ABSTRACT
Neuroblastoma is a highly lethal childhood tumor derived from differentiation-arrested neural crest cells1,2. Like all cancers, its growth is fueled by metabolites obtained from either circulation or local biosynthesis3,4. Neuroblastomas depend on local polyamine biosynthesis, with the inhibitor difluoromethylornithine showing clinical activity5. Here we show that such inhibition can be augmented by dietary restriction of upstream amino acid substrates, leading to disruption of oncogenic protein translation, tumor differentiation, and profound survival gains in the TH-MYCN mouse model. Specifically, an arginine/proline-free diet decreases the polyamine precursor ornithine and augments tumor polyamine depletion by difluoromethylornithine. This polyamine depletion causes ribosome stalling, unexpectedly specifically at adenosine-ending codons. Such codons are selectively enriched in cell cycle genes and low in neuronal differentiation genes. Thus, impaired translation of these codons, induced by the diet-drug combination, favors a pro-differentiation proteome. These results suggest that the genes of specific cellular programs have evolved hallmark codon usage preferences that enable coherent translational rewiring in response to metabolic stresses, and that this process can be targeted to activate differentiation of pediatric cancers.
ABSTRACT
The Ccr4-Not complex is a conserved multi protein complex with diverse roles in the mRNA life cycle. Recently we determined that the Not1 and Not4 subunits of Ccr4-Not inversely regulate mRNA solubility and thereby impact dynamics of co-translation events. One mRNA whose solubility is limited by Not4 is MMF1 encoding a mitochondrial matrix protein. In this work we uncover a mechanism that limits MMF1 overexpression and depends upon its co-translational targeting to the mitochondria. We have named this mechanism Mito-ENCay. This mechanism relies on Not4 promoting ribosome pausing during MMF1 translation, and hence the co-translational docking of the MMF1 mRNA to mitochondria via the mitochondrial targeting sequence of the Mmf1 nascent chain, the Egd1 chaperone, the Om14 mitochondrial outer membrane protein and the co-translational import machinery. Besides co-translational Mitochondrial targeting, Mito-ENCay depends upon Egd1 ubiquitination by Not4, the Caf130 subunit of the Ccr4-Not complex, the mitochondrial outer membrane protein Cis1, autophagy and no-go-decay.
Subject(s)
Saccharomyces cerevisiae Proteins , Autophagy/genetics , Membrane Proteins/genetics , Mitochondria/genetics , Mitochondria/metabolism , Mitochondrial Proteins/genetics , Mitochondrial Proteins/metabolism , Ribosomes/genetics , Ribosomes/metabolism , RNA, Messenger/genetics , RNA, Messenger/metabolism , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae/metabolism , Saccharomyces cerevisiae Proteins/genetics , Saccharomyces cerevisiae Proteins/metabolism , UbiquitinationABSTRACT
BACKGROUND: The Ccr4-Not complex is mostly known as the major eukaryotic deadenylase. However, several studies have uncovered roles of the complex, in particular of the Not subunits, unrelated to deadenylation and relevant for translation. In particular, the existence of Not condensates that regulate translation elongation dynamics has been reported. Typical studies that evaluate translation efficiency rely on soluble extracts obtained after the disruption of cells and ribosome profiling. Yet cellular mRNAs in condensates can be actively translated and may not be present in such extracts. RESULTS: In this work, by analyzing soluble and insoluble mRNA decay intermediates in yeast, we determine that insoluble mRNAs are enriched for ribosomes dwelling at non-optimal codons compared to soluble mRNAs. mRNA decay is higher for soluble RNAs, but the proportion of co-translational degradation relative to the overall mRNA decay is higher for insoluble mRNAs. We show that depletion of Not1 and Not4 inversely impacts mRNA solubilities and, for soluble mRNAs, ribosome dwelling according to codon optimality. Depletion of Not4 solubilizes mRNAs with lower non-optimal codon content and higher expression that are rendered insoluble by Not1 depletion. By contrast, depletion of Not1 solubilizes mitochondrial mRNAs, which are rendered insoluble upon Not4 depletion. CONCLUSIONS: Our results reveal that mRNA solubility defines the dynamics of co-translation events and is oppositely regulated by Not1 and Not4, a mechanism that we additionally determine may already be set by Not1 promoter association in the nucleus.
Subject(s)
Ribosomes , Saccharomyces cerevisiae Proteins , Saccharomyces cerevisiae , Codon/metabolism , Protein Biosynthesis , Ribosomes/metabolism , RNA Stability , RNA, Messenger/genetics , RNA, Messenger/metabolism , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae/metabolism , Solubility , Saccharomyces cerevisiae Proteins/genetics , Saccharomyces cerevisiae Proteins/metabolismABSTRACT
In this work, we show that Not4 and Not5 from the Ccr4-Not complex modulate translation elongation dynamics and change ribosome A-site dwelling occupancy in a codon-dependent fashion. These codon-specific changes in not5Δ cells are very robust and independent of codon position within the mRNA, the overall mRNA codon composition, or changes of mRNA expression levels. They inversely correlate with codon-specific changes in cells depleted for eIF5A and positively correlate with those in cells depleted for ribosome-recycling factor Rli1. Not5 resides in punctate loci, co-purifies with ribosomes and Rli1, but not with eIF5A, and limits mRNA solubility. Overexpression of wild-type or non-complementing Rli1 and loss of Rps7A ubiquitination enable Not4 E3 ligase-dependent translation of polyarginine stretches. We propose that Not4 and Not5 modulate translation elongation dynamics to produce a soluble proteome by Rps7A ubiquitination, dynamic condensates that limit mRNA solubility and exclude eIF5A, and a moonlighting function of Rli1.
Subject(s)
ATP-Binding Cassette Transporters/metabolism , Eukaryotic Initiation Factor-5/metabolism , Peptide Chain Elongation, Translational , Peptide Initiation Factors/metabolism , RNA-Binding Proteins/metabolism , Repressor Proteins/metabolism , Ribosome Subunits, Small/metabolism , Ribosomes/metabolism , Saccharomyces cerevisiae Proteins/metabolism , Saccharomyces cerevisiae/metabolism , Transcription Factors/metabolism , Ubiquitin-Protein Ligases/metabolism , ATP-Binding Cassette Transporters/genetics , Eukaryotic Initiation Factor-5/genetics , Gene Expression Regulation, Fungal , Peptide Initiation Factors/genetics , RNA, Fungal/genetics , RNA, Fungal/metabolism , RNA, Messenger/genetics , RNA, Messenger/metabolism , RNA-Binding Proteins/genetics , Repressor Proteins/genetics , Ribosome Subunits, Small/genetics , Ribosomes/genetics , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae Proteins/genetics , Signal Transduction , Transcription Factors/genetics , Ubiquitin-Protein Ligases/genetics , Ubiquitination , Eukaryotic Translation Initiation Factor 5AABSTRACT
Antibiotic resistance is a serious problem which may be caused by bacterial dormancy. It has been suggested that bacterial toxin-antitoxin systems induce dormancy. We analyzed the genome-wide role of Staphylococcus aureus endoribonuclease toxin MazF using RNA-Seq, Ribo-Seq and quantitative proteomics. We characterized changes in transcriptome, translatome and proteome caused by MazF, and proposed that MazF decreases translation directly by cleaving mRNAs, and indirectly, by decreasing translation factors and by promoting ribosome hibernation. Important pathways affected during the early stage of MazF induction were identified: MazF increases cell wall thickness and decreases cell division; MazF activates SsrA-system which rescues stalled ribosomes, appearing as a result of MazF mRNA cleavage. These pathways may be promising targets for new antibacterial drugs that prevent bacteria dormancy. Finally, we described the overall impact of MazF on S. aureus cell physiology, and propose one of the mechanisms by which MazF might regulate cellular changes leading to dormancy.
Subject(s)
Bacterial Toxins/metabolism , Endoribonucleases/physiology , Staphylococcus aureus/genetics , Staphylococcus aureus/metabolism , Bacterial Toxins/biosynthesis , Cell Division/genetics , Cell Wall/genetics , Cell Wall/metabolism , Endoribonucleases/biosynthesis , Endoribonucleases/metabolism , Protein Biosynthesis , Proteome , Staphylococcus aureus/enzymology , TranscriptomeABSTRACT
A crucial bacterial strategy to avoid killing by antibiotics is to enter a growth arrested state, yet the molecular mechanisms behind this process remain elusive. The conditional overexpression of mazF, the endoribonuclease toxin of the MazEF toxin-antitoxin system in Staphylococcus aureus, is one approach to induce bacterial growth arrest, but its targets remain largely unknown. We used overexpression of mazF and high-throughput sequence analysis following the exact mapping of non-phosphorylated transcriptome ends (nEMOTE) technique to reveal in vivo toxin cleavage sites on a global scale. We obtained a catalogue of MazF cleavage sites and unearthed an extended MazF cleavage specificity that goes beyond the previously reported one. We correlated transcript cleavage and abundance in a global transcriptomic profiling during mazF overexpression. We observed that MazF affects RNA molecules involved in ribosome biogenesis, cell wall synthesis, cell division and RNA turnover and thus deliver a plausible explanation for how mazF overexpression induces stasis. We hypothesize that autoregulation of MazF occurs by directly modulating the MazEF operon, such as the rsbUVW genes that regulate the sigma factor SigB, including an observed cleavage site on the MazF mRNA that would ultimately play a role in entry and exit from bacterial stasis.
Subject(s)
DNA-Binding Proteins/genetics , Endoribonucleases/genetics , Escherichia coli Proteins/genetics , Staphylococcus aureus/genetics , Toxin-Antitoxin Systems/genetics , Anti-Bacterial Agents/pharmacology , Cell Proliferation/drug effects , DNA-Binding Proteins/chemistry , Escherichia coli/genetics , Humans , Operon/genetics , RNA, Messenger/genetics , Staphylococcal Infections/drug therapy , Staphylococcal Infections/genetics , Staphylococcal Infections/microbiology , Staphylococcus aureus/drug effects , Staphylococcus aureus/pathogenicity , Substrate Specificity , Transcriptome/geneticsABSTRACT
Cancer therapy is limited, in part, by lack of specificity. Thus, identifying molecules that are selectively expressed by, and relevant for, cancer cells is of paramount medical importance. Here, we show that peptidyl-prolyl-cis-trans-isomerase (PPIase) FK506-binding protein 10 (FKBP10)-positive cells are present in cancer lesions but absent in the healthy parenchyma of human lung. FKBP10 expression negatively correlates with survival of lung cancer patients, and its downregulation causes a dramatic diminution of lung tumor burden in mice. Mechanistically, our results from gain- and loss-of-function assays show that FKBP10 boosts cancer growth and stemness via its PPIase activity. Also, FKBP10 interacts with ribosomes, and its downregulation leads to reduction of translation elongation at the beginning of open reading frames (ORFs), particularly upon insertion of proline residues. Thus, our data unveil FKBP10 as a cancer-selective molecule with a key role in translational reprogramming, stem-like traits, and growth of lung cancer.
Subject(s)
Lung Neoplasms/metabolism , Lung Neoplasms/pathology , Protein Biosynthesis , Tacrolimus Binding Proteins/metabolism , Animals , Carcinogenesis/pathology , Cell Line, Tumor , Cell Proliferation , Down-Regulation , Mice, Inbred NOD , Mice, SCID , Neoplastic Stem Cells/metabolism , Neoplastic Stem Cells/pathology , Peptidylprolyl Isomerase/metabolism , Proto-Oncogene Proteins p21(ras)/metabolism , Ribosomes/metabolismABSTRACT
Bacterial cells respond to environmental stresses by modulating their gene expression and adjusting their proteome. In Staphylococcus aureus, selective degradation by ClpP protease eliminates damaged proteins and regulates the abundance of functional proteins such as many important stress-induced transcriptional regulators. Degradation by ClpP requires the unfolding activity of partner Clp ATPases, such as ClpX and ClpC, and assistance of substrate-specific adaptor proteins such as YjbH and TrfA. Herein, we demonstrated that YjbH is aggregated in response to growth stress stimuli, such as oxidative and antibiotic stresses. In consequence, its function as an adaptor protein is compromised. YjbH controls the degradation of the stress-induced transcriptional regulator, Spx. Aggregated YjbH cannot assist Spx degradation, which results in Spx accumulation. We discovered that depending on the stress stimulus, Spx can be soluble or insoluble, and, consequently, transcriptionally active or inactive. Therefore, Spx accumulation and solubility are key components governing activation of Spx-dependent genes. Spx positively regulates expression of a ClpCP adaptor protein TrfA. TrfA in turn is required for degradation of MazE antitoxin, the unstable component of the MazEF toxin-antitoxin system, that neutralizes the endoribonuclease activity of MazF toxin. Bacterial toxin-antitoxin systems are associated with dormancy and tolerance to antibiotics that are related to chronic and relapsing infections, and it is at present a key unresolved problem in medicine. MazF activity was linked to growth stasis, yet the precise environmental signals that trigger MazE degradation and MazF activation are poorly understood. Here we propose a model where YjbH serves as a sensor of environmental stresses for downstream regulation of MazEF activity. YjbH aggregation, soluble Spx, and TrfA, coordinately control MazE antitoxin levels and consequently MazF toxin activity. This model implies that certain stress conditions culminate in modulation of MazF activity resulting in growth stasis during in vivo infections.
ABSTRACT
Staphylococcus aureus is a major human pathogen and represents a clinical challenge because of widespread antibiotic resistance. Methicillin resistant Staphylococcus aureus (MRSA) is particularly problematic and originates by the horizontal acquisition of mecA encoding PBP2a, an extracellular membrane anchored transpeptidase, which confers resistance to ß-lactam antibiotics by allosteric gating of its active site channel. Herein, we show that dual disruption of PrsA, a lipoprotein chaperone displaying anti-aggregation activity, together with HtrA1, a membrane anchored chaperone/serine protease, resulted in severe and synergistic attenuation of PBP2a folding that restores sensitivity to ß-lactams such as oxacillin. Purified PBP2a has a pronounced unfolding transition initiating at physiological temperatures that leads to irreversible precipitation and complete loss of activity. The concordance of genetic and biochemical data highlights the necessity for extracellular protein folding factors governing MRSA ß-lactam resistance. Targeting the PBP2a folding pathway represents a particularly attractive adjuvant strategy to combat antibiotic resistance.
Subject(s)
Bacterial Proteins/genetics , High-Temperature Requirement A Serine Peptidase 1/genetics , Lipoproteins/genetics , Membrane Proteins/genetics , Methicillin-Resistant Staphylococcus aureus/genetics , Penicillin-Binding Proteins/genetics , Staphylococcal Infections/microbiology , beta-Lactam Resistance , Anti-Bacterial Agents/pharmacology , Bacterial Proteins/chemistry , Bacterial Proteins/metabolism , High-Temperature Requirement A Serine Peptidase 1/metabolism , Humans , Lipoproteins/metabolism , Membrane Proteins/metabolism , Methicillin-Resistant Staphylococcus aureus/drug effects , Microbial Sensitivity Tests , Models, Molecular , Penicillin-Binding Proteins/chemistry , Protein Conformation , Structure-Activity RelationshipABSTRACT
The assembly of large multimeric complexes in the crowded cytoplasm is challenging. Here we reveal a mechanism that ensures accurate production of the yeast proteasome, involving ribosome pausing and co-translational assembly of Rpt1 and Rpt2. Interaction of nascent Rpt1 and Rpt2 then lifts ribosome pausing. We show that the N-terminal disordered domain of Rpt1 is required to ensure efficient ribosome pausing and association of nascent Rpt1 protein complexes into heavy particles, wherein the nascent protein complexes escape ribosome quality control. Immunofluorescence and in situ hybridization studies indicate that Rpt1- and Rpt2-encoding messenger RNAs co-localize in these particles that contain, and are dependent on, Not1, the scaffold of the Ccr4-Not complex. We refer to these particles as Not1-containing assemblysomes, as they are smaller than and distinct from other RNA granules such as stress granules and GW- or P-bodies. Synthesis of Rpt1 with ribosome pausing and Not1-containing assemblysome induction is conserved from yeast to human cells.
Subject(s)
Proteasome Endopeptidase Complex/metabolism , Adenosine Triphosphatases/genetics , Adenosine Triphosphatases/metabolism , Algorithms , Cell Cycle Proteins/genetics , Cell Cycle Proteins/metabolism , Cell Line, Tumor , Genome, Fungal/genetics , Humans , In Situ Hybridization , Male , Oligonucleotides/genetics , Oligonucleotides/metabolism , Proteasome Endopeptidase Complex/genetics , Protein Binding , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae/metabolism , Saccharomyces cerevisiae Proteins/genetics , Saccharomyces cerevisiae Proteins/metabolism , Transcription Factors/genetics , Transcription Factors/metabolismABSTRACT
The Ccr4-Not complex is an essential multi-subunit protein complex that plays a fundamental role in eukaryotic mRNA metabolism and has a multitude of different roles that impact eukaryotic gene expression . It has a conserved core of three Not proteins, the Ccr4 protein, and two Ccr4 associated factors, Caf1 and Caf40. A fourth Not protein, Not4, is conserved, but is only a stable subunit of the complex in yeast. Certain subunits have been duplicated during evolution, with functional divergence, such as Not3 in yeast, and Ccr4 or Caf1 in human. However the complex includes only one homolog for each protein. In addition, species-specific subunits are part of the complex, such as Caf130 in yeast or Not10 and Not11 in human. Two conserved catalytic functions are associated with the complex, deadenylation and ubiquitination . The complex adopts an L-shaped structure, in which different modules are bound to a large Not1 scaffold protein. In this chapter we will summarize our current knowledge of the architecture of the complex and of the structure of its constituents.
Subject(s)
Multiprotein Complexes/chemistry , Multiprotein Complexes/metabolism , Nuclear Receptor Subfamily 4, Group A, Member 2/chemistry , Nuclear Receptor Subfamily 4, Group A, Member 2/metabolism , Receptors, CCR4/chemistry , Receptors, CCR4/metabolism , Humans , Protein Binding , Ribonucleases/chemistry , Ribonucleases/metabolism , Saccharomyces cerevisiae/chemistry , Saccharomyces cerevisiae/metabolism , Saccharomyces cerevisiae Proteins/chemistry , Saccharomyces cerevisiae Proteins/metabolism , Transcription Factors/metabolismABSTRACT
The current understanding of gene expression considers transcription and translation to be independent processes. Challenging this notion, we found that translation efficiency is determined during transcription elongation through the imprinting of mRNAs with Not1, the central scaffold of the Ccr4-Not complex. We determined that another subunit of the complex, Not5, defines Not1 binding to specific mRNAs, particularly those produced from ribosomal protein genes. This imprinting mechanism specifically regulates ribosomal protein gene expression, which in turn determines the translational capacity of cells. We validate our model by SILAC and polysome profiling experiments. As a proof of concept, we demonstrate that enhanced translation compensates for transcriptional elongation stress. Taken together, our data indicate that in addition to defining mRNA stability, components of the Ccr4-Not imprinting complex regulate RNA translatability, thus ensuring global gene expression homeostasis.
Subject(s)
Cell Cycle Proteins/metabolism , Protein Biosynthesis , Ribonucleases/metabolism , Saccharomyces cerevisiae Proteins/metabolism , Saccharomyces cerevisiae/cytology , Saccharomyces cerevisiae/genetics , Transcription Elongation, Genetic , Transcription Factors/metabolism , Cell Cycle Proteins/genetics , Cell Nucleus/metabolism , Gene Expression Regulation, Fungal , Genes, Fungal , Protein Binding/genetics , RNA, Fungal/metabolism , RNA, Messenger/genetics , RNA, Messenger/metabolism , Ribosomal Proteins/metabolism , Saccharomyces cerevisiae/metabolism , Saccharomyces cerevisiae Proteins/genetics , Stress, Physiological/genetics , Transcription Factors/genetics , Transcriptome/genetics , Up-Regulation/geneticsABSTRACT
Recent studies have suggested that a sub-complex of RNA polymerase II composed of Rpb4 and Rpb7 couples the nuclear and cytoplasmic stages of gene expression by associating with newly made mRNAs in the nucleus, and contributing to their translation and degradation in the cytoplasm. Here we show by yeast two hybrid and co-immunoprecipitation experiments, followed by ribosome fractionation and fluorescent microscopy, that a subunit of the Ccr4-Not complex, Not5, is essential in the nucleus for the cytoplasmic functions of Rpb4. Not5 interacts with Rpb4; it is required for the presence of Rpb4 in polysomes, for interaction of Rpb4 with the translation initiation factor eIF3 and for association of Rpb4 with mRNAs. We find that Rpb7 presence in the cytoplasm and polysomes is much less significant than that of Rpb4, and that it does not depend upon Not5. Hence Not5-dependence unlinks the cytoplasmic functions of Rpb4 and Rpb7. We additionally determine with RNA immunoprecipitation and native gel analysis that Not5 is needed in the cytoplasm for the co-translational assembly of RNA polymerase II. This stems from the importance of Not5 for the association of the R2TP Hsp90 co-chaperone with polysomes translating RPB1 mRNA to protect newly synthesized Rpb1 from aggregation. Hence taken together our results show that Not5 interconnects translation and transcription.
Subject(s)
Protein Biosynthesis , RNA, Messenger/genetics , Saccharomyces cerevisiae Proteins/genetics , Transcription Factors/genetics , Transcription, Genetic , Cell Nucleus/genetics , Cytoplasm/genetics , Eukaryotic Initiation Factor-3/genetics , Gene Expression Regulation, Fungal , Polyribosomes/genetics , RNA Polymerase II/genetics , RNA Stability , Saccharomyces cerevisiae , Saccharomyces cerevisiae Proteins/metabolism , Transcription Factors/metabolismABSTRACT
Cotranslational quality control (QC) is the mechanism by which the cell checks the integrity of newly synthesized proteins and mRNAs. In the event of mistakes these molecules are degraded. The Ccr4-Not complex has been proposed to play a role in this process. It contains both deadenylation and ubiquitination activities, thus it may target both aberrant proteins and mRNAs. Deadenylation is the first step in mRNA degradation. In yeast it is performed by the Ccr4 subunit of the Ccr4-Not complex. Another complex subunit, namely Not4, is a RING E3 ligase and it provides the ubiquitination activity of the complex. It was found associated with translating ribosomes. Thus, it has been suggested that Not4 is involved in ribosome-associated ubiquitination and degradation of aberrant peptides. However, several other E3 ligases have been associated with peptide ubiquitination on the ribosome and the relevance of Not4 in this process remains unclear. In this review we summarize the recent data and suggest a role for Not4 in cotranslational protein QC.
ABSTRACT
Eukaryotic cells control their proteome by regulating protein production and protein clearance. Protein production is determined to a large extent by mRNA levels, whereas protein degradation depends mostly upon the proteasome. Dysfunction of the proteasome leads to the accumulation of non-functional proteins that can aggregate, be toxic for the cell, and, in extreme cases, lead to cell death. mRNA levels are controlled by their rates of synthesis and degradation. Recent evidence indicates that these rates have oppositely co-evolved to ensure appropriate mRNA levels. This opposite co-evolution has been correlated with the mutations in the Ccr4-Not complex. Consistently, the deadenylation enzymes responsible for the rate-limiting step in eukaryotic mRNA degradation, Caf1 and Ccr4, are subunits of the Ccr4-Not complex. Another subunit of this complex is a RING E3 ligase, Not4. It is essential for cellular protein solubility and has been proposed to be involved in co-translational quality control. An open question has been whether this role of Not4 resides strictly in the regulation of the deadenylation module of the Ccr4-Not complex. However, Not4 is important for proper assembly of the proteasome, and the Ccr4-Not complex may have multiple functional modules that participate in protein quality control in different ways. In this work we studied how the functions of the Caf1/Ccr4 and Not4 modules are connected. We concluded that Not4 plays a role in protein quality control independently of the Ccr4 deadenylase, and that it is involved in clearance of aberrant proteins at least in part via the proteasome.
Subject(s)
Ribonucleases/metabolism , Saccharomyces cerevisiae Proteins/metabolism , Saccharomyces cerevisiae/enzymology , Ubiquitin-Protein Ligases/metabolism , Azetidinecarboxylic Acid/pharmacology , Cathepsin A/metabolism , Enzyme Stability/drug effects , Gene Deletion , Mutant Proteins/metabolism , Phenotype , Polyribosomes/drug effects , Polyribosomes/metabolism , Polyubiquitin/metabolism , Proteasome Endopeptidase Complex/metabolism , Protein Biosynthesis/drug effects , Protein Structure, Quaternary , Proteolysis/drug effects , Repressor Proteins , Saccharomyces cerevisiae/drug effects , Substrate Specificity/drug effects , Temperature , Ubiquitination/drug effectsABSTRACT
The Ccr4-Not complex is a conserved multi-subunit complex in eukaryotes that carries 2 enzymatic activities: ubiquitination mediated by the Not4 RING E3 ligase and deadenylation mediated by the Ccr4 and Caf1 orthologs. This complex has been implicated in all aspects of the mRNA life cycle, from synthesis of mRNAs in the nucleus to their degradation in the cytoplasm. More recently the complex has also been implicated in many aspects of the life cycle of proteins, from quality control during synthesis of peptides, to assembly of protein complexes and protein degradation. Consistently, the Ccr4-Not complex is found both in the nucleus, where it is connected to transcribing ORFs, and in the cytoplasm, where it was revealed to be both associated with translating ribosomes and in RNA processing bodies. This functional and physical presence of the Ccr4-Not complex at all stages of gene expression raises the question of its fundamental role. This review will summarize recent evidence designing the Not3/5 module of the Ccr4-Not complex as a functional module involved in coordination of the regulation of gene expression between the nucleus and the cytoplasm.
Subject(s)
Receptors, CCR4/metabolism , Saccharomyces cerevisiae Proteins/metabolism , Transcription Factors/metabolism , Animals , Cell Nucleus/metabolism , Cytoplasm/metabolism , Gene Expression Regulation, Fungal , Humans , Protein Binding , RNA, Messenger/metabolism , Receptors, CCR4/chemistry , Receptors, CCR4/genetics , Saccharomyces cerevisiae/metabolism , Saccharomyces cerevisiae Proteins/chemistry , Saccharomyces cerevisiae Proteins/genetics , Transcription Factors/chemistry , Transcription Factors/geneticsABSTRACT
In this study, we determine that Saccharomyces cerevisiae Not4 E3 ligase ubiquitinates Rps7A in vivo and in vitro, but not its paralogue, Rps7B. Ubiquitinated Rps7A is detectable only in 80S and polysomes, but not in free 40S fractions. A different role of the Rps7 paralogues in vivo is supported by the observation that the deletion of Rps7A but not Rps7B is sensitive to translational inhibitors and leads to an accumulation of aggregated proteins. An important accumulation of aggregated proteins that include ribosomal proteins and ribosome-associated chaperones is also observed in cells lacking Not4. A contribution of Not4 to ribosomal function extending beyond Rps7A ubiquitination is supported by the observation that the deletion of Not4 displays a synthetic slow growth phenotype when combined with the deletion of either one of the two Rps7 paralogues. Not4 is detectable in polysome fractions, as are other subunits of the Ccr4-Not complex such as Not5. The optimal presence of Not5 in polysomes is dependent upon Not4 and the deletion of Not5 leads to a dramatic reduction of polysomes. These results lead us to suggest that Not4 contributes to normal polysome levels and is important for cellular protein solubility maybe in part by ubiquitination of Rps7A.
Subject(s)
Polyribosomes/metabolism , Saccharomyces cerevisiae Proteins/metabolism , Saccharomyces cerevisiae/enzymology , Transcription Factors/metabolism , Amino Acid Sequence , Gene Deletion , Molecular Sequence Data , Repressor Proteins , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae Proteins/genetics , Transcription Factors/genetics , Ubiquitin-Protein Ligases/genetics , Ubiquitin-Protein Ligases/metabolism , UbiquitinationABSTRACT
The Ccr4-Not complex is a unique, essential and conserved multi-subunit complex that acts at the level of many different cellular functions to regulate gene expression. Two enzymatic activities, namely ubiquitination and deadenylation, are provided by different subunits of the complex. However, studies over the last decade have demonstrated a tantalizing multi-functionality of this complex that extends well beyond its identified enzymatic activities. Most of our initial knowledge about the Ccr4-Not complex stemmed from studies in yeast, but an increasing number of reports on this complex in other species are emerging. In this review we will discuss the structure and composition of the complex, and describe the different cellular functions with which the Ccr4-Not complex has been connected in different organisms. Finally, based upon our current state of knowledge, we will propose a model to explain how one complex can provide such multi-functionality. This model suggests that the Ccr4-Not complex might function as a "chaperone platform".
Subject(s)
Gene Expression Regulation , Receptors, CCR4/physiology , Adenosine Monophosphate/metabolism , Animals , DNA Repair , Humans , Models, Biological , Molecular Chaperones/metabolism , Receptors, CCR4/chemistry , Transcription, Genetic , UbiquitinationABSTRACT
In this study we determine that the Not4 E3 ligase is important for proteasome integrity. Consequently, deletion of Not4 leads to an accumulation of polyubiquitinated proteins and reduced levels of free ubiquitin. In the absence of Not4, the proteasome regulatory particle (RP) and core particle (CP) form salt-resistant complexes, and all other forms of RPs are unstable. Not4 can associate with RP species present in purified proteasome holoenzyme but not with purified RP. Additionally, Not4 interacts with Ecm29, a protein that stabilizes the proteasome. Interestingly, Ecm29 is identified in RP species that are inactive and not detectable in cells lacking Not4. In the absence of Not4, Ecm29 interacts less well with the proteasome and becomes ubiquitinated and degraded. Our results characterize Ecm29 as a proteasome chaperone whose appropriate interaction with the proteasome requires Not4.