Structural and functional analysis of Nro1/Ett1: a protein involved in translation termination in S. cerevisiae and in O2-mediated gene control in S. pombe

Monitoring translation in all reading frames downstream of weak stop codons provides mechanistic insights into the impact of nucleotide and cellular contexts

A stop codon entering the ribosome A-site is normally decoded by release factors that induce release of the polypeptide. Certain factors influence the efficiency of the termination which is in competition with elongation in either the same (readthrough) or an alternative (frameshifting) reading frame. To gain insight into the competition between these processes, we monitored translation in parallel from all three reading frames downstream of stop codons while changing the nucleotide context of termination sites or altering cellular conditions (polyamine levels). We found that P-site codon identity can have a major impact on the termination efficiency of the OPRL1 stop signal, whereas for the OAZ1 ORF1 stop signal, the P-site codon mainly influences the reading frame of non-terminating ribosomes. Changes to polyamine levels predominantly influence the termination efficiency of the OAZ1 ORF1 stop signal. In contrast, increasing polyamine levels stimulate readthrough of the OPRL1 stop signal by enhancing near-cognate decoding rather than by decreasing termination efficiency. Thus, by monitoring the four competing processes occurring at stop codons we were able to determine which is the most significantly affected upon perturbation. This approach may be useful for the interrogation of other recoding phenomena where alternative decoding processes compete with standard decoding.

The COP9 signalosome mediates the Spt23 regulated fatty acid desaturation and ergosterol biosynthesis

The COP9 signalosome (CSN) is a conserved eukaryotic complex, essential for vitality in all multicellular organisms and critical for the turnover of key cellular proteins through catalytic and non-catalytic activities. Saccharomyces cerevisiae is a powerful model organism for studying fundamental aspects of the CSN complex, since it includes a conserved enzymatic core but lacks non-catalytic activities, probably explaining its non-essentiality for life. A previous transcriptomic analysis of an S. cerevisiae strain deleted in the CSN5/RRI1 gene, encoding to the CSN catalytic subunit, revealed a downregulation of genes involved in lipid metabolism. We now show that the S. cerevisiae CSN holocomplex is essential for cellular lipid homeostasis. Defects in CSN assembly or activity lead to decreased quantities of ergosterol and unsaturated fatty acids (UFA); vacuole defects; diminished lipid droplets (LDs) size; and to accumulation of endoplasmic reticulum (ER) stress. The molecular mechanism behind these findings depends on CSN involvement in upregulating mRNA expression of SPT23. Spt23 is a novel activator of lipid desaturation and ergosterol biosynthesis. Our data reveal for the first time a functional link between the CSN holocomplex and Spt23. Moreover, CSN-dependent upregulation of SPT23 transcription is necessary for the fine-tuning of lipid homeostasis and for cellular health.

Purification of cross-linked RNA-protein complexes by phenol-toluol extraction

Recent methodological advances allowed the identification of an increasing number of RNA-binding proteins (RBPs) and their RNA-binding sites. Most of those methods rely, however, on capturing proteins associated to polyadenylated RNAs which neglects RBPs bound to non-adenylate RNA classes (tRNA, rRNA, pre-mRNA) as well as the vast majority of species that lack poly-A tails in their mRNAs (including all archea and bacteria). We have developed the Phenol Toluol extraction (PTex) protocol that does not rely on a specific RNA sequence or motif for isolation of cross-linked ribonucleoproteins (RNPs), but rather purifies them based entirely on their physicochemical properties. PTex captures RBPs that bind to RNA as short as 30 nt, RNPs directly from animal tissue and can be used to simplify complex workflows such as PAR-CLIP. Finally, we provide a global RNA-bound proteome of human HEK293 cells and the bacterium Salmonella Typhimurium.

Eukaryotic translational termination efficiency is influenced by the 3' nucleotides within the ribosomal mRNA channel

When a stop codon is at the 80S ribosomal A site, there are six nucleotides (+4 to +9) downstream that are inferred to be occupying the mRNA channel. We examined the influence of these downstream nucleotides on translation termination success or failure in mammalian cells at the three stop codons. The expected hierarchy in the intrinsic fidelity of the stop codons (UAA>UAG>>UGA) was observed, with highly influential effects on termination readthrough mediated by nucleotides at position +4 and position +8. A more complex influence was observed from the nucleotides at positions +5 and +6. The weakest termination contexts were most affected by increases or decreases in the concentration of the decoding release factor (eRF1), indicating that eRF1 binding to these signals was rate-limiting. When termination efficiency was significantly reduced by cognate suppressor tRNAs, the observed influence of downstream nucleotides was maintained. There was a positive correlation between experimentally measured signal strength and frequency of the signal in eukaryotic genomes, particularly in Saccharomyces cerevisiae and Drosophila melanogaster. We propose that termination efficiency is not only influenced by interrogation of the stop signal directly by the release factor, but also by downstream ribosomal interactions with the mRNA nucleotides in the entry channel.

Prolyl dihydroxylation of unassembled uS12/Rps23 regulates fungal hypoxic adaptation

The prolyl-3,4-dihydroxylase Ofd1 and nuclear import adaptor Nro1 regulate the hypoxic response in fission yeast by controlling activity of the sterol regulatory element-binding protein transcription factor Sre1. Here, we identify an extra-ribosomal function for uS12/Rps23 central to this regulatory system. Nro1 binds Rps23, and Ofd1 dihydroxylates Rps23 P62 in complex with Nro1. Concurrently, Nro1 imports Rps23 into the nucleus for assembly into 40S ribosomes. Low oxygen inhibits Ofd1 hydroxylase activity and stabilizes the Ofd1-Rps23-Nro1 complex, thereby sequestering Ofd1 from binding Sre1, which is then free to activate hypoxic gene expression. In vitro studies demonstrate that Ofd1 directly binds Rps23, Nro1, and Sre1 through a consensus binding sequence. Interestingly, Rps23 expression modulates Sre1 activity by changing the Rps23 substrate pool available to Ofd1. To date, oxygen is the only known signal to Sre1, but additional nutrient signals may tune the hypoxic response through control of unassembled Rps23 or Ofd1 activity.

Animals, plants, and fungi need oxygen to release energy within their cells and for other chemical reactions. Enzymes that use oxygen typically become less active when less oxygen is available, and this makes them well suited to help cells sense oxygen. These enzymes include oxygenases, some of which modify proteins by adding oxygen to specific sites in a reaction called hydroxylation. Oxygenases control how mammals adapt to low levels of oxygen – a condition referred to as hypoxia. These enzymes achieve this by hydroxylating a protein – specifically a transcription factor – that turns on genes for survival in low oxygen. Cells quickly destroy the hydroxylated transcription factor but when oxygen is limiting, it remains unmodified. This means that, rather than being destroyed, the transcription factor binds DNA, and activates genes that keep the cells alive and growing in low oxygen.

In fission yeast, an oxygenase called Ofd1 controls the activity of a transcription factor called Sre1. Yeast requires Sre1 to grow when oxygen is limiting. Exactly how Ofd1 regulates Sre1 is unknown, but the mechanism is different from that in mammals because regulation of gene expression does not need Sre1 to be hydroxylated.

Now, Clasen et al. report that Ofd1 actually hydroxylates another protein called Rps23. This protein is one of about 80 that form the cell’s protein-building machinery, the ribosome. It turns out that, before Rps23 becomes part of the ribosome, it binds Ofd1 in a complex with other proteins. The multi-protein complex then acts to hydroxylate and transport Rps23 into the nucleus, where ribosomes are built and where the cell stores its DNA. When little oxygen is around, Ofd1 cannot hydroxylate Rps23. This stops the complex from falling apart and traps Ofd1 away from the transcription factor Sre1. When not bound by Ofd1, Sre1 is free to turn on genes that allow growth at low levels of oxygen. Finally, Clasen et al. show that more unassembled Rps23 means less Ofd1 is available to inhibit Sre1, which controls the yeast cell’s response to hypoxia.

Humans have proteins similar to Ofd1 and Rps23. As such, this pathway for sensing oxygen in yeast may occur in humans too. Further work is now needed to explore if other enzymes that hydroxylate ribosomal proteins work in a similar way.

Sterol Regulatory Element Binding Protein (Srb1) Is Required for Hypoxic Adaptation and Virulence in the Dimorphic Fungus Histoplasma capsulatum

The Histoplasma capsulatum sterol regulatory element binding protein (SREBP), Srb1 is a member of the basic helix-loop-helix (bHLH), leucine zipper DNA binding protein family of transcription factors that possess a unique tyrosine (Y) residue instead of an arginine (R) residue in the bHLH region. We have determined that Srb1 message levels increase in a time dependent manner during growth under oxygen deprivation (hypoxia). To further understand the role of Srb1 during infection and hypoxia, we silenced the gene encoding Srb1 using RNA interference (RNAi); characterized the resulting phenotype, determined its response to hypoxia, and its ability to cause disease within an infected host. Silencing of Srb1 resulted in a strain of H. capsulatum that is incapable of surviving in vitro hypoxia. We found that without complete Srb1 expression, H. capsulatum is killed by murine macrophages and avirulent in mice given a lethal dose of yeasts. Additionally, silencing Srb1 inhibited the hypoxic upregulation of other known H. capsulatum hypoxia-responsive genes (HRG), and genes that encode ergosterol biosynthetic enzymes. Consistent with these regulatory functions, Srb1 silenced H. capsulatum cells were hypersensitive to the antifungal azole drug itraconazole. These data support the theory that the H. capsulatum SREBP is critical for hypoxic adaptation and is required for H. capsulatum virulence.

Structure of the ribosomal oxygenase OGFOD1 provides insights into the regio- and stereoselectivity of prolyl hydroxylases

Post-translational ribosomal protein hydroxylation is catalyzed by 2-oxoglutarate (2OG) and ferrous iron dependent oxygenases, and occurs in prokaryotes and eukaryotes. OGFOD1 catalyzes trans-3 prolyl hydroxylation at Pro62 of the small ribosomal subunit protein uS12 (RPS23) and is conserved from yeasts to humans. We describe crystal structures of the human uS12 prolyl 3-hydroxylase (OGFOD1) and its homolog from Saccharomyces cerevisiae (Tpa1p): OGFOD1 in complex with the broad-spectrum 2OG oxygenase inhibitors; N-oxalylglycine (NOG) and pyridine-2,4-dicarboxylate (2,4-PDCA) to 2.1 and 2.6 Å resolution, respectively; and Tpa1p in complex with NOG, 2,4-PDCA, and 1-chloro-4-hydroxyisoquinoline-3-carbonylglycine (a more selective prolyl hydroxylase inhibitor) to 2.8, 1.9, and 1.9 Å resolution, respectively. Comparison of uS12 hydroxylase structures with those of other prolyl hydroxylases, including the human hypoxia-inducible factor (HIF) prolyl hydroxylases (PHDs), reveals differences between the prolyl 3- and prolyl 4-hydroxylase active sites, which can be exploited for developing selective inhibitors of the different subfamilies.

Edc3 function in yeast and mammals is modulated by interaction with NAD-related compounds

The control of mRNA translation and degradation is mediated in part by a set of proteins that can inhibit translation and promote decapping, as well as function in the assembly of cytoplasmic mRNP granules referred to as processing bodies (P-bodies). The conserved enhancer of mRNA decapping 3 (Edc3) protein functions to promote both decapping and P-body assembly. Crystal structures of the YjeF_N domain in hEdc3 identified a putative binding site for a small molecule. Structure modeling of the human Edc3 Yjef_N along with other Yjef_N-containing proteins suggests that this molecule is related to NAD(H). We now show human Edc3 directly binds NADH. We also show that human and yeast Edc3 chemically modify NAD in vitro. Mutations that are predicted to disrupt the binding and/or hydrolysis of an NAD-related molecule by yeast and human Edc3 affect the control of mRNA degradation and/or P-body composition in vivo. This suggests that the interaction of Edc3 with an NAD-related molecule affects its function in the regulation of mRNA translation and degradation and provides a possible mechanism to couple the energetics of the cell to posttranscriptional control. Moreover, this provides a unique example of and lends strength to the postulated connection of metabolites, enzymes, and RNA.

Hypoxia and gene expression in eukaryotic microbes

The response of eukaryotic microbes to low-oxygen (hypoxic) conditions is strongly regulated at the level of transcription. Comparative analysis shows that some of the transcriptional regulators (such as the sterol regulatory element-binding proteins, or SREBPs) are of ancient origin and probably regulate sterol synthesis in most eukaryotic microbes. However, in some fungi SREBPs have been replaced by a zinc-finger transcription factor (Upc2). Nuclear localization of fungal SREBPs is determined by regulated proteolysis, either by site-specific proteases or by an E3 ligase complex and the proteasome. The exact mechanisms of oxygen sensing are not fully characterized but involve responding to low levels of heme and/or sterols and possibly to levels of nitric oxide and reactive oxygen species. Changes in central carbon metabolism (glycolysis and respiration) are a core hypoxic response in some, but not all, fungal species. Adaptation to hypoxia is an important virulence characteristic of pathogenic fungi.

Methylation of class I translation termination factors: structural and functional aspects

During protein synthesis, release of polypeptide from the ribosome occurs when an in frame termination codon is encountered. Contrary to sense codons, which are decoded by tRNAs, stop codons present in the A-site are recognized by proteins named class I release factors, leading to the release of newly synthesized proteins. Structures of these factors bound to termination ribosomal complexes have recently been obtained, and lead to a better understanding of stop codon recognition and its coordination with peptidyl-tRNA hydrolysis in bacteria. Release factors contain a universally conserved GGQ motif which interacts with the peptidyl-transferase centre to allow peptide release. The Gln side chain from this motif is methylated, a feature conserved from bacteria to man, suggesting an important biological role. However, methylation is catalysed by completely unrelated enzymes. The function of this motif and its post-translational modification will be discussed in the context of recent structural and functional studies.