@article {6591, title = {The PP2A-like phosphatase Ppg1 mediates assembly of the Far complex to balance gluconeogenic outputs and enables adaptation to glucose depletion.}, journal = {PLoS Genet}, volume = {20}, year = {2024}, month = {2024 Mar}, pages = {e1011202}, abstract = {

To sustain growth in changing nutrient conditions, cells reorganize outputs of metabolic networks and appropriately reallocate resources. Signaling by reversible protein phosphorylation can control such metabolic adaptations. In contrast to kinases, the functions of phosphatases that enable metabolic adaptation as glucose depletes are poorly studied. Using a Saccharomyces cerevisiae deletion screen, we identified the PP2A-like phosphatase Ppg1 as required for appropriate carbon allocations towards gluconeogenic outputs-trehalose, glycogen, UDP-glucose, UDP-GlcNAc-after glucose depletion. This Ppg1 function is mediated via regulation of the assembly of the Far complex-a multi-subunit complex that tethers to the ER and mitochondrial outer membranes forming localized signaling hubs. The Far complex assembly is Ppg1 catalytic activity-dependent. Ppg1 regulates the phosphorylation status of multiple ser/thr residues on Far11 to enable the proper assembly of the Far complex. The assembled Far complex is required to maintain gluconeogenic outputs after glucose depletion. Glucose in turn regulates Far complex amounts. This Ppg1-mediated Far complex assembly, and Ppg1-Far complex dependent control of gluconeogenic outputs enables adaptive growth under glucose depletion. Our study illustrates how protein dephosphorylation is required for the assembly of a multi-protein scaffold present in localized cytosolic pools, to thereby alter gluconeogenic flux and enable cells to metabolically adapt to nutrient fluctuations.

}, keywords = {Glucose, Phosphorylation, Protein Phosphatase 2, Saccharomyces cerevisiae, Saccharomyces cerevisiae Proteins, Signal Transduction}, issn = {1553-7404}, doi = {10.1371/journal.pgen.1011202}, author = {Niphadkar, Shreyas and Karinje, Lavanya and Laxman, Sunil} } @article {3342, title = {Methionine uptake via the SLC43A2 transporter is essential for regulatory T-cell survival.}, journal = {Life Sci Alliance}, volume = {5}, year = {2022}, month = {2022 Sep 09}, abstract = {

Cell death, survival, or growth decisions in T-cell subsets depend on interplay between cytokine-dependent and metabolic processes. The metabolic requirements of T-regulatory cells (Tregs) for their survival and how these are satisfied remain unclear. Herein, we identified a necessary requirement of methionine uptake and usage for Tregs survival upon IL-2 deprivation. Activated Tregs have high methionine uptake and usage to S-adenosyl methionine, and this uptake is essential for Tregs survival in conditions of IL-2 deprivation. We identify a solute carrier protein SLC43A2 transporter, regulated in a Notch1-dependent manner that is necessary for this methionine uptake and Tregs viability. Collectively, we uncover a specifically regulated mechanism of methionine import in Tregs that is required for cells to adapt to cytokine withdrawal. We highlight the need for methionine availability and metabolism in contextually regulating cell death in this immunosuppressive population of T cells.

}, keywords = {Interleukin-2, Methionine, Racemethionine, Solute Carrier Proteins, T-Lymphocytes, Regulatory}, issn = {2575-1077}, doi = {10.26508/lsa.202201663}, author = {Saini, Neetu and Naaz, Afsana and Metur, Shree Padma and Gahlot, Pinki and Walvekar, Adhish and Dutta, Anupam and Davathamizhan, Umamaheswari and Sarin, Apurva and Laxman, Sunil} } @article {2268, title = {The bacterial social network and beyond.}, journal = {Nat Rev Mol Cell Biol}, volume = {22}, year = {2021}, month = {2021 Jul}, pages = {443}, issn = {1471-0080}, doi = {10.1038/s41580-021-00369-3}, author = {Laxman, Sunil} } @article {2374, title = {Bend or break: how biochemically versatile molecules enable metabolic division of labor in clonal microbial communities.}, journal = {Genetics}, volume = {219}, year = {2021}, month = {2021 Oct 02}, abstract = {

In fluctuating nutrient environments, isogenic microbial cells transition into "multicellular" communities composed of phenotypically heterogeneous cells, showing functional specialization. In fungi (such as budding yeast), phenotypic heterogeneity is often described in the context of cells switching between different morphotypes (e.g., yeast to hyphae/pseudohyphae or white/opaque transitions in Candida albicans). However, more fundamental forms of metabolic heterogeneity are seen in clonal Saccharomyces cerevisiae communities growing in nutrient-limited conditions. Cells within such communities exhibit contrasting, specialized metabolic states, and are arranged in distinct, spatially organized groups. In this study, we explain how such an organization can stem from self-organizing biochemical reactions that depend on special metabolites. These metabolites exhibit plasticity in function, wherein the same metabolites are metabolized and utilized for distinct purposes by different cells. This in turn allows cell groups to function as specialized, interdependent cross-feeding systems which support distinct metabolic processes. Exemplifying a system where cells exhibit either gluconeogenic or glycolytic states, we highlight how available metabolites can drive favored biochemical pathways to produce new, limiting resources. These new resources can themselves be consumed or utilized distinctly by cells in different metabolic states. This thereby enables cell groups to sustain contrasting, even apparently impossible metabolic states with stable transcriptional and metabolic signatures for a given environment, and divide labor in order to increase community fitness or survival. We speculate on possible evolutionary implications of such metabolic specialization and division of labor in isogenic microbial communities.

}, issn = {1943-2631}, doi = {10.1093/genetics/iyab109}, author = {Varahan, Sriram and Laxman, Sunil} } @article {2241, title = {Cycles, sources, and sinks: Conceptualizing how phosphate balance modulates carbon flux using yeast metabolic networks.}, journal = {Elife}, volume = {10}, year = {2021}, month = {2021 Feb 05}, abstract = {

Phosphates are ubiquitous molecules that enable critical intracellular biochemical reactions. Therefore, cells have elaborate responses to phosphate limitation. Our understanding of long-term transcriptional responses to phosphate limitation is extensive. Contrastingly, a systems-level perspective presenting unifying biochemical concepts to interpret how phosphate balance is critically coupled to (and controls) metabolic information flow is missing. To conceptualize such processes, utilizing yeast metabolic networks we categorize phosphates utilized in metabolism into cycles, sources and sinks. Through this, we identify metabolic reactions leading to putative phosphate sources or sinks. With this conceptualization, we illustrate how mass action driven flux towards sources and sinks enable cells to manage phosphate availability during transient/immediate phosphate limitations. We thereby identify how intracellular phosphate availability will predictably alter specific nodes in carbon metabolism, and determine signature cellular metabolic states. Finally, we identify a need to understand intracellular phosphate pools, in order to address mechanisms of phosphate regulation and restoration.

}, issn = {2050-084X}, doi = {10.7554/eLife.63341}, author = {Gupta, Ritu and Laxman, Sunil} } @article {2269, title = {Kog1/Raptor mediates metabolic rewiring during nutrient limitation by controlling SNF1/AMPK activity.}, journal = {Sci Adv}, volume = {7}, year = {2021}, month = {2021 Apr}, abstract = {

In changing environments, cells modulate resource budgeting through distinct metabolic routes to control growth. Accordingly, the TORC1 and SNF1/AMPK pathways operate contrastingly in nutrient replete or limited environments to maintain homeostasis. The functions of TORC1 under glucose and amino acid limitation are relatively unknown. We identified a modified form of the yeast TORC1 component Kog1/Raptor, which exhibits delayed growth exclusively during glucose and amino acid limitations. Using this, we found a necessary function for Kog1 in these conditions where TORC1 kinase activity is undetectable. Metabolic flux and transcriptome analysis revealed that Kog1 controls SNF1-dependent carbon flux apportioning between glutamate/amino acid biosynthesis and gluconeogenesis. Kog1 regulates SNF1/AMPK activity and outputs and mediates a rapamycin-independent activation of the SNF1 targets Mig1 and Cat8. This enables effective glucose derepression, gluconeogenesis activation, and carbon allocation through different pathways. Therefore, Kog1 centrally regulates metabolic homeostasis and carbon utilization during nutrient limitation by managing SNF1 activity.

}, issn = {2375-2548}, doi = {10.1126/sciadv.abe5544}, author = {Rashida, Zeenat and Srinivasan, Rajalakshmi and Cyanam, Meghana and Laxman, Sunil} } @article {2248, title = {Allosteric inhibition of MTHFR prevents futile SAM cycling and maintains nucleotide pools in one-carbon metabolism.}, journal = {J Biol Chem}, volume = {295}, year = {2020}, month = {2020 11 20}, pages = {16037-16057}, abstract = {

Methylenetetrahydrofolate reductase (MTHFR) links the folate cycle to the methionine cycle in one-carbon metabolism. The enzyme is known to be allosterically inhibited by SAM for decades, but the importance of this regulatory control to one-carbon metabolism has never been adequately understood. To shed light on this issue, we exchanged selected amino acid residues in a highly conserved stretch within the regulatory region of yeast MTHFR to create a series of feedback-insensitive, deregulated mutants. These were exploited to investigate the impact of defective allosteric regulation on one-carbon metabolism. We observed a strong growth defect in the presence of methionine. Biochemical and metabolite analysis revealed that both the folate and methionine cycles were affected in these mutants, as was the transsulfuration pathway, leading also to a disruption in redox homeostasis. The major consequences, however, appeared to be in the depletion of nucleotides. C isotope labeling and metabolic studies revealed that the deregulated MTHFR cells undergo continuous transmethylation of homocysteine by methyltetrahydrofolate (CHTHF) to form methionine. This reaction also drives SAM formation and further depletes ATP reserves. SAM was then cycled back to methionine, leading to futile cycles of SAM synthesis and recycling and explaining the necessity for MTHFR to be regulated by SAM. The study has yielded valuable new insights into the regulation of one-carbon metabolism, and the mutants appear as powerful new tools to further dissect out the intersection of one-carbon metabolism with various pathways both in yeasts and in humans.

}, keywords = {Adenosine Triphosphate, Allosteric Regulation, Humans, Methylation, Methylenetetrahydrofolate Reductase (NADPH2), S-Adenosylmethionine, Saccharomyces cerevisiae, Saccharomyces cerevisiae Proteins}, issn = {1083-351X}, doi = {10.1074/jbc.RA120.015129}, author = {Bhatia, Muskan and Thakur, Jyotika and Suyal, Shradha and Oniel, Ruchika and Chakraborty, Rahul and Pradhan, Shalini and Sharma, Monika and Sengupta, Shantanu and Laxman, Sunil and Masakapalli, Shyam Kumar and Bachhawat, Anand Kumar} } @article {1985, title = {Anabolic SIRT4 Exerts Retrograde Control over TORC1 Signaling by Glutamine Sparing in the Mitochondria.}, journal = {Mol Cell Biol}, volume = {40}, year = {2020}, month = {2020 Jan 03}, abstract = {

Anabolic and catabolic signaling mediated via mTOR and AMPK (AMP-activated kinase) have to be intrinsically coupled to mitochondrial functions for maintaining homeostasis and mitigate cellular/organismal stress. Although glutamine is known to activate mTOR, whether and how differential mitochondrial utilization of glutamine impinges on mTOR signaling has been less explored. Mitochondrial SIRT4, which unlike other sirtuins is induced in a fed state, is known to inhibit catabolic signaling/pathways through the AMPK-PGC1α/SIRT1-peroxisome proliferator-activated receptor α (PPARα) axis and negatively regulate glutamine metabolism via the tricarboxylic acid cycle. However, physiological significance of SIRT4 functions during a fed state is still unknown. Here, we establish SIRT4 as key anabolic factor that activates TORC1 signaling and regulates lipogenesis, autophagy, and cell proliferation. Mechanistically, we demonstrate that the ability of SIRT4 to inhibit anaplerotic conversion of glutamine to α-ketoglutarate potentiates TORC1. Interestingly, we also show that mitochondrial glutamine sparing or utilization is critical for differentially regulating TORC1 under fed and fasted conditions. Moreover, we conclusively show that differential expression of SIRT4 during fed and fasted states is vital for coupling mitochondrial energetics and glutamine utilization with anabolic pathways. These significant findings also illustrate that SIRT4 integrates nutrient inputs with mitochondrial retrograde signals to maintain a balance between anabolic and catabolic pathways.

}, issn = {1098-5549}, doi = {10.1128/MCB.00212-19}, author = {Shaw, Eisha and Talwadekar, Manasi and Rashida, Zeenat and Mohan, Nitya and Acharya, Aishwarya and Khatri, Subhash and Laxman, Sunil and Kolthur-Seetharam, Ullas} } @article {2203, title = {Genome-scale reconstruction of Gcn4/ATF4 networks driving a growth program.}, journal = {PLoS Genet}, volume = {16}, year = {2020}, month = {2020 12}, pages = {e1009252}, abstract = {

Growth and starvation are considered opposite ends of a spectrum. To sustain growth, cells use coordinated gene expression programs and manage biomolecule supply in order to match the demands of metabolism and translation. Global growth programs complement increased ribosomal biogenesis with sufficient carbon metabolism, amino acid and nucleotide biosynthesis. How these resources are collectively managed is a fundamental question. The role of the Gcn4/ATF4 transcription factor has been best studied in contexts where cells encounter amino acid starvation. However, high Gcn4 activity has been observed in contexts of rapid cell proliferation, and the roles of Gcn4 in such growth contexts are unclear. Here, using a methionine-induced growth program in yeast, we show that Gcn4/ATF4 is the fulcrum that maintains metabolic supply in order to sustain translation outputs. By integrating matched transcriptome and ChIP-Seq analysis, we decipher genome-wide direct and indirect roles for Gcn4 in this growth program. Genes that enable metabolic precursor biosynthesis indispensably require Gcn4; contrastingly ribosomal genes are partly repressed by Gcn4. Gcn4 directly binds promoter-regions and transcribes a subset of metabolic genes, particularly driving lysine and arginine biosynthesis. Gcn4 also globally represses lysine and arginine enriched transcripts, which include genes encoding the translation machinery. The Gcn4 dependent lysine and arginine supply thereby maintains the synthesis of the translation machinery. This is required to maintain translation capacity. Gcn4 consequently enables metabolic-precursor supply to bolster protein synthesis, and drive a growth program. Thus, we illustrate how growth and starvation outcomes are both controlled using the same Gcn4 transcriptional outputs that function in distinct contexts.

}, keywords = {Basic-Leucine Zipper Transcription Factors, Cell Proliferation, Gene Expression Regulation, Fungal, Gene Regulatory Networks, Genome, Fungal, Ribosomes, Saccharomyces cerevisiae, Saccharomyces cerevisiae Proteins, Transcriptional Activation}, issn = {1553-7404}, doi = {10.1371/journal.pgen.1009252}, author = {Srinivasan, Rajalakshmi and Walvekar, Adhish S and Rashida, Zeenat and Seshasayee, Aswin and Laxman, Sunil} } @article {2144, title = {Methylated PP2A stabilizes Gcn4 to enable a methionine-induced anabolic program.}, journal = {J Biol Chem}, year = {2020}, month = {2020 Oct 29}, abstract = {

Methionine, through S-adenosylmethionine, activates a multifaceted growth program in which ribosome biogenesis, carbon metabolism, amino acid and nucleotide biosynthesis are induced. This growth program requires the activity of the Gcn4 transcription factor (called ATF4 in mammals), which facilitates the supply of metabolic precursors that are essential for anabolism. However, how Gcn4 itself is regulated in the presence of methionine is unknown. Here, we discover that Gcn4 protein levels are increased by methionine, despite conditions of high cell growth and translation (where the roles of Gcn4 are not well studied). We demonstrate that this mechanism of Gcn4 induction is independent of transcription, as well as the conventional Gcn2/eIF2α-mediated increased translation of Gcn4. Instead, when methionine is abundant, Gcn4 phosphorylation is decreased, which reduces its ubiquitination and therefore degradation. Gcn4 is dephosphorylated by the protein phosphatase PP2A; our data show that when methionine is abundant, the conserved methyltransferase Ppm1 methylates and alters the activity of the catalytic subunit of PP2A, shifting the balance of Gcn4 towards a dephosphorylated, stable state. The absence of Ppm1 or the loss of the PP2A methylation destabilizes Gcn4 even when methionine is abundant, leading to collapse of the Gcn4-dependent anabolic program. These findings reveal a novel, methionine-dependent signaling and regulatory axis. Here methionine directs a conserved methyltransferase Ppm1, via its target phosphatase PP2A, to selectively stabilize Gcn4. Through this, cells conditionally modify a major phosphatase to stabilize a metabolic master-regulator and drive anabolism.

}, issn = {1083-351X}, doi = {10.1074/jbc.RA120.014248}, author = {Walvekar, Adhish S and Kadamur, Ganesh and Sreedharan, Sreesa and Gupta, Ritu and Srinivasan, Rajalakshmi and Laxman, Sunil} } @article {2205, title = {Methylated PP2A stabilizes Gcn4 to enable a methionine-induced anabolic program.}, journal = {J Biol Chem}, volume = {295}, year = {2020}, month = {2020 Dec 25}, pages = {18390-18405}, abstract = {

Methionine, through S-adenosylmethionine, activates a multifaceted growth program in which ribosome biogenesis, carbon metabolism, and amino acid and nucleotide biosynthesis are induced. This growth program requires the activity of the Gcn4 transcription factor (called ATF4 in mammals), which facilitates the supply of metabolic precursors that are essential for anabolism. However, how Gcn4 itself is regulated in the presence of methionine is unknown. Here, we discover that Gcn4 protein levels are increased by methionine, despite conditions of high cell growth and translation (in which the roles of Gcn4 are not well-studied). We demonstrate that this mechanism of Gcn4 induction is independent of transcription, as well as the conventional Gcn2/eIF2α-mediated increased translation of Gcn4. Instead, when methionine is abundant, Gcn4 phosphorylation is decreased, which reduces its ubiquitination and therefore degradation. Gcn4 is dephosphorylated by the protein phosphatase 2A (PP2A); our data show that when methionine is abundant, the conserved methyltransferase Ppm1 methylates and alters the activity of the catalytic subunit of PP2A, shifting the balance of Gcn4 toward a dephosphorylated, stable state. The absence of Ppm1 or the loss of the PP2A methylation destabilizes Gcn4 even when methionine is abundant, leading to collapse of the Gcn4-dependent anabolic program. These findings reveal a novel, methionine-dependent signaling and regulatory axis. Here methionine directs the conserved methyltransferase Ppm1 via its target phosphatase PP2A to selectively stabilize Gcn4. Through this, cells conditionally modify a major phosphatase to stabilize a metabolic master regulator and drive anabolism.

}, issn = {1083-351X}, doi = {10.1074/jbc.RA120.014248}, author = {Walvekar, Adhish S and Kadamur, Ganesh and Sreedharan, Sreesa and Gupta, Ritu and Srinivasan, Rajalakshmi and Laxman, Sunil} } @article {2108, title = {A novel polyubiquitin chain linkage formed by viral Ubiquitin is resistant to host deubiquitinating enzymes.}, journal = {Biochem J}, volume = {477}, year = {2020}, month = {2020 Jun 26}, pages = {2193-2219}, abstract = {

The Baculoviridae family of viruses encode a viral Ubiquitin (vUb) gene. Though the vUb is homologous to the host eukaryotic Ubiquitin (Ub), its preservation in the viral genome indicates unique functions that are not compensated by the host Ub. We report the structural, biophysical, and biochemical properties of the vUb from Autographa californica multiple nucleo-polyhedrosis virus (AcMNPV). The packing of central helix α1 to the beta-sheet β1-β5 is different between vUb and Ub. Consequently, its stability is lower compared with Ub. However, the surface properties, ubiquitination activity, and the interaction with Ubiquitin-binding domains are similar between vUb and Ub. Interestingly, vUb forms atypical polyubiquitin chain linked by lysine at the 54th position (K54), and the deubiquitinating enzymes are ineffective against the K54-linked polyubiquitin chains. We propose that the modification of host/viral proteins with the K54-linked chains is an effective way selected by the virus to protect the vUb signal from host DeUbiquitinases.

}, issn = {1470-8728}, doi = {10.1042/BCJ20200289}, author = {Negi, Hitendra and Reddy, Pothula Purushotham and Vengayil, Vineeth and Patole, Chhaya and Laxman, Sunil and Das, Ranabir} } @article {2150, title = {The Rad53-Spt21 and Tel1 axes couple glucose tolerance to histone dosage and subtelomeric silencing.}, journal = {Nat Commun}, volume = {11}, year = {2020}, month = {2020 08 19}, pages = {4154}, abstract = {

The DNA damage response (DDR) coordinates DNA metabolism with nuclear and non-nuclear processes. The DDR kinase Rad53 controls histone degradation to assist DNA repair. However, Rad53 deficiency causes histone-dependent growth defects in the absence of DNA damage, pointing out unknown physiological functions of the Rad53-histone axis. Here we show that histone dosage control by Rad53 ensures metabolic homeostasis. Under physiological conditions, Rad53 regulates histone levels through inhibitory phosphorylation of the transcription factor Spt21 on Ser276. Rad53-Spt21 mutants display severe glucose dependence, caused by excess histones through two separable mechanisms: dampening of acetyl-coenzyme A-dependent carbon metabolism through histone hyper-acetylation, and Sirtuin-mediated silencing of starvation-induced subtelomeric domains. We further demonstrate that repression of subtelomere silencing by physiological Tel1 and Rpd3 activities coveys tolerance to glucose restriction. Our findings identify DDR mutations, histone imbalances and aberrant subtelomeric chromatin as interconnected causes of glucose dependence, implying that DDR kinases coordinate metabolism and epigenetic changes.

}, keywords = {Acetylation, Ataxia Telangiectasia Mutated Proteins, Cell Cycle Proteins, Checkpoint Kinase 2, DNA Damage, DNA Repair, Gene Silencing, Glucose, Histone Deacetylases, Histones, Intracellular Signaling Peptides and Proteins, Mutation, Phosphorylation, Protein-Serine-Threonine Kinases, Saccharomyces cerevisiae, Saccharomyces cerevisiae Proteins, Serine, Telomere, Transcription Factors}, issn = {2041-1723}, doi = {10.1038/s41467-020-17961-4}, author = {Bruhn, Christopher and Ajazi, Arta and Ferrari, Elisa and Lanz, Michael Charles and Batrin, Renaud and Choudhary, Ramveer and Walvekar, Adhish and Laxman, Sunil and Longhese, Maria Pia and Fabre, Emmanuelle and Smolka, Marcus Bustamente and Foiani, Marco} } @article {2204, title = {Resource plasticity-driven carbon-nitrogen budgeting enables specialization and division of labor in a clonal community.}, journal = {Elife}, volume = {9}, year = {2020}, month = {2020 09 02}, abstract = {

Previously, we found that in glucose-limited colonies, metabolic constraints drive cells into groups exhibiting gluconeogenic or glycolytic states. In that study, threshold amounts of trehalose - a limiting, produced carbon-resource, controls the emergence and self-organization of cells exhibiting the glycolytic state, serving as a carbon source that fuels glycolysis (Varahan et al., 2019). We now discover that the plasticity of use of a non-limiting resource, aspartate, controls both resource production and the emergence of heterogeneous cell states, based on differential metabolic budgeting. In gluconeogenic cells, aspartate is a carbon source for trehalose production, while in glycolytic cells using trehalose for carbon, aspartate is predominantly a nitrogen source for nucleotide synthesis. This metabolic plasticity of aspartate enables carbon-nitrogen budgeting, thereby driving the biochemical self-organization of distinct cell states. Through this organization, cells in each state exhibit true division of labor, providing growth/survival advantages for the whole community.

}, issn = {2050-084X}, doi = {10.7554/eLife.57609}, author = {Varahan, Sriram and Sinha, Vaibhhav and Walvekar, Adhish and Krishna, Sandeep and Laxman, Sunil} } @article {1934, title = {The E3 ubiquitin ligase Pib1 regulates effective gluconeogenic shutdown upon glucose availability.}, journal = {J Biol Chem}, year = {2019}, month = {2019 Oct 11}, abstract = {

Cells use multiple mechanisms to regulate their metabolic states in response to changes in their nutrient environment. One example is the response of cells to glucose. In S. cerevisiae growing in glucose-depleted medium, the re-availability of glucose leads to the downregulation of gluconeogenesis, and the activation of glycolysis, leading to {\textquoteright}glucose repression{\textquoteright}. However, our knowledge of the mechanisms mediating the glucose dependent downregulation of the gluconeogenic transcription factors is limited. Using a major gluconeogenic transcription factor Rds2 as a candidate, here we identify a novel role for the E3 ubiquitin ligase Pib1 in regulating the stability and degradation of Rds2. Glucose addition to cells growing in glucose limitation results in rapid ubiquitination of Rds2, followed by its proteasomal degradation. Through in vivo and in vitro experiments, we establish Pib1 as the ubiquitin E3 ligase that regulates Rds2 ubiquitination and stability. Notably, this Pib1 mediated Rds2 ubiquitination, followed by proteasomal degradation, is specific to the presence of glucose. This Pib1 mediated ubiquitination of Rds2 depends on the phosphorylation state of Rds2, suggesting a cross-talk between ubiquitination and phosphorylation to achieve a metabolic state change. Using stable-isotope based metabolic flux experiments we find that the loss of Pib1 results in an imbalanced gluconeogenic state, regardless of glucose availability. Pib1 is required for complete glucose repression, and enables cells to optimally grow in competitive environments when glucose becomes re-available. Our results reveal the existence of a Pib1 mediated regulatory program that mediates glucose-repression when glucose availability is restored.

}, issn = {1083-351X}, doi = {10.1074/jbc.RA119.009822}, author = {Vengayil, Vineeth and Rashida, Zeenat and Laxman, Sunil} } @article {1741, title = {Metabolic constraints drive self-organization of specialized cell groups.}, journal = {Elife}, volume = {8}, year = {2019}, month = {2019 Jun 26}, abstract = {

How phenotypically distinct states in isogenic cell populations appear and stably co-exist remains unresolved. We find that within a mature, clonal yeast colony developing in low glucose, cells arrange into metabolically disparate cell groups. Using this system, we model and experimentally identify metabolic constraints sufficient to drive such self-assembly. Beginning in a uniformly gluconeogenic state, cells exhibiting a contrary, high pentose phosphate pathway activity state, spontaneously appear and proliferate, in a spatially constrained manner. Gluconeogenic cells in the colony produce and provide a resource, which we identify as trehalose. Above threshold concentrations of external trehalose, cells switch to the new metabolic state and proliferate. A self-organized system establishes, where cells in this new state are sustained by trehalose consumption, which thereby restrains other cells in the trehalose producing, gluconeogenic state. Our work suggests simple physico-chemical principles that determine how isogenic cells spontaneously self-organize into structured assemblies in complimentary, specialized states.

}, issn = {2050-084X}, doi = {10.7554/eLife.46735}, author = {Varahan, Sriram and Walvekar, Adhish and Sinha, Vaibhhav and Krishna, Sandeep and Laxman, Sunil} } @article {1988, title = {Methionine at the Heart of Anabolism and Signaling: Perspectives From Budding Yeast.}, journal = {Front Microbiol}, volume = {10}, year = {2019}, month = {2019}, pages = {2624}, abstract = {

Studies using a fungal model, , have been instrumental in advancing our understanding of sulfur metabolism in eukaryotes. Sulfur metabolites, particularly methionine and its derivatives, induce anabolic programs in yeast, and drive various processes integral to metabolism (one-carbon metabolism, nucleotide synthesis, and redox balance). Thereby, methionine also connects these processes with autophagy and epigenetic regulation. The direct involvement of methionine-derived metabolites in diverse chemistries such as transsulfuration and methylation reactions comes from the elegant positioning and safe handling of sulfur through these molecules. In this mini-review, we highlight studies from yeast that reveal how this amino acid holds a unique position in both metabolism and cell signaling, and illustrate cell fate decisions that methionine governs. We further discuss the interconnections between sulfur and NADPH metabolism, and highlight critical nodes around methionine metabolism that are promising for antifungal drug development.

}, issn = {1664-302X}, doi = {10.3389/fmicb.2019.02624}, author = {Walvekar, Adhish S and Laxman, Sunil} } @article {1743, title = {A tRNA modification balances carbon and nitrogen metabolism by regulating phosphate homeostasis.}, journal = {Elife}, volume = {8}, year = {2019}, month = {2019 Jul 01}, abstract = {

Cells must appropriately sense and integrate multiple metabolic resources to commit to proliferation. Here, we report that cells regulate carbon and nitrogen metabolic homeostasis through tRNA U-thiolation. Despite amino acid sufficiency, tRNA-thiolation deficient cells appear amino acid starved. In these cells, carbon flux towards nucleotide synthesis decreases, and trehalose synthesis increases, resulting in a starvation-like metabolic signature. Thiolation mutants have only minor translation defects. However, in these cells phosphate homeostasis genes are strongly down-regulated, resulting in an effectively phosphate-limited state. Reduced phosphate enforces a metabolic switch, where glucose-6-phosphate is routed towards storage carbohydrates. Notably, trehalose synthesis, which releases phosphate and thereby restores phosphate availability, is central to this metabolic rewiring. Thus, cells use thiolated tRNAs to perceive amino acid sufficiency, balance carbon and amino acid metabolic flux and grow optimally, by controlling phosphate availability. These results further biochemically explain how phosphate availability determines a switch to a {\textquoteright}starvation-state{\textquoteright}.

}, issn = {2050-084X}, doi = {10.7554/eLife.44795}, author = {Gupta, Ritu and Walvekar, Adhish and Liang, Shun and Rashida, Zeenat and Shah, Premal and Laxman, Sunil} } @article {1987, title = {tRNA wobble-uridine modifications as amino acid sensors and regulators of cellular metabolic state.}, journal = {Curr Genet}, year = {2019}, month = {2019 Nov 22}, abstract = {

Cells must appropriately sense available nutrients and accordingly regulate their metabolic outputs, to survive. This mini-review considers the idea that conserved chemical modifications of wobble (U34) position tRNA uridines enable cells to sense nutrients and regulate their metabolic state. tRNA wobble uridines are chemically modified at the 2- and 5- positions, with a thiol (s2), and (commonly) a methoxycarbonylmethyl (mcm5) modification, respectively. These modifications reflect sulfur amino acid (methionine and cysteine) availability. The loss of these modifications has minor translation defects. However, they result in striking phenotypes consistent with an altered metabolic state. Using yeast, we recently discovered that the s2 modification regulates overall carbon and nitrogen metabolism, dependent on methionine availability. The loss of this modification results in rewired carbon (glucose) metabolism. Cells have reduced carbon flux towards the pentose phosphate pathway and instead increased flux towards storage carbohydrates-primarily trehalose, along with reduced nucleotide synthesis, and perceived amino acid starvation signatures. Remarkably, this metabolic rewiring in the s2U mutants is caused by mechanisms leading to intracellular phosphate limitation. Thus this U34 tRNA modification responds to methionine availability and integratively regulates carbon and nitrogen homeostasis, wiring cells to a {\textquoteright}growth{\textquoteright} state. We interpret the importance of U34 modifications in the context of metabolic sensing and anabolism, emphasizing their intimate coupling to methionine metabolism.

}, issn = {1432-0983}, doi = {10.1007/s00294-019-01045-y}, author = {Gupta, Ritu and Laxman, Sunil} } @article {1596, title = {Methionine coordinates a hierarchically organized anabolic program enabling proliferation.}, journal = {Mol Biol Cell}, year = {2018}, month = {2018 Oct 24}, pages = {mbcE18080515}, abstract = {

Methionine availability during overall amino acid limitation metabolically reprograms cells to support proliferation, the underlying basis for which remains unclear. Here, we construct the organization of this methionine mediated anabolic program, using yeast. Combining comparative transcriptome analysis, biochemical and metabolic flux based approaches, we discover that methionine rewires overall metabolic outputs by increasing the activity of a key regulatory node. This comprises of: the pentose phosphate pathway (PPP) coupled with reductive biosynthesis, the glutamate dehydrogenase (GDH) dependent synthesis of glutamate/glutamine, and pyridoxal-5-phosphate (PLP) dependent transamination capacity. This PPP-GDH-PLP node provides the required cofactors and/or substrates for subsequent rate-limiting reactions in the synthesis of amino acids, and therefore nucleotides. These rate-limiting steps in amino acid biosynthesis are also induced in a methionine-dependent manner. This thereby results in a biochemical cascade establishing a hierarchically organized anabolic program. For this methionine mediated anabolic program to be sustained, cells co-opt a "starvation stress response" regulator, Gcn4p. Collectively, our data suggest a hierarchical metabolic framework explaining how methionine mediates an anabolic switch.

}, issn = {1939-4586}, doi = {10.1091/mbc.E18-08-0515}, author = {Walvekar, Adhish S and Srinivasan, Rajalakshmi and Gupta, Ritu and Laxman, Sunil} } @article {1593, title = {A minimal "push-pull" bistability model explains oscillations between quiescent and proliferative cell states.}, journal = {Mol Biol Cell}, volume = {29}, year = {2018}, month = {2018 Sep 15}, pages = {2243-2258}, abstract = {

A minimal model for oscillating between quiescent and growth/proliferation states, dependent on the availability of a central metabolic resource, is presented. From the yeast metabolic cycles, metabolic oscillations in oxygen consumption are represented as transitions between quiescent and growth states. We consider metabolic resource availability, growth rates, and switching rates (between states) to model a relaxation oscillator explaining transitions between these states. This frustrated bistability model reveals a required communication between the metabolic resource that determines oscillations and the quiescent and growth state cells. Cells in each state reflect memory, or hysteresis of their current state, and "push-pull" cells from the other state. Finally, a parsimonious argument is made for a specific central metabolite as the controller of switching between quiescence and growth states. We discuss how an oscillator built around the availability of such a metabolic resource is sufficient to generally regulate oscillations between growth and quiescence through committed transitions.

}, issn = {1939-4586}, doi = {10.1091/mbc.E18-01-0017}, author = {Krishna, Sandeep and Laxman, Sunil} } @article {1594, title = {A versatile LC-MS/MS approach for comprehensive, quantitative analysis of central metabolic pathways.}, journal = {Wellcome Open Res}, volume = {3}, year = {2018}, month = {2018}, pages = {122}, abstract = {

Liquid chromatography-mass spectrometry (LC-MS/MS) based approaches are widely used for the identification and quantitation of specific metabolites, and are a preferred approach towards analyzing cellular metabolism. Most methods developed come with specific requirements such as unique columns, ion-pairing reagents and pH conditions, and typically allow measurements in a specific pathway alone. Here, we present a single column-based set of methods for simultaneous coverage of multiple pathways, primarily focusing on central carbon, amino acid, and nucleotide metabolism. We further demonstrate the use of this method for quantitative, stable isotope-based metabolic flux experiments, expanding its use beyond steady-state level measurements of metabolites. The expected kinetics of label accumulation pertinent to the pathway under study are presented with some examples. The methods discussed here are broadly applicable, minimize the need for multiple chromatographic resolution methods, and highlight how simple labeling experiments can be valuable in facilitating a comprehensive understanding of the metabolic state of cells.

}, issn = {2398-502X}, doi = {10.12688/wellcomeopenres.14832.1}, author = {Walvekar, Adhish and Rashida, Zeenat and Maddali, Hemanth and Laxman, Sunil} } @article {1157, title = {Conceptualizing Eukaryotic Metabolic Sensing and Signaling.}, journal = {J Indian Inst Sci}, volume = {97}, year = {2017}, month = {2017 Mar}, pages = {59-77}, abstract = {

For almost all cells, nutrient availability, from glucose to amino acids, dictates their growth or developmental programs. This nutrient availability is closely coupled to the overall intracellular metabolic state of the cell. Therefore, cells have evolved diverse, robust and versatile modules to sense intracellular metabolic states, activate signaling outputs and regulate outcomes to these states. Yet, signaling and metabolism have been viewed as important but separate. This short review attempts to position aspects of intracellular signaling from a metabolic perspective, highlighting how conserved, core principles of metabolic sensing and signaling can emerge from an understanding of metabolic regulation. I briefly explain the nature of metabolic sensors, using the example of the AMP activated protein kinase (AMPK) as an "energy sensing" hub. Subsequently, I explore how specific central metabolites, particularly acetyl-CoA, but also -adenosyl methionine and SAICAR, can act as signaling molecules. I extensively illustrate the nature of a metabolic signaling hub using the specific example of the Target of Rapamycin Complex 1 (TORC1), and amino acid sensing. A highlight is the emergence of the lysosome/vacuole as a metabolic and signaling hub. Finally, the need to expand our understanding of the intracellular dynamics (in concentration and localization) of several metabolites, and their signaling hubs is emphasized.

}, issn = {0970-4140}, doi = {10.1007/s41745-016-0013-1}, author = {Laxman, Sunil} } @article {1164, title = {Thiol trapping and metabolic redistribution of sulfur metabolites enable cells to overcome cysteine overload.}, journal = {Microb Cell}, volume = {4}, year = {2017}, month = {2017 Mar 27}, pages = {112-126}, abstract = {

Cysteine is an essential requirement in living organisms. However, due to its reactive thiol side chain, elevated levels of intracellular cysteine can be toxic and therefore need to be rapidly eliminated from the cellular milieu. In mammals and many other organisms, excess cysteine is believed to be primarily eliminated by the cysteine dioxygenase dependent oxidative degradation of cysteine, followed by the removal of the oxidative products. However, other mechanisms of tackling excess cysteine are also likely to exist, but have not thus far been explored. In this study, we use , which naturally lacks a cysteine dioxygenase, to investigate mechanisms for tackling cysteine overload. Overexpressing the high affinity cysteine transporter, , enabled yeast cells to rapidly accumulate high levels of intracellular cysteine. Using targeted metabolite analysis, we observe that cysteine is initially rapidly interconverted to non-reactive cystine . A time course revealed that cells systematically convert excess cysteine to inert thiol forms; initially to cystine, and subsequently to cystathionine, S-Adenosyl-L-homocysteine (SAH) and S-Adenosyl L-methionine (SAM), in addition to eventually accumulating glutathione (GSH) and polyamines. Microarray based gene expression studies revealed the upregulation of arginine/ornithine biosynthesis a few hours after the cysteine overload, and suggest that the non-toxic, non-reactive thiol based metabolic products are eventually utilized for amino acid and polyamine biogenesis, thereby enabling cell growth. Thus, cells can handle potentially toxic amounts of cysteine by a combination of thiol trapping, metabolic redistribution to non-reactive thiols and subsequent consumption for anabolism.

}, issn = {2311-2638}, doi = {10.15698/mic2017.04.567}, author = {Deshpande, Anup Arunrao and Bhatia, Muskan and Laxman, Sunil and Bachhawat, Anand Kumar} } @article {618, title = {Decoding the stem cell quiescence cycle--lessons from yeast for regenerative biology.}, journal = {J Cell Sci}, volume = {128}, year = {2015}, month = {2015 Dec 15}, pages = {4467-74}, abstract = {

In the past decade, major advances have occurred in the understanding of mammalian stem cell biology, but roadblocks (including gaps in our fundamental understanding) remain in translating this knowledge to regenerative medicine. Interestingly, a close analysis of the Saccharomyces cerevisiae literature leads to an appreciation of how much yeast biology has contributed to the conceptual framework underpinning our understanding of stem cell behavior, to the point where such insights have been internalized into the realm of the known. This Opinion article focuses on one such example, the quiescent adult mammalian stem cell, and examines concepts underlying our understanding of quiescence that can be attributed to studies in yeast. We discuss the metabolic, signaling and gene regulatory events that control entry and exit into quiescence in yeast. These processes and events retain remarkable conservation and conceptual parallels in mammalian systems, and collectively suggest a regulated program beyond the cessation of cell division. We argue that studies in yeast will continue to not only reveal fundamental concepts in quiescence, but also leaven progress in regenerative medicine.

}, issn = {1477-9137}, doi = {10.1242/jcs.177758}, author = {Dhawan, Jyotsna and Laxman, Sunil} }