphorylation of eIF2alpha by methylglyoxal occurred independently of the target of rapamycin (TOR) pathway.

Control of Glycolytic Flux by AMPK and p53-mediated Signaling Pathways in Tumor Cells Grown at Low pH

Introduction: Tumor cells grow in nutrient and oxygen deprived microenvironments and adapt to the suboptimal growth conditions by altering metabolic pathways. This adaptation process characteristically results in a tumor phenotype that displays upregulated Hif-1α anaerobic glycolysis, chronic acidification, reduced rate of overall protein synthesis, lower rate of cell proliferation and aggressive invasive characteristics. Most transplantable tumors exhibit a pHe of 6.7- 7.0; the DB-1 melanoma xenografts used here have a pHe=6.7. Understanding tumor cell reaction to the microenvironment is a critical factor in predicting the tumor response to radiotherapy. The glucose regulatory molecule, 6-Phosphofructo-2-Kinase/Fructose-2,6- Biphosphatase Isoform-3 (PFKFB3), is a bifunctional enzyme central to glycolytic flux and downstream of the metabolic stress sensor AMP-activated protein kinase (AMPK), which we show activates an isoform of phosphofructokinase (PFK-2). Radiation Research Society (RRS) 8th Annual Meeting September 25-29, Maui, HI