Upf1 and Upf2 proteins mediate normal yeast mRNA degradation when translation initiation is limited

CLUH regulates mitochondrial metabolism by controlling translation and decay of target mRNAs

Mitochondria are essential organelles that host crucial metabolic pathways and produce adenosine triphosphate. The mitochondrial proteome is heterogeneous among tissues and can dynamically change in response to different metabolic conditions. Although the transcriptional programs that govern mitochondrial biogenesis and respiratory function are well known, posttranscriptional regulatory mechanisms remain unclear. In this study, we show that the cytosolic RNA-binding protein clustered mitochondria homologue (CLUH) regulates the expression of a mitochondrial protein network supporting key metabolic programs required under nutrient deprivation. CLUH exerts its function by controlling the stability and translation of target messenger RNAs. In the absence of Cluh, mitochondria are severely depleted of crucial enzymes involved in catabolic energy-converting pathways. CLUH preserves oxidative mitochondrial function and glucose homeostasis, thus preventing death at the fetal-neonatal transition. In the adult liver, CLUH ensures maximal respiration capacity and the metabolic response to starvation. Our results shed new light on the posttranscriptional mechanisms controlling the expression of mitochondrial proteins and suggest novel strategies to tailor mitochondrial function to physiological and pathological conditions.

Interrelations between translation and general mRNA degradation in yeast

Messenger RNA (mRNA) degradation is an important element of gene expression that can be modulated by alterations in translation, such as reductions in initiation or elongation rates. Reducing translation initiation strongly affects mRNA degradation by driving mRNA toward the assembly of a decapping complex, leading to decapping. While mRNA stability decreases as a consequence of translational inhibition, in apparent contradiction several external stresses both inhibit translation initiation and stabilize mRNA. A key difference in these processes is that stresses induce multiple responses, one of which stabilizes mRNAs at the initial and rate-limiting step of general mRNA decay. Because this increase in mRNA stability is directly induced by stress, it is independent of the translational effects of stress, which provide the cell with an opportunity to assess its response to changing environmental conditions. After assessment, the cell can store mRNAs, reinitiate their translation or, alternatively, embark on a program of enhanced mRNA decay en masse. Finally, recent results suggest that mRNA decay is not limited to non-translating messages and can occur when ribosomes are not initiating but are still elongating on mRNA. This review will discuss the models for the mechanisms of these processes and recent developments in understanding the relationship between translation and general mRNA degradation, with a focus on yeast as a model system.

How to cite this article: WIREs RNA 2014, 5:747–763. doi: 10.1002/wrna.1244

Deciphering the rules by which 5'-UTR sequences affect protein expression in yeast

The 5'-untranslated region (5'-UTR) of mRNAs contains elements that affect expression, yet the rules by which these regions exert their effect are poorly understood. Here, we studied the impact of 5'-UTR sequences on protein levels in yeast, by constructing a large-scale library of mutants that differ only in the 10 bp preceding the translational start site of a fluorescent reporter. Using a high-throughput sequencing strategy, we obtained highly accurate measurements of protein abundance for over 2,000 unique sequence variants. The resulting pool spanned an approximately sevenfold range of protein levels, demonstrating the powerful consequences of sequence manipulations of even 1-10 nucleotides immediately upstream of the start codon. We devised computational models that predicted over 70% of the measured expression variability in held-out sequence variants. Notably, a combined model of the most prominent features successfully explained protein abundance in an additional, independently constructed library, whose nucleotide composition differed greatly from the library used to parameterize the model. Our analysis reveals the dominant contribution of the start codon context at positions -3 to -1, mRNA secondary structure, and out-of-frame upstream AUGs (uAUGs) to phenotypic diversity, thereby advancing our understanding of how protein levels are modulated by 5'-UTR sequences, and paving the way toward predictably tuning protein expression through manipulations of 5'-UTRs.

Expression proteomics of UPF1 knockdown in HeLa cells reveals autoregulation of hnRNP A2/B1 mediated by alternative splicing resulting in nonsense-mediated mRNA decay

Background

In addition to acting as an RNA quality control pathway, nonsense-mediated mRNA decay (NMD) plays roles in regulating normal gene expression. In particular, the extent to which alternative splicing is coupled to NMD and the roles of NMD in regulating uORF containing transcripts have been a matter of debate.

Results

In order to achieve a greater understanding of NMD regulated gene expression we used 2D-DiGE proteomics technology to examine the changes in protein expression induced in HeLa cells by UPF1 knockdown. QPCR based validation of the corresponding mRNAs, in response to both UPF1 knockdown and cycloheximide treatment, identified 17 bona fide NMD targets. Most of these were associated with bioinformatically predicted NMD activating features, predominantly upstream open reading frames (uORFs). Strikingly, however, the majority of transcripts up-regulated by UPF1 knockdown were either insensitive to, or even down-regulated by, cycloheximide treatment. Furthermore, the mRNA abundance of several down-regulated proteins failed to change upon UPF1 knockdown, indicating that UPF1's role in regulating mRNA and protein abundance is more complex than previously appreciated. Among the bona fide NMD targets, we identified a highly conserved AS-NMD event within the 3' UTR of the HNRNPA2B1 gene. Overexpression of GFP tagged hnRNP A2 resulted in a decrease in endogenous hnRNP A2 and B1 mRNA with a concurrent increase in the NMD sensitive isoforms.

Conclusions

Despite the large number of changes in protein expression upon UPF1 knockdown, a relatively small fraction of them can be directly attributed to the action of NMD on the corresponding mRNA. From amongst these we have identified a conserved AS-NMD event within HNRNPA2B1 that appears to mediate autoregulation of HNRNPA2B1 expression levels.

Initiation-mediated mRNA decay in yeast affects heat-shock mRNAs, and works through decapping and 5'-to-3' hydrolysis

The degradation of mRNA in the yeast Saccharomyces cerevisiae takes place through several related pathways. In the most general mRNA-decay pathway, that of poly(A)-dependent decay, the normal shortening of the poly(A) tail on an mRNA molecule by deadenylation triggers mRNA decapping by the enzyme Dcp1p, followed by exonucleolytic digestion by Xrn1p. A specialized mRNA-decay pathway, termed nonsense-mediated decay, comes into play for mRNAs that contain an early nonsense codon. This pathway operates through the Upf proteins in addition to Dcp1p and Xrn1p. Previously, we identified a different specialized mRNA-decay pathway, the initiation-mediated decay pathway, and showed that it affects two Hsp70 heat-shock mRNAs under conditions of slowed translation initiation. Here we report that initiation-mediated mRNA decay also works through the Dcp1 and Xrn1 enzymes, and requires ongoing transcription by RNA polymerase II. We show that several other heat-shock mRNAs, including two from the Hsp90 gene family and three more from the Hsp70 gene family, are also subject to initiation-mediated decay, whereas a variety of non-heat-shock mRNAs are not affected.

An NMD pathway in yeast involving accelerated deadenylation and exosome-mediated 3'-->5' degradation

Eukaryotic mRNAs containing premature termination codons are subjected to accelerated turnover, known as nonsense-mediated decay (NMD). Recognition of translation termination events as premature requires a surveillance complex, which includes the RNA helicase Upf1p. In Saccharomyces cerevisiae, NMD provokes rapid decapping followed by 5'-->3' exonucleolytic decay. Here we report an alternative, decapping-independent NMD pathway involving deadenylation and subsequent 3'-->5' exonucleolytic decay. Accelerated turnover via this pathway required Upf1p and was blocked by the translation inhibitor cycloheximide. Degradation of the deadenylated mRNA required the Rrp4p and Ski7p components of the cytoplasmic exosome complex, as well as the putative RNA helicase Ski2p. We conclude that recognition of NMD substrates by the Upf surveillance complex can target mRNAs to rapid deadenylation and exosome-mediated degradation.

Identification of a human decapping complex associated with hUpf proteins in nonsense-mediated decay

Decapping is a key step in general and regulated mRNA decay. In Saccharomyces cerevisiae it constitutes a rate-limiting step in the nonsense-mediated decay pathway that rids cells of mRNAs containing premature termination codons. Here two human decapping enzymes are identified, hDcp1a and hDcp2, as well as a homolog of hDcp1a, termed hDcp1b. Transiently expressed hDcp1a and hDcp2 proteins localize primarily to the cytoplasm and form a complex in human cell extracts. hDcp1a and hDcp2 copurify with decapping activity, an activity sensitive to mutation of critical hDcp residues. Importantly, coimmunoprecipitation assays demonstrate that hDcp1a and hDcp2 interact with the nonsense-mediated decay factor hUpf1, both in the presence and in the absence of the other hUpf proteins, hUpf2, hUpf3a, and hUpf3b. These data suggest that a human decapping complex may be recruited to mRNAs containing premature termination codons by the hUpf proteins.

The abundant retinal protein of the Chlamydomonas eye is not the photoreceptor for phototaxis and photophobic responses

The chlamyopsin gene (cop) encodes the most abundant eyespot protein in the unicellular green alga Chlamydomonas reinhardtii. This opsin-related protein (COP) binds retinal and was thought to be the photoreceptor controlling photomovement responses via a set of photoreceptor currents. Unfortunately, opsin-deficient mutants are not available and targeted disruption of non-selectable nuclear genes is not yet possible in any green alga. Here we show that intron-containing gene fragments directly linked to their intron-less antisense counterpart provide efficient post-transcriptional gene silencing (PTGS) in C. reinhardtii, thus allowing an efficient reduction of a specific gene product in a green alga. In opsin-deprived transformants, flash-induced photoreceptor currents (PC) are left unchanged. Moreover, photophobic responses as studied by motion analysis and phototaxis tested in a light-scattering assay were indistinguishable from the responses of untransformed wild-type cells. We conclude that phototaxis and photophobic responses in C. reinhardtii are triggered by an as yet unidentified rhodopsin species.

Characterization of a general stabilizer element that blocks deadenylation-dependent mRNA decay

mRNA degradation is a regulated process that can play an important role in determining the level of expression of specific genes. The rate at which a specific mRNA is degraded depends largely on specific cis-acting sequences located throughout the transcript. cis-Acting destabilizer sequences that promote increased rates of decay have been identified in several short-lived mRNAs. However, little is known about elements that promote stability, known as stabilizer elements (STEs), and how they function. The work presented here describes the characterization of a STE in the PGK1 transcript. The PGK1 stabilizer element (P-STE) has been delineated to a 64-nucleotide sequence from the coding region that can stabilize a chimeric transcript containing the instability elements from the 3'-untranslated region of the MFA2 transcript. The P-STE is located within the PGK1 coding region and functions when located in the translated portion of the transcript and at a minimum distance from the 3'-untranslated region. These results further support the link between translation and mRNA degradation. A conserved sequence in the TEF1/2 transcript has been identified that also functions as a STE, suggesting that this sequence element maybe a general stability determinant found in other yeast mRNAs.

Mechanisms and control of mRNA decapping in Saccharomyces cerevisiae

The process of mRNA turnover is a critical component of the regulation of gene expression. In the past few years a discrete set of pathways for the degradation of polyadenylated mRNAs in eukaryotic cells have been described. A major pathway of mRNA degradation in yeast occurs by deadenylation of the mRNA, which leads to a decapping reaction, thereby exposing the mRNA to rapid 5' to 3' exonucleolytic degradation. A critical step in this pathway is decapping, since it effectively terminates the existence of the mRNA and is the site of numerous control inputs. In this review, we discuss the properties of the decapping enzyme and how its activity is regulated to give rise to differential mRNA turnover. A key point is that decapping appears to be controlled by access of the enzyme to the cap structure in a competition with the translation initiation complex. Strikingly, several proteins required for mRNA decapping show interactions with the translation machinery and suggest possible mechanisms for the triggering of mRNA decapping.

Eukaryote-specific domains in translation initiation factors: implications for translation regulation and evolution of the translation system

Computational analysis of sequences of proteins involved in translation initiation in eukaryotes reveals a number of specific domains that are not represented in bacteria or archaea. Most of these eukaryote-specific domains are known or predicted to possess an alpha-helical structure, which suggests that such domains are easier to invent in the course of evolution than are domains of other structural classes. A previously undetected, conserved region predicted to form an alpha-helical domain is delineated in the initiation factor eIF4G, in Nonsense-mediated mRNA decay 2 protein (NMD2/UPF2), in the nuclear cap-binding CBP80, and in other, poorly characterized proteins, which is named the NIC (NMD2, eIF4G, CBP80) domain. Biochemical and mutagenesis data on NIC-containing proteins indicate that this predicted domain is one of the central adapters in the regulation of mRNA processing, translation, and degradation. It is demonstrated that, in the course of eukaryotic evolution, initiation factor eIF4G, of which NIC is the core, conserved portion, has accreted several additional, distinct predicted domains such as MI (MA-3 and eIF4G ) and W2, which probably was accompanied by acquisition of new regulatory interactions.

Inhibition of mRNA turnover in yeast by an xrn1 mutation enhances the requirement for eIF4E binding to eIF4G and for proper capping of transcripts by Ceg1p

Null mutants of XRN1, encoding the major cytoplasmic exoribonuclease in yeast, are viable but accumulate decapped, deadenylated transcripts. A screen for mutations synthetic lethal with xrn1Delta identified a mutation in CDC33, encoding eIF4E. This mutation (glutamate to glycine at position 72) affected a highly conserved residue involved in interaction with eIF4G. Synthetic lethality between xrn1 and cdc33 was not relieved by high-copy expression of eIF4G or by disruption of the yeast eIF4E binding protein Caf20p. High-copy expression of a mutant eIF4G defective for eIF4E binding resulted in a dominant negative phenotype in an xrn1 mutant, indicating the importance of this interaction in an xrn1 mutant. Another allele of CDC33, cdc33-1, along with mutations in CEG1, encoding the nuclear guanylyltransferase, were also synthetic lethal with xrn1Delta, whereas mutations in PRT1, encoding a subunit of eIF3, were not. Mutations in CDC33, CEG1, PRT1, PAB1, and TIF4631, encoding eIF4G1, have been shown to lead to destabilization of mRNAs. Although such destabilization in cdc33, ceg1, and pab1 mutants can be partially suppressed by an xrn1 mutation, we observed synthetic lethality between xrn1 and either cdc33 or ceg1 and no suppression of the inviability of a pab1 null mutation by xrn1Delta. Thus, the inhibition of mRNA turnover by blocking Xrn1p function does not suppress the lethality of defects upstream in the turnover pathway but it does enhance the requirement for (7)mG caps and for proper formation of the eIF4E/eIF4G cap recognition complex.

Mutations in translation initiation factors lead to increased rates of deadenylation and decapping of mRNAs in Saccharomyces cerevisiae

The turnover of most mRNAs in Saccharomyces cerevisiae begins with deadenylation followed by decapping and 5'-->3' exonucleolytic digestion. An important question involves the mechanisms that allow particular mRNAs to exhibit different rates of both deadenylation and decapping. Since the cap structure plays a critical role in the assembly of translation initiation factors, we hypothesized that the status of the cytoplasmic cap binding complex would affect the rate of decapping. To test this hypothesis, we examined mRNA decay rates in yeast strains that were defective in several translation initiation factors that are part of the cap binding complex. These experiments yielded three significant observations. First, any mutation known to inhibit translation initiation also increased the rate of decapping. Second, decapping still occurred only after deadenylation, suggesting that the ability of the poly(A) tail to inhibit decapping does not require efficient translation of the transcript. Third, mutants with defects in translation initiation factors also showed an increase in the rate of deadenylation, suggesting that the rate of deadenylation may be controlled primarily by the translation status of the transcript. These results argue that the nature of the translation initiation complex is a critical factor in determining the mRNA half-life. This view also implies that some cis-acting sequences that modulate mRNA decay rate do so by affecting the translation status of the transcript.

The yeast protein complex containing cdc68 and pob3 mediates core-promoter repression through the cdc68 N-terminal domain

Transcription of nuclear genes usually involves trans-activators, whereas repression is exerted by chromatin. For several genes the transcription mediated by trans-activators and the repression mediated by chromatin depend on the CP complex, a recently described abundant yeast nuclear complex of the Pob3 and Cdc68/Spt16 proteins. We report that the N-terminal third of the Saccharomyces cerevisiae Cdc68 protein is dispensable for gene activation but necessary for the maintenance of chromatin repression. The absence of this 300-residue N-terminal domain also decreases the need for the Swi/Snf chromatin-remodeling complex in transcription and confers an Spt- effect characteristic of chromatin alterations. The repression domain, and indeed the entire Cdc68 protein, is highly conserved, as shown by the sequence of the Cdc68 functional homolog from the yeast Kluyveromyces lactis and by database searches. The repression-defective (truncated) form of Cdc68 is stable but less active at high temperatures, whereas the known point-mutant form of Cdc68, encoded by three independent mutant alleles, alters the N-terminal repression domain and destabilizes the mutant protein.

Posttranscriptional control of gene expression in yeast

Studies of the budding yeast Saccharomyces cerevisiae have greatly advanced our understanding of the posttranscriptional steps of eukaryotic gene expression. Given the wide range of experimental tools applicable to S. cerevisiae and the recent determination of its complete genomic sequence, many of the key challenges of the posttranscriptional control field can be tackled particularly effectively by using this organism. This article reviews the current knowledge of the cellular components and mechanisms related to translation and mRNA decay, with the emphasis on the molecular basis for rate control and gene regulation. Recent progress in characterizing translation factors and their protein-protein and RNA-protein interactions has been rapid. Against the background of a growing body of structural information, the review discusses the thermodynamic and kinetic principles that govern the translation process. As in prokaryotic systems, translational initiation is a key point of control. Modulation of the activities of translational initiation factors imposes global regulation in the cell, while structural features of particular 5' untranslated regions, such as upstream open reading frames and effector binding sites, allow for gene-specific regulation. Recent data have revealed many new details of the molecular mechanisms involved while providing insight into the functional overlaps and molecular networking that are apparently a key feature of evolving cellular systems. An overall picture of the mechanisms governing mRNA decay has only very recently begun to develop. The latest work has revealed new information about the mRNA decay pathways, the components of the mRNA degradation machinery, and the way in which these might relate to the translation apparatus. Overall, major challenges still to be addressed include the task of relating principles of posttranscriptional control to cellular compartmentalization and polysome structure and the role of molecular channelling in these highly complex expression systems.