Differential chromatin proteomics of the MMS-induced DNA damage response in yeast

PARylation of 14-3-3 proteins controls the virulence of Magnaporthe oryzae

Magnaporthe oryzae is a devastating fungal pathogen that causes the rice blast disease worldwide. The post-translational modification of ADP-ribosylation holds significant importance in various fundamental biological processes. However, the specific function of this modification in M. oryzae remains unknown. This study revealed that Poly(ADP-ribosyl)ation (PARylation) executes a critical function in M. oryzae. M. oryzae Poly(ADP-ribose) polymerase 1 (PARP1) exhibits robust PARylation activity. Disruption of PARylation by PARP1 knock-out or chemical inhibition reveals its involvement in M. oryzae virulence, particularly in appressorium formation. Furthermore, we identified two M. oryzae 14-3-3 proteins, GRF1 and GRF2, as substrates of PARP1. Deletion of GRF1 or GRF2 results in delayed and dysfunctional appressorium, diminished plant penetration, and reduced virulence of the fungus. Biochemical and genetic evidence suggest that PARylation of 14-3-3s is essential for its function in M. oryzae virulence. Moreover, PARylation regulates 14-3-3 dimerization and is required for the activation of the mitogen-activated protein kinases (MAPKs), Pmk1 and Mps1. GRF1 interacts with both Mst7 and Pmk1, and bridges their interaction in a PARylation-dependent manner. This study unveils a distinctive mechanism that PARylation of 14-3-3 proteins controls appressorium formation through MAPK activation, and could facilitate the development of new strategies of rice blast disease control.

Mitochondrial superoxide dismutase Sod2 suppresses nuclear genome instability during oxidative stress

Oxidative stress can damage DNA and thereby contribute to genome instability. To avoid an imbalance or overaccumulation of reactive oxygen species (ROS), cells are equipped with antioxidant enzymes that scavenge excess ROS. Cells lacking the RecQ-family DNA helicase Sgs1, which contributes to homology-dependent DNA break repair and chromosome stability, are known to accumulate ROS, but the origin and consequences of this oxidative stress phenotype are not fully understood. Here, we show that the sgs1 mutant exhibits elevated mitochondrial superoxide, increased mitochondrial mass, and accumulation of recombinogenic DNA lesions that can be suppressed by antioxidants. Increased mitochondrial mass in the sgs1Δ mutant is accompanied by increased mitochondrial branching, which was also inducible in wildtype cells by replication stress. Superoxide dismutase Sod2 genetically interacts with Sgs1 in the suppression of nuclear chromosomal rearrangements under paraquat (PQ)-induced oxidative stress. PQ-induced chromosome rearrangements in the absence of Sod2 are promoted by Rad51 recombinase and the polymerase subunit Pol32. Finally, the dependence of chromosomal rearrangements on the Rev1/Pol ζ mutasome suggests that under oxidative stress successful DNA synthesis during DNA break repair depends on translesion DNA synthesis.

Yeast UPS1 deficiency leads to UVC radiation sensitivity and shortened lifespan

UPS1/YLR193C of Saccharomyces cerevisiae (S. cerevisiae) encodes a mitochondrial intermembrane space protein. A previous study found that Ups1p is needed for normal mitochondrial morphology and that UPS1 deficiency disrupts the intramitochondrial transport of phosphatidic acid in yeast cells and leads to an altered unfolded protein response and mTORC1 signaling activation. In this paper, we first provide evidence showing that the UPS1 gene is involved in the UVC-induced DNA damage response and aging. We show that UPS1 deficiency leads to sensitivity to ultraviolet C (UVC) radiation and that this effect is accompanied by elevated DNA damage, increased intracellular ROS levels, abnormal mitochondrial respiratory function, an increased early apoptosis rate, and shortened replicative lifespan and chronological lifespan. Moreover, we show that overexpression of the DNA damage-induced checkpoint gene RAD9 effectively eliminates the senescence-related defects observed in the UPS1-deficient strain. Collectively, these results suggest a novel role for UPS1 in the UVC-induced DNA damage response and aging.

Proteome analysis of Saccharomyces cerevisiae after methyl methane sulfonate (MMS) treatment

The treatment of methyl methane sulfonate (MMS) increases sensitivity to the DNA damage which, further leads to the cell death followed by a cell cycle delay. Delay in the cell cycle is because of the change in global transcription regulation which results into proteome change. There are several microarray studies on the transcriptome changes after MMS treatment, but very few studies are reported related to proteome change. The proteome analysis in this report identified subgroups of proteins, belonging to known cell cycle regulators, metabolic pathways and protein folding. About 53 proteins were identified by MS/MS and found that 36 of them were induced, 10 were repressed and few of them showed insignificant change. Our results indicated the change in the interactome as well as phosphorylation status of carboxy terminal domain (CTD) of RNA Polymerase II (RNAP-II) after MMS treatment. The RNAP-II complex was affinity purified and ~1640 peptides were identified using nano LC/MS corresponding to 27 interacting proteins along with the twelve RNAP-II subunit. These identified proteins participated in the repair of the damage, changes the function of the main energetic pathways and the carbon flux in various end products. The main metabolic enzymes in the glycolysis, pyruvate phosphate and amino acid biosynthesis pathways showed significant change. Our results indicate that DNA damage is somehow related to these pathways and is co-regulated simultaneously.

KEOPS complex promotes homologous recombination via DNA resection

KEOPS complex is one of the most conserved protein complexes in eukaryotes. It plays important roles in both telomere uncapping and tRNA N⁶-threonylcarbamoyladenosine (t⁶A) modification in budding yeast. But whether KEOPS complex plays any roles in DNA repair remains unknown. Here, we show that KEOPS complex plays positive roles in both DNA damage response and homologous recombination-mediated DNA repair independently of its t⁶A synthesis function. Additionally, KEOPS displays DNA binding activity in vitro, and is recruited to the chromatin at DNA breaks in vivo, suggesting a direct role of KEOPS in DSB repair. Mechanistically, KEOPS complex appears to promote DNA end resection through facilitating the association of Exo1 and Dna2 with DNA breaks. Interestingly, inactivation of both KEOPS and Mre11/Rad50/Xrs2 (MRX) complexes results in synergistic defect in DNA resection, revealing that KEOPS and MRX have some redundant functions in DNA resection. Thus we uncover a t⁶A-independent role of KEOPS complex in DNA resection, and propose that KEOPS might be a DSB sensor to assist cells in maintaining chromosome stability.

Gene Expression Noise Produces Cell-to-Cell Heterogeneity in Eukaryotic Homologous Recombination Rate

Variation in gene expression among genetically identical individual cells (called gene expression noise) directly contributes to phenotypic diversity. Whether such variation can impact genome stability and lead to variation in genotype remains poorly explored. We addressed this question by investigating whether noise in the expression of genes affecting homologous recombination (HR) activity either directly (RAD52) or indirectly (RAD27) confers cell-to-cell heterogeneity in HR rate in Saccharomyces cerevisiae. Using cell sorting to isolate subpopulations with various expression levels, we show that spontaneous HR rate is highly heterogeneous from cell-to-cell in clonal populations depending on the cellular amount of proteins affecting HR activity. Phleomycin-induced HR is even more heterogeneous, showing that RAD27 expression variation strongly affects the rate of recombination from cell-to-cell. Strong variations in HR rate between subpopulations are not correlated to strong changes in cell cycle stage. Moreover, this heterogeneity occurs even when simultaneously sorting cells at equal expression level of another gene involved in DNA damage response (BMH1) that is upregulated by DNA damage, showing that the initiating DNA damage is not responsible for the observed heterogeneity in HR rate. Thus gene expression noise seems mainly responsible for this phenomenon. Finally, HR rate non-linearly scales with Rad27 levels showing that total amount of HR cannot be explained solely by the time- or population-averaged Rad27 expression. Altogether, our data reveal interplay between heterogeneity at the gene expression and genetic levels in the production of phenotypic diversity with evolutionary consequences from microbial to cancer cell populations.

Transcriptional regulation of the genes involved in protein metabolism and processing in Saccharomyces cerevisiae

Topological analysis of large networks, which focus on a specific biological process or on related biological processes, where functional coherence exists among the interacting members, may provide a wealth of insight into cellular functionality. This work presents an unbiased systems approach to analyze genetic, transcriptional regulatory and physical interaction networks of yeast genes possessing such functional coherence to gain novel biological insight. The present analysis identified only a few transcriptional regulators amongst a large gene cohort associated with the protein metabolism and processing in yeast. These transcription factors are not functionally required for the maintenance of these tasks in growing cells. Rather, they are involved in rewiring gene transcription in response to such major challenges as starvation, hypoxia, DNA damage, heat shock or the accumulation of unfolded proteins. Indeed, only a subset of these proteins were captured empirically in the nuclear-enriched fraction of non-stressed yeast cells, suggesting that the transcriptional regulation of protein metabolism and processing in yeast is primarily concerned with maintaining cellular robustness in the face of threat by either internal or external stressors.

CTT1 overexpression increases the replicative lifespan of MMS-sensitive Saccharomyces cerevisiae deficient in KSP1

Ksplp is a nuclear-localized Ser/Thr kinase that is not essential for the vegetative growth of yeast. A global gene function analysis in yeast suggested that Ksplp was involved in the oxidative stress response; however, the underlying mechanism remains unclear. Here, we showed that KSP1-deficient yeast cells exhibit hypersensitivity to the DNA alkylating agent methyl methanesulphonate (MMS), and treatment of the KSP1-deficient strain with MMS could trigger abnormal mitochondrial membrane potential and up-regulate reactive oxygen species (ROS) production. In addition, the mRNA expression level of the catalase gene CTT1 (which encodes cytosolic catalase) and total catalase activity were strongly down-regulated in the KSP1-deleted strain compared with those in wild-type cells. Moreover, the KSP1 deficiency also leads to a shortened replicative lifespan, which could be restored by the increased expression of CTT1. On the other hand, KSP1-overexpressed (KSP1OX) yeast cells exhibited increased resistance towards MMS, an effect that was, at least in part, CTT1 independent. Collectively, these findings highlight the involvement of Ksplp in the DNA damage response and implicate Ksplp as a modulator of the replicative lifespan.

An account of fungal 14-3-3 proteins

14-3-3s are a group of relatively low molecular weight, acidic, dimeric, protein(s) conserved from single-celled yeast to multicellular vertebrates including humans. Despite lacking catalytic activity, these proteins have been shown to be involved in multiple cellular processes. Apart from their role in normal cellular physiology, recently these proteins have been implicated in various medical consequences. In this present review, fungal 14-3-3 protein localization, interactions, transcription, regulation, their role in the diverse cellular process including DNA duplication, cell cycle, protein trafficking or secretion, apoptosis, autophagy, cell viability under stress, gene expression, spindle positioning, role in carbon metabolism have been discussed. In the end, I also highlighted various roles of yeasts 14-3-3 proteins in tabular form. Thus this review with primary emphasis on yeast will help in appreciating the significance of 14-3-3 proteins in cell physiology.

Proteomics to study DNA-bound and chromatin-associated gene regulatory complexes

High-resolution mass spectrometry (MS)-based proteomics is a powerful method for the identification of soluble protein complexes and large-scale affinity purification screens can decode entire protein interaction networks. In contrast, protein complexes residing on chromatin have been much more challenging, because they are difficult to purify and often of very low abundance. However, this is changing due to recent methodological and technological advances in proteomics. Proteins interacting with chromatin marks can directly be identified by pulldowns with synthesized histone tails containing posttranslational modifications (PTMs). Similarly, pulldowns with DNA baits harbouring single nucleotide polymorphisms or DNA modifications reveal the impact of those DNA alterations on the recruitment of transcription factors. Accurate quantitation – either isotope-based or label free – unambiguously pinpoints proteins that are significantly enriched over control pulldowns. In addition, protocols that combine classical chromatin immunoprecipitation (ChIP) methods with mass spectrometry (ChIP-MS) target gene regulatory complexes in their in-vivo context. Similar to classical ChIP, cells are crosslinked with formaldehyde and chromatin sheared by sonication or nuclease digested. ChIP-MS baits can be proteins in tagged or endogenous form, histone PTMs, or lncRNAs. Locus-specific ChIP-MS methods would allow direct purification of a single genomic locus and the proteins associated with it. There, loci can be targeted either by artificial DNA-binding sites and corresponding binding proteins or via proteins with sequence specificity such as TAL or nuclease deficient Cas9 in combination with a specific guide RNA. We predict that advances in MS technology will soon make such approaches generally applicable tools in epigenetics.

The contribution of mass spectrometry-based proteomics to understanding epigenetics

Chromatin is a macromolecular complex composed of DNA and histones that regulate gene expression and nuclear architecture. The concerted action of DNA methylation, histone post-translational modifications and chromatin-associated proteins control the epigenetic regulation of the genome, ultimately determining cell fate and the transcriptional outputs of differentiated cells. Deregulation of this complex machinery leads to disease states, and exploiting epigenetic drugs is becoming increasingly attractive for therapeutic intervention. Mass spectrometry (MS)-based proteomics emerged as a powerful tool complementary to genomic approaches for epigenetic research, allowing the unbiased and comprehensive analysis of histone post-translational modifications and the characterization of chromatin constituents and chromatin-associated proteins. Furthermore, MS holds great promise for epigenetic biomarker discovery and represents a useful tool for deconvolution of epigenetic drug targets. Here, we will provide an overview of the applications of MS-based proteomics in various areas of chromatin biology.

Determinants of tolerance to inhibitors in hardwood spent sulfite liquor in genome shuffled Pachysolen tannophilus strains

Genome shuffling was used to obtain Pachysolen tannophilus mutants with improved tolerance to inhibitors in hardwood spent sulfite liquor (HW SSL). Genome shuffled strains (GHW301, GHW302 and GHW303) grew at higher concentrations of HW SSL (80 % v/v) compared to the HW SSL UV mutant (70 % v/v) and the wild-type (WT) strain (50 % v/v). In defined media containing acetic acid (0.70-0.90 % w/v), GHW301, GHW302 and GHW303 exhibited a shorter lag compared to the acetic acid UV mutant, while the WT did not grow. Genome shuffled strains produced more ethanol than the WT at higher concentrations of HW SSL and an aspen hydrolysate. To identify the genetic basis of inhibitor tolerance, whole genome sequencing was carried out on GHW301, GHW302 and GHW303 and compared to the WT strain. Sixty single nucleotide variations were identified that were common to all three genome shuffled strains. Of these, 40 were in gene sequences and 20 were within 5 bp-1 kb either up or downstream of protein encoding genes. Based on the mutated gene products, mutations were grouped into functional categories and affected a variety of cellular functions, demonstrating the complexity of inhibitor tolerance in yeast. Sequence analysis of UV mutants (UAA302 and UHW303) from which GHW301, GHW302 and GHW303 were derived, confirmed the success of our cross-mating based genome shuffling strategy. Whole-genome sequencing analysis allowed identification of potential gene targets for tolerance to inhibitors in lignocellulosic hydrolysates.

Chromatin enrichment for proteomics

During interphase, chromatin hosts fundamental cellular processes, such as gene expression, DNA replication and DNA damage repair. To analyze chromatin on a proteomic scale, we have developed chromatin enrichment for proteomics (ChEP), which is a simple biochemical procedure that enriches interphase chromatin in all its complexity. It enables researchers to take a 'snapshot' of chromatin and to isolate and identify even transiently bound factors. In ChEP, cells are fixed with formaldehyde; subsequently, DNA together with all cross-linked proteins is isolated by centrifugation under denaturing conditions. This approach enables the analysis of global chromatin composition and its changes, which is in contrast with existing chromatin enrichment procedures, which either focus on specific chromatin loci (e.g., affinity purification) or are limited in specificity, such as the analysis of the chromatin pellet (i.e., analysis of all insoluble nuclear material). ChEP takes half a day to complete and requires no specialized laboratory skills or equipment. ChEP enables the characterization of chromatin response to drug treatment or physiological processes. Beyond proteomics, ChEP may preclear chromatin for chromatin immunoprecipitation (ChIP) analyses.

Tools and strategies for DNA damage interactome analysis

DNA is the target of multiple endogenous and exogenous agents generating chemical lesions on the double helix. Cellular DNA damage response pathways rely on a myriad of proteins interacting with DNA alterations. The cartography of this interactome currently includes well known actors of chromatin remodelling, DNA repair or proteins hijacked from their natural functions such as transcription factors. In order to go further into the characterisation of these protein networks, proteomics-based methods began to be used in the early 2000s. The strategies are diverse and include mainly (i) damaged DNA molecules used as targets on protein microarrays, (ii) damaged DNA probes used to trap within complex cellular extracts proteins that are then separated and identified by proteomics, (iii) identification of chromatin- bound proteins after a genotoxic stress, or (iv) identification of proteins associated with other proteins already known to be part of DNA damage interactome. All these approaches have already been performed to find new proteins recognizing oxidised bases, abasic sites, strand breaks or crosslinks generated by anticancer drugs such as nitrogen mustards and platinating agents. Identified interactions are generally confirmed using complementary methods such as electromobility shift assays or surface plasmon resonance. These strategies allowed, for example, demonstration of interactions between cisplatin-DNA crosslinks and PARP-1 or the protein complex PTW/PP. The next challenging step will be to understand the biological repercussions of these newly identified interactions which may help to unravel new mechanisms involved in genetic toxicology, discover new cellular responses to anticancer drugs or identify new biomarkers and therapeutic targets.