Roles of the HEAT repeat proteins Utp10 and Utp20 in 40S ribosome maturation

Surface Proteome of Extracellular Vesicles and Correlation Analysis Reveal Breast Cancer Biomarkers

Breast cancer (BC) is the second most frequently diagnosed cancer and accounts for approximately 25% of new cancer cases in Canadian women. Using biomarkers as a less-invasive BC diagnostic method is currently under investigation but is not ready for practical application in clinical settings. During the last decade, extracellular vesicles (EVs) have emerged as a promising source of biomarkers because they contain cancer-derived proteins, RNAs, and metabolites. In this study, EV proteins from small EVs (sEVs) and medium EVs (mEVs) were isolated from BC MDA-MB-231 and MCF7 and non-cancerous breast epithelial MCF10A cell lines and then analyzed by two approaches: global proteomic analysis and enrichment of EV surface proteins by Sulfo-NHS-SS-Biotin labeling. From the first approach, proteomic profiling identified 2459 proteins, which were subjected to comparative analysis and correlation network analysis. Twelve potential biomarker proteins were identified based on cell line-specific expression and filtered by their predicted co-localization with known EV marker proteins, CD63, CD9, and CD81. This approach resulted in the identification of 11 proteins, four of which were further investigated by Western blot analysis. The presence of transmembrane serine protease matriptase (ST14), claudin-3 (CLDN3), and integrin alpha-7 (ITGA7) in each cell line was validated by Western blot, revealing that ST14 and CLDN3 may be further explored as potential EV biomarkers for BC. The surface labeling approach enriched proteins that were not identified using the first approach. Ten potential BC biomarkers (Glutathione S-transferase P1 (GSTP1), Elongation factor 2 (EEF2), DEAD/H box RNA helicase (DDX10), progesterone receptor (PGR), Ras-related C3 botulinum toxin substrate 2 (RAC2), Disintegrin and metalloproteinase domain-containing protein 10 (ADAM10), Aconitase 2 (ACO2), UTP20 small subunit processome component (UTP20), NEDD4 binding protein 2 (N4BP2), Programmed cell death 6 (PDCD6)) were selected from surface proteins commonly identified from MDA-MB-231 and MCF7, but not identified in MCF10A EVs. In total, 846 surface proteins were identified from the second approach, of which 11 were already known as BC markers. This study supports the proposition that Evs are a rich source of known and novel biomarkers that may be used for non-invasive detection of BC. Furthermore, the presented datasets could be further explored for the identification of potential biomarkers in BC.

Ribosome biogenesis factors-from names to functions

The assembly of ribosomal subunits is a highly orchestrated process that involves a huge cohort of accessory factors. Most eukaryotic ribosome biogenesis factors were first identified by genetic screens and proteomic approaches of pre-ribosomal particles in Saccharomyces cerevisiae. Later, research on human ribosome synthesis not only demonstrated that the requirement for many of these factors is conserved in evolution, but also revealed the involvement of additional players, reflecting a more complex assembly pathway in mammalian cells. Yet, it remained a challenge for the field to assign a function to many of the identified factors and to reveal their molecular mode of action. Over the past decade, structural, biochemical, and cellular studies have largely filled this gap in knowledge and led to a detailed understanding of the molecular role that many of the players have during the stepwise process of ribosome maturation. Such detailed knowledge of the function of ribosome biogenesis factors will be key to further understand and better treat diseases linked to disturbed ribosome assembly, including ribosomopathies, as well as different types of cancer.

Production of nascent ribosome precursors within the nucleolar microenvironment of Saccharomyces cerevisiae

35S rRNA transcripts include a 5'-external transcribed spacer followed by rRNAs of the small and large ribosomal subunits. Their processing yields massive precursors that include dozens of assembly factor proteins. In Saccharomyces cerevisiae, nucleolar assembly factors form 2 coaxial layers/volumes around ribosomal DNA. Most of these factors are cyclically recruited from a latent state to an operative state, and are extensively conserved. The layers match, at least approximately, known subcompartments found in higher eukaryotic cells. ∼80% of assembly factors are essential. The number of copies of these assembly factors is comparable to the number of nascent transcripts. Moreover, they exhibit "isoelectric balance," with RNA-binding candidate "nucleator" assembly factors being notably basic. The physical properties of pre-small subunit and pre-large subunit assembly factors are similar, as are their 19 motif signatures detected by hierarchical clustering, unlike motif signatures of the 5'-external transcribed spacer rRNP. Additionally, many assembly factors lack shared motifs. Taken together with the progression of rRNP composition during subunit maturation, and the realization that the ribosomal DNA cable is initially bathed in a subunit-nonspecific assembly factor reservoir/microenvironment, we propose a "3-step subdomain assembly model": Step (1): predominantly basic assembly factors sequentially nucleate sites along nascent rRNA; Step (2): the resulting rRNPs recruit numerous less basic assembly factors along with notably basic ribosomal proteins; Step (3): rRNPs in nearby subdomains consolidate. Cleavages of rRNA then promote release of rRNPs to the nucleoplasm, likely facilitated by the persistence of assembly factors that were already associated with nucleolar precursors.

Synergistic defects in pre-rRNA processing from mutations in the U3-specific protein Rrp9 and U3 snoRNA

U3 snoRNA and the associated Rrp9/U3-55K protein are essential for 18S rRNA production by the SSU-processome complex. U3 and Rrp9 are required for early pre-rRNA cleavages at sites A0, A1 and A2, but the mechanism remains unclear. Substitution of Arg 289 in Rrp9 to Ala (R289A) specifically reduced cleavage at sites A1 and A2. Surprisingly, R289 is located on the surface of the Rrp9 β-propeller structure opposite to U3 snoRNA. To understand this, we first characterized the protein-protein interaction network of Rrp9 within the SSU-processome. This identified a direct interaction between the Rrp9 β-propeller domain and Rrp36, the strength of which was reduced by the R289A substitution, implicating this interaction in the observed processing phenotype. The Rrp9 R289A mutation also showed strong synergistic negative interactions with mutations in U3 that destabilize the U3/pre-rRNA base-pair interactions or reduce the length of their linking segments. We propose that the Rrp9 β-propeller and U3/pre-rRNA binding cooperate in the structure or stability of the SSU-processome. Additionally, our analysis of U3 variants gave insights into the function of individual segments of the 5′-terminal 72-nt sequence of U3. We interpret these data in the light of recently reported SSU-processome structures.

The role of Candida albicans in root caries biofilms: an RNA-seq analysis

Objective

This study sought to analyze the gene expression of Candida albicans in sound root surface and root caries lesions, exploring its role in root caries pathogenesis.

Methodology

The differential gene expression of C. albicans and the specific genes related to cariogenic traits were studied in association with samples of biofilm collected from exposed sound root surface (SRS, n=10) and from biofilm and carious dentin of active root carious lesions (RC, n=9). The total microbial RNA was extracted, and the cDNA libraries were prepared and sequenced on the Illumina Hi-Seq2500. Unique reads were mapped to 163 oral microbial reference genomes including two chromosomes of C. albicans SC5314 (14,217 genes). The putative presence of C. albicans was estimated (sum of reads/total number of genes≥1) in each sample. Count data were normalized (using the DESeq method package) to analyze differential gene expression (using the DESeq2R package) applying the Benjamini-Hochberg correction (FDR<0.05).

Results

Two genes (CaO19.610, FDR=0.009; CaO19.2506, FDR=0.018) were up-regulated on SRS, and their functions are related to biofilm formation. Seven genes ( UTP20 , FDR=0.018; ITR1 , FDR=0.036; DHN6 , FDR=0.046; CaO19.7197 , FDR=0.046; CaO19.7838 , FDR=0.046; STT4 , FDR=0.046; GUT1 , FDR=0.046) were up-regulated on RC and their functions are related to metabolic activity, sugar transport, stress tolerance, invasion and pH regulation. The use of alternative carbon sources, including lactate, and the ability to form hypha may be a unique trait of C. albicans influencing biofilm virulence.

Conclusions

C. albicans is metabolically active in SRS and RC biofilm, with different roles in health and disease.

Formation of mammalian preribosomes proceeds from intermediate to composed state during ribosome maturation

In eukaryotes, ribosome assembly is a rate-limiting step in ribosomal biogenesis that takes place in a distinctive subnuclear organelle, the nucleolus. How ribosomes get assembled at the nucleolar site by forming initial preribosomal complexes remains poorly characterized. In this study, using several human and murine cell lines, we developed a method for isolation of native mammalian preribosomal complexes by lysing cell nuclei through mild sonication. A sucrose gradient fractionation of the nuclear lysate resolved several ribonucleoprotein (RNP) complexes containing rRNAs and ribosomal proteins. Characterization of the RNP complexes with MS-based protein identification and Northern blotting-based rRNA detection approaches identified two types of preribosomes we named here as intermediate preribosomes (IPRibs) and composed preribosome (CPRib). IPRib complexes comprised large preribosomes (105S to 125S in size) containing the rRNA modification factors and premature rRNAs. We further observed that a distinctive CPRib complex consists of an 85S preribosome assembled with mature rRNAs and a ribosomal biogenesis factor, Ly1 antibody-reactive (LYAR), that does not associate with premature rRNAs and rRNA modification factors. rRNA-labeling experiments uncovered that IPRib assembly precedes CPRib complex formation. We also found that formation of the preribosomal complexes is nutrient-dependent because the abundances of IPRib and CPRib decreased substantially when cells were either deprived of amino acids or exposed to an mTOR kinase inhibitor. These findings indicate that preribosomes form via dynamic and nutrient-dependent processing events and progress from an intermediate to a composed state during ribosome maturation.

Impaired ribosome biogenesis: mechanisms and relevance to cancer and aging

The biosynthesis of ribosomes is a complex process that requires the coordinated action of many factors and a huge energy investment from the cell. Ribosomes are essential for protein production, and thus for cellular survival, growth and proliferation. Ribosome biogenesis is initiated in the nucleolus and includes: the synthesis and processing of ribosomal RNAs, assembly of ribosomal proteins, transport to the cytoplasm and association of ribosomal subunits. The disruption of ribosome biogenesis at various steps, with either increased or decreased expression of different ribosomal components, can promote cell cycle arrest, senescence or apoptosis. Additionally, interference with ribosomal biogenesis is often associated with cancer, aging and age-related degenerative diseases. Here, we review current knowledge on impaired ribosome biogenesis, discuss the main factors involved in stress responses under such circumstances and focus on examples with clinical relevance.

Turnover of aberrant pre-40S pre-ribosomal particles is initiated by a novel endonucleolytic decay pathway

Ribosome biogenesis requires more than 200 trans-acting factors to achieve the correct production of the two mature ribosomal subunits. Here, we have identified Efg1 as a novel, nucleolar ribosome biogenesis factor in Saccharomyces cerevisiae that is directly linked to the surveillance of pre-40S particles. Depletion of Efg1 impairs early pre-rRNA processing, leading to a strong decrease in 18S rRNA and 40S subunit levels and an accumulation of the aberrant 23S rRNA. Using Efg1 as bait, we revealed a novel degradation pathway of the 23S rRNA. Co-immunoprecipitation experiments showed that Efg1 is a component of 90S pre-ribosomes, as it is associated with the 35S pre-rRNA and U3 snoRNA, but has stronger affinity for 23S pre-rRNA and its novel degradation intermediate 11S rRNA. 23S is cleaved at a new site, Q1, within the 18S sequence by the endonuclease Utp24, generating 11S and 17S' rRNA. Both of these cleavage products are targeted for degradation by the TRAMP/exosome complexes. Therefore, the Q1 site defines a novel endonucleolytic cleavage site of ribosomal RNA exclusively dedicated to surveillance of pre-ribosomal particles.

Perturbation of RNA Polymerase I transcription machinery by ablation of HEATR1 triggers the RPL5/RPL11-MDM2-p53 ribosome biogenesis stress checkpoint pathway in human cells

Ribosome biogenesis is an energy consuming process which takes place mainly in the nucleolus. By producing ribosomes to fuel protein synthesis, it is tightly connected with cell growth and cell cycle control. Perturbation of ribosome biogenesis leads to the activation of p53 tumor suppressor protein promoting processes like cell cycle arrest, apoptosis or senescence. This ribosome biogenesis stress pathway activates p53 through sequestration of MDM2 by a subset of ribosomal proteins (RPs), thereby stabilizing p53. Here, we identify human HEATR1, as a nucleolar protein which positively regulates ribosomal RNA (rRNA) synthesis. Downregulation of HEATR1 resulted in cell cycle arrest in a manner dependent on p53. Moreover, depletion of HEATR1 also caused disruption of nucleolar structure and activated the ribosomal biogenesis stress pathway - RPL5 / RPL11 dependent stabilization and activation of p53. These findings reveal an important role for HEATR1 in ribosome biogenesis and further support the concept that perturbation of ribosome biosynthesis results in p53-dependent cell cycle checkpoint activation, with implications for human pathologies including cancer.

Molecular architecture of the 90S small subunit pre-ribosome

Eukaryotic small ribosomal subunits are first assembled into 90S pre-ribosomes. The complete 90S is a gigantic complex with a molecular mass of approximately five megadaltons. Here, we report the nearly complete architecture of Saccharomyces cerevisiae 90S determined from three cryo-electron microscopy single particle reconstructions at 4.5 to 8.7 angstrom resolution. The majority of the density maps were modeled and assigned to specific RNA and protein components. The nascent ribosome is assembled into isolated native-like substructures that are stabilized by abundant assembly factors. The 5' external transcribed spacer and U3 snoRNA nucleate a large subcomplex that scaffolds the nascent ribosome. U3 binds four sites of pre-rRNA, including a novel site on helix 27 but not the 3' side of the central pseudoknot, and crucially organizes the 90S structure. The 90S model provides significant insight into the principle of small subunit assembly and the function of assembly factors.

DOI:http://dx.doi.org/10.7554/eLife.22086.001

Evidence for cohesin sliding along budding yeast chromosomes

The ring-shaped cohesin complex is thought to topologically hold sister chromatids together from their synthesis in S phase until chromosome segregation in mitosis. How cohesin stably binds to chromosomes for extended periods, without impeding other chromosomal processes that also require access to the DNA, is poorly understood. Budding yeast cohesin is loaded onto DNA by the Scc2-Scc4 cohesin loader at centromeres and promoters of active genes, from where cohesin translocates to more permanent places of residence at transcription termination sites. Here we show that, at the GAL2 and MET17 loci, pre-existing cohesin is pushed downstream along the DNA in response to transcriptional gene activation, apparently without need for intermittent dissociation or reloading. We observe translocation intermediates and find that the distribution of most chromosomal cohesin is shaped by transcription. Our observations support a model in which cohesin is able to slide laterally along chromosomes while maintaining topological contact with DNA. In this way, stable cohesin binding to DNA and enduring sister chromatid cohesion become compatible with simultaneous underlying chromosomal activities, including but maybe not limited to transcription.

Immature large ribosomal subunits containing the 7S pre-rRNA can engage in translation in Saccharomyces cerevisiae

Evolution has provided eukaryotes with mechanisms that impede immature and/or aberrant ribosomes to engage in translation. These mechanisms basically either prevent the nucleo-cytoplasmic export of these particles or, once in the cytoplasm, the release of associated assembly factors, which interfere with the binding of translation initiation factors and/or the ribosomal subunit joining. We have previously shown that aberrant yeast 40S ribosomal subunits containing the 20S pre-rRNA can engage in translation. In this study, we describe that cells harbouring the dob1-1 allele, encoding a mutated version of the exosome-assisting RNA helicase Mtr4, accumulate otherwise nuclear pre-60S ribosomal particles containing the 7S pre-rRNA in the cytoplasm. Polysome fractionation analyses revealed that these particles are competent for translation and do not induce elongation stalls. This phenomenon is rather specific since most mutations in other exosome components or co-factors, impairing the 3' end processing of the mature 5.8S rRNA, accumulate 7S pre-rRNAs in the nucleus. In addition, we confirm that pre-60S ribosomal particles containing either 5.8S + 30 or 5.8S + 5 pre-rRNAs also engage in translation elongation. We propose that 7S pre-rRNA processing is not strictly required for pre-60S r-particle export and that, upon arrival in the cytoplasm, there is no specific mechanism to prevent translation by premature pre-60S r-particles containing 3' extended forms of mature 5.8S rRNA.

RNA Export through the NPC in Eukaryotes

In eukaryotic cells, RNAs are transcribed in the nucleus and exported to the cytoplasm through the nuclear pore complex. The RNA molecules that are exported from the nucleus into the cytoplasm include messenger RNAs (mRNAs), ribosomal RNAs (rRNAs), transfer RNAs (tRNAs), small nuclear RNAs (snRNAs), micro RNAs (miRNAs), and viral mRNAs. Each RNA is transported by a specific nuclear export receptor. It is believed that most of the mRNAs are exported by Nxf1 (Mex67 in yeast), whereas rRNAs, snRNAs, and a certain subset of mRNAs are exported in a Crm1/Xpo1-dependent manner. tRNAs and miRNAs are exported by Xpot and Xpo5. However, multiple export receptors are involved in the export of some RNAs, such as 60S ribosomal subunit. In addition to these export receptors, some adapter proteins are required to export RNAs. The RNA export system of eukaryotic cells is also used by several types of RNA virus that depend on the machineries of the host cell in the nucleus for replication of their genome, therefore this review describes the RNA export system of two representative viruses. We also discuss the NPC anchoring-dependent mRNA export factors that directly recruit specific genes to the NPC.

Rrp12 and the Exportin Crm1 participate in late assembly events in the nucleolus during 40S ribosomal subunit biogenesis

During the biogenesis of small ribosomal subunits in eukaryotes, the pre-40S particles formed in the nucleolus are rapidly transported to the cytoplasm. The mechanisms underlying the nuclear export of these particles and its coordination with other biogenesis steps are mostly unknown. Here we show that yeast Rrp12 is required for the exit of pre-40S particles to the cytoplasm and for proper maturation dynamics of upstream 90S pre-ribosomes. Due to this, in vivo elimination of Rrp12 leads to an accumulation of nucleoplasmic 90S to pre-40S transitional particles, abnormal 35S pre-rRNA processing, delayed elimination of processing byproducts, and no export of intermediate pre-40S complexes. The exportin Crm1 is also required for the same pre-ribosome maturation events that involve Rrp12. Thus, in addition to their implication in nuclear export, Rrp12 and Crm1 participate in earlier biosynthetic steps that take place in the nucleolus. Our results indicate that, in the 40S subunit synthesis pathway, the completion of early pre-40S particle assembly, the initiation of byproduct degradation and the priming for nuclear export occur in an integrated manner in late 90S pre-ribosomes.

An overview of pre-ribosomal RNA processing in eukaryotes

Ribosomal RNAs are the most abundant and universal noncoding RNAs in living organisms. In eukaryotes, three of the four ribosomal RNAs forming the 40S and 60S subunits are borne by a long polycistronic pre-ribosomal RNA. A complex sequence of processing steps is required to gradually release the mature RNAs from this precursor, concomitant with the assembly of the 79 ribosomal proteins. A large set of trans-acting factors chaperone this process, including small nucleolar ribonucleoparticles. While yeast has been the gold standard for studying the molecular basis of this process, recent technical advances have allowed to further define the mechanisms of ribosome biogenesis in animals and plants. This renewed interest for a long-lasting question has been fueled by the association of several genetic diseases with mutations in genes encoding both ribosomal proteins and ribosome biogenesis factors, and by the perspective of new anticancer treatments targeting the mechanisms of ribosome synthesis. A consensus scheme of pre-ribosomal RNA maturation is emerging from studies in various kinds of eukaryotic organisms. However, major differences between mammalian and yeast pre-ribosomal RNA processing have recently come to light. WIREs RNA 2015, 6:225–242. doi: 10.1002/wrna.1269

Crucial role of the Rcl1p-Bms1p interaction for yeast pre-ribosomal RNA processing

The essential Rcl1p and Bms1p proteins form a complex required for 40S ribosomal subunit maturation. Bms1p is a GTPase and Rcl1p has been proposed to catalyse the endonucleolytic cleavage at site A₂ separating the pre-40S and pre-60S maturation pathways. We determined the 2.0 Å crystal structure of Bms1p associated with Rcl1p. We demonstrate that Rcl1p nuclear import depends on Bms1p and that the two proteins are loaded into pre-ribosomes at a similar stage of the maturation pathway and remain present within pre-ribosomes after cleavage at A₂. Importantly, GTP binding to Bms1p is not required for the import in the nucleus nor for the incorporation of Rcl1p into pre-ribosomes, but is essential for early pre-rRNA processing. We propose that GTP binding to Bms1p and/or GTP hydrolysis may induce conformational rearrangements within the Bms1p-Rcl1p complex allowing the interaction of Rcl1p with its RNA substrate.

Rrp5 binding at multiple sites coordinates pre-rRNA processing and assembly

In vivo UV crosslinking identified numerous preribosomal RNA (pre-rRNA) binding sites for the large, highly conserved ribosome synthesis factor Rrp5. Intramolecular complementation has shown that the C-terminal domain (CTD) of Rrp5 is required for pre-rRNA cleavage at sites A0–A2 on the pathway of 18S rRNA synthesis, whereas the N-terminal domain (NTD) is required for A3 cleavage on the pathway of 5.8S/25S rRNA synthesis. The CTD was crosslinked to sequences flanking A2 and to the snoRNAs U3, U14, snR30, and snR10, which are required for cleavage at A0–A2. The NTD was crosslinked to sequences flanking A3 and to the RNA component of ribonuclease MRP, which cleaves site A3. Rrp5 could also be directly crosslinked to several large structural proteins and nucleoside triphosphatases. A key role in coordinating preribosomal assembly and processing was confirmed by chromatin spreads. Following depletion of Rrp5, cotranscriptional cleavage was lost and preribosome compaction greatly reduced.

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Rrp5 binds multiple dispersed sites in the pre-rRNA

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The NTD and CTD of Rrp5 each bind adjacent to sites of cleavages that require them

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Rrp5 directly binds large, structural proteins and NTPases

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Rrp5 is required for preribosome compaction

Ribosome biogenesis in the yeast Saccharomyces cerevisiae

Ribosomes are highly conserved ribonucleoprotein nanomachines that translate information in the genome to create the proteome in all cells. In yeast these complex particles contain four RNAs (>5400 nucleotides) and 79 different proteins. During the past 25 years, studies in yeast have led the way to understanding how these molecules are assembled into ribosomes in vivo. Assembly begins with transcription of ribosomal RNA in the nucleolus, where the RNA then undergoes complex pathways of folding, coupled with nucleotide modification, removal of spacer sequences, and binding to ribosomal proteins. More than 200 assembly factors and 76 small nucleolar RNAs transiently associate with assembling ribosomes, to enable their accurate and efficient construction. Following export of preribosomes from the nucleus to the cytoplasm, they undergo final stages of maturation before entering the pool of functioning ribosomes. Elaborate mechanisms exist to monitor the formation of correct structural and functional neighborhoods within ribosomes and to destroy preribosomes that fail to assemble properly. Studies of yeast ribosome biogenesis provide useful models for ribosomopathies, diseases in humans that result from failure to properly assemble ribosomes.

TRAMP Stimulation of Exosome

In order to control and/or enhance the specificity and activity of nuclear surveillance and degradation, exosomes cooperate with the polyadenylation complex called TRAMP. Two forms of TRAMP operate in budding yeast, TRAMP4 and TRAMP5. They oligoadenylate defective or precursor forms of RNAs and promote trimming or complete degradation by exosomes. TRAMPs target a wide variety of nuclear transcripts. The known substrates include the noncoding RNAs originating from pervasive transcription from diverse parts of the yeast genome. Although TRAMP and exosomes can be triggered to a subset of their targets via the RNA-binding complex Nrd1, it is still not completely understood how TRAMP recognizes other aberrant RNAs. The existence of TRAMP-like complexes in other organisms indicates the importance of nuclear surveillance for general cell biology. In this chapter, we review the current understanding of TRAMP function and substrate repertoire. We discuss the advances in TRAMP biochemistry with respect to its catalytic activities and RNA recognition. Finally, we speculate about the possible mechanisms by which TRAMP activates exosomes.

Ski2-like RNA helicase structures: common themes and complex assemblies

Ski2-like RNA helicases are large multidomain proteins involved in a variety of RNA processing and degradation events. Recent structures of Mtr4, Ski2 and Brr2 provide our first view of these intricate helicases. Here we review these structures, which reveal a conserved ring-like architecture that extends beyond the canonical RecA domains to include a winged helix and ratchet domain. Comparison of apo- and RNA-bound Mtr4 structures suggests a role for the winged helix domain as a molecular hub that coordinates RNA interacting events throughout the helicase. Unique accessory domains provide expanded diversity and functionality to each Ski2-like family member. A common theme is the integration of Ski2-like RNA helicases into larger protein assemblies. We describe the central role of Mtr4 and Ski2 in formation of complexes that activate RNA decay by the eukaryotic exosome. The current structures provide clues into what promises to be a fascinating view of these dynamic assemblies.

Trm112 is required for Bud23-mediated methylation of the 18S rRNA at position G1575

Posttranscriptional and posttranslational modification of macromolecules is known to fine-tune their functions. Trm112 is unique, acting as an activator of both tRNA and protein methyltransferases. Here we report that in Saccharomyces cerevisiae, Trm112 is required for efficient ribosome synthesis and progression through mitosis. Trm112 copurifies with pre-rRNAs and with multiple ribosome synthesis trans-acting factors, including the 18S rRNA methyltransferase Bud23. Consistent with the known mechanisms of activation of methyltransferases by Trm112, we found that Trm112 interacts directly with Bud23 in vitro and that it is required for its stability in vivo. Consequently, trm112Δ cells are deficient for Bud23-mediated 18S rRNA methylation at position G1575 and for small ribosome subunit formation. Bud23 failure to bind nascent preribosomes activates a nucleolar surveillance pathway involving the TRAMP complexes, leading to preribosome degradation. Trm112 is thus active in rRNA, tRNA, and translation factor modification, ideally placing it at the interface between ribosome synthesis and function.

Nuclear noncoding RNA surveillance: is the end in sight?

Nuclear noncoding RNA (ncRNA) surveillance pathways play key roles in shaping the steady-state transcriptomes of eukaryotic cells. Defective and unneeded ncRNAs are primarily degraded by exoribonucleases that rely on protein cofactors to identify these RNAs. Recent studies have begun to elucidate both the mechanisms by which these cofactors recognize aberrant RNAs and the features that mark RNAs for degradation. One crucial RNA determinant is the presence of an accessible end; in addition, the failure of aberrant RNAs to fold into compact structures and assemble with specific binding proteins probably also contributes to their recognition and subsequent degradation. To date, ncRNA surveillance has been most extensively studied in budding yeast. However, mammalian cells possess nucleases and cofactors that have no known yeast counterparts, indicating that RNA surveillance pathways may be more complex in metazoans. Importantly, there is evidence that the failure of ncRNA surveillance pathways contributes to human disease.

Interaction profiling identifies the human nuclear exosome targeting complex

The RNA exosome is a conserved degradation machinery, which obtains full activity only when associated with cofactors. The most prominent activator of the yeast nuclear exosome is the RNA helicase Mtr4p, acting in the context of the Trf4p/Air2p/Mtr4p polyadenylation (TRAMP) complex. The existence of a similar activator(s) in humans remains elusive. By establishing an interaction network of the human nuclear exosome, we identify the trimeric Nuclear Exosome Targeting (NEXT) complex, containing hMTR4, the Zn-knuckle protein ZCCHC8, and the putative RNA binding protein RBM7. ZCCHC8 and RBM7 are excluded from nucleoli, and consistently NEXT is specifically required for the exosomal degradation of promoter upstream transcripts (PROMPTs). We also detect putative homolog TRAMP subunits hTRF4-2 (Trf4p) and ZCCHC7 (Air2p) in hRRP6 and hMTR4 precipitates. However, at least ZCCHC7 function is restricted to nucleoli. Our results suggest that human nuclear exosome degradation pathways comprise modules of spatially organized cofactors that diverge from the yeast model.

A functional interface at the rDNA connects rRNA synthesis, pre-rRNA processing and nucleolar surveillance in budding yeast

Ribogenesis is a multistep error-prone process that is actively monitored by quality control mechanisms. How ribosomal RNA synthesis, pre-rRNA processing and nucleolar surveillance are integrated is unclear. Nor is it understood how defective ribosomes are recognized. We report in budding yeast that, in vivo, the interaction between the transcription elongation factor Spt5 and Rpa190, the largest subunit of RNA polymerase (Pol) I, requires the Spt5 C-terminal region (CTR), a conserved and highly repetitive domain that is reminiscent of the RNA Pol II C-terminal domain (CTD). We show that this sequence is also required for the interaction between Spt5 and Nrd1, an RNA specific binding protein, and an exosome cofactor. Both the Spt4-Spt5, and the Nrd1-Nab3 complexes interact functionally with Rrp6, and colocalize at the rDNA. Mutations in the RNA binding domain of Nrd1, but not in its RNA Pol II CTD-interacting domain, and mutations in the RRM of Nab3 led to the accumulation of normal and aberrant polyadenylated pre-rRNAs. Altogether these results indicate that Nrd1-Nab3 contributes to recruiting the nucleolar surveillance to elongating polymerases to survey nascent rRNA transcripts.

Elucidation of the assembly events required for the recruitment of Utp20, Imp4 and Bms1 onto nascent pre-ribosomes

The 90S pre-ribosome, also known as the small subunit (SSU) processome, is a large multisubunit particle required for the production of the 18S rRNA from a pre-rRNA precursor. Recently, it has been shown that the formation of this particle entails the initial association of the tUTP subunit with the nascent pre-RNA and, subsequently, the binding of Rrp5/UTP-C and U3 snoRNP/UTP-B subunits in two independent assembly branches. However, the mode of assembly of other 90S pre-ribosome components remains obscure as yet. In this study, we have investigated the assembly of three proteins (Utp20, Imp4 and Bms1) previously regarded as potential nucleating factors of the 90S particle. Here, we demonstrate that the loading of those three proteins onto the pre-rRNA takes place independently of Rrp5/UTP-C and, instead, occurs downstream of the tUTP and U3/UTP-B subcomplexes. We also demonstrate that Bms1 and Utp20 are required for the recruitment of a subset of proteins to nascent pre-ribosomes. Finally, we show that proteins associated through secondary steps condition the stability of the two assembly branches in partially assembled pre-ribosomes. These results provide new information about the functional relationships among 90S particle components and the events that are required for their stepwise incorporation onto the primary pre-rRNA.

Utp25p, a nucleolar Saccharomyces cerevisiae protein, interacts with U3 snoRNP subunits and affects processing of the 35S pre-rRNA

In eukaryotes, pre-rRNA processing depends on a large number of nonribosomal trans-acting factors that form intriguingly organized complexes. Two intermediate complexes, pre-40S and pre-60S, are formed at the early stages of 35S pre-rRNA processing and give rise to the mature ribosome subunits. Each of these complexes contains specific pre-rRNAs, some ribosomal proteins and processing factors. The novel yeast protein Utp25p has previously been identified in the nucleolus, an indication that this protein could be involved in ribosome biogenesis. Here we show that Utp25p interacts with the SSU processome proteins Sas10p and Mpp10p, and affects 18S rRNA maturation. Depletion of Utp25p leads to accumulation of the pre-rRNA 35S and the aberrant rRNA 23S, and to a severe reduction in 40S ribosomal subunit levels. Our results indicate that Utp25p is a novel SSU processome subunit involved in pre-40S maturation.

[<< Catch me if you can >>: how the structural and functional integrity of eukaryotic RNA molecules is monitored by surveillance mechanisms]

Cellular RNAs are invariably organized in ribonucleoprotein particles, or RNPs, regardless of their size, structure or function. RNPs are monitored by active surveillance mechanisms for their structural and functional integrity at every single step of their "life". A limited number of key endoRNase and/or exoRNase activities are recruited to multiple metabolic pathways by specific adaptors. These trans-acting factors are often endowed with synthesis activities in the formation of mature RNA termini, as well as degradation and surveillance activities. Quality control mechanisms are robust because they are partially redundant. The actual mechanisms that discriminate aberrant RNAs from normal RNAs are still loosely defined. Surveillance is essential to cellular homeostasis and has been linked to several human diseases.

A 'garbage can' for ribosomes: how eukaryotes degrade their ribosomes

Ribosome synthesis is a major metabolic activity that involves hundreds of individual reactions, each of which is error-prone. Ribosomal insults occur in cis (alteration in rRNA sequences) and in trans (failure to bind to, or loss of, an assembly factor or ribosomal protein). In addition, specific growth conditions, such as starvation, require that excess ribosomes are turned over efficiently. Recent work indicates that cells evolved multiple strategies to recognize specifically, and target for clearance, ribosomes that are structurally and/or functionally deficient, as well as in excess. This surveillance is active at every step of the ribosome synthesis pathway and on mature ribosomes, involves nearly entirely different mechanisms for the small and large subunits, and requires specialized subcellular organelles.

Evolutionarily conserved function of RRP36 in early cleavages of the pre-rRNA and production of the 40S ribosomal subunit

Ribosome biogenesis in eukaryotes is a major cellular activity mobilizing the products of over 200 transcriptionally coregulated genes referred to as the rRNA and ribosome biosynthesis regulon. We investigated the function of an essential, uncharacterized gene of this regulon, renamed RRP36. We show that the Rrp36p protein is nucleolar and interacts with 90S and pre-40S preribosomal particles. Its depletion affects early cleavages of the 35S pre-rRNA and results in a rapid decrease in mature 18S rRNA levels. Rrp36p is a novel component of the 90S preribosome, the assembly of which has been suggested to result from the stepwise incorporation of several modules, including the tUTP/UTP-A, PWP2/UTP-B, and UTP-C subcomplexes. We show that Rrp36p depletion does not impair the incorporation of these subcomplexes and the U3 small nucleolar RNP into preribosomes. In contrast, depletion of components of the UTP-A or UTP-B modules, but not Rrp5p, prevents Rrp36p recruitment and reduces its accumulation levels. In parallel, we studied the human orthologue of Rrp36p in HeLa cells, and we show that the function of this protein in early cleavages of the pre-rRNA has been conserved through evolution in eukaryotes.

The Noc-domain containing C-terminus of Noc4p mediates both formation of the Noc4p-Nop14p submodule and its incorporation into the SSU processome

Noc1p, Noc3p and Noc4p are eukaryotic proteins which play essential roles in yeast ribosome biogenesis and contain a homologous stretch of about 45 aminoacids (Noc-domain) of unknown function. Yeast Noc4p is a component of the small ribosomal subunit (SSU) processome, can be isolated as a stable Noc4p-Nop14p SSU-processome submodule from yeast cells, and is required for nuclear steps of small ribosomal subunit rRNA maturation. We expressed a series of mutated alleles of NOC4 in yeast cells and analysed whether the corresponding protein variants support vegetative growth, interact with Nop14p, and are incorporated into the SSU-processome. The data reveal that the essential C-terminus of Noc4p which contains 237 aminoacids including the Noc-domain represents a protein-protein interaction module. It is required and sufficient for its association with Nop14p and several nuclear precursors of the small ribosomal subunit. The N-terminal Noc4-part seems to be targeted to pre-ribosomes via the C-terminus of Noc4p and plays there an essential role in SSU-processome function. Replacement of the Noc4p-Noc-domain by its homologues Noc1p-counterpart results in a hybrid Noc4p variant which fails to associate with Nop14p and pre-ribosomes. On the other hand, exchange of 6 amino acids in the Noc1-Noc-domain of this hybrid Noc4p protein is sufficient to restore its essential in vivo functions. These data suggest that Noc-domains of Noc1p and Noc4p share a common structural backbone in which diverging amino acids play crucial roles in mediating specific regulated interactions. Our analysis allows us to distinguish between different functions of certain domains within Noc4p and contribute to the understanding of how incorporation of Noc4p into ribosomal precursors is coupled to rRNA processing and maturation of the small ribosomal subunit.

Powering through ribosome assembly

Ribosome assembly is required for cell growth in all organisms. Classic in vitro work in bacteria has led to a detailed understanding of the biophysical, thermodynamic, and structural basis for the ordered and correct assembly of ribosomal proteins on ribosomal RNA. Furthermore, it has enabled reconstitution of active subunits from ribosomal RNA and proteins in vitro. Nevertheless, recent work has shown that eukaryotic ribosome assembly requires a large macromolecular machinery in vivo. Many of these assembly factors such as ATPases, GTPases, and kinases hydrolyze nucleotide triphosphates. Because these enzymes are likely regulatory proteins, much work to date has focused on understanding their role in the assembly process. Here, we review these factors, as well as other sources of energy, and their roles in the ribosome assembly process. In addition, we propose roles of energy-releasing enzymes in the assembly process, to explain why energy is used for a process that occurs largely spontaneously in bacteria. Finally, we use literature data to suggest testable models for how these enzymes could be used as targets for regulation of ribosome assembly.

A novel small-subunit processome assembly intermediate that contains the U3 snoRNP, nucleolin, RRP5, and DBP4

Eukaryotic 18S rRNA processing is mediated by the small subunit (SSU) processome, a machine comprised of the U3 small nucleolar RNP (U3 snoRNP), tUTP, bUTP, MPP10, and BMS1/RCL1 subcomplexes. We report that the human SSU processome is a dynamic structure with the recruitment and release of subcomplexes occurring during the early stages of ribosome biogenesis. A novel 50S U3 snoRNP accumulated when either pre-rRNA transcription was blocked or the tUTP proteins were depleted. This complex did not contain the tUTP, bUTP, MPP10, and BMS1/RCL1 subcomplexes but was associated with the RNA-binding proteins nucleolin and RRP5 and the RNA helicase DBP4. Our data suggest that the 50S U3 snoRNP is an SSU assembly intermediate that is likely recruited to the pre-rRNA through the RNA-binding proteins nucleolin and RRP5. We predict that nucleolin is only transiently associated with the SSU processome and likely leaves the complex not long after 50S U3 snoRNP recruitment. The nucleolin-binding site potentially overlaps that of several other key factors, and we propose that this protein must leave the SSU processome for pre-rRNA processing to occur.

The nuclear poly(A) polymerase and Exosome cofactor Trf5 is recruited cotranscriptionally to nucleolar surveillance

Terminal balls detected at the 5'-end of nascent ribosomal transcripts act as pre-rRNA processing complexes and are detected in all eukaryotes examined, resulting in illustrious Christmas tree images. Terminal balls (also known as SSU-processomes) compaction reflects the various stages of cotranscriptional ribosome assembly. Here, we have followed SSU-processome compaction in vivo by use of a chromatin immunoprecipitation (Ch-IP) approach and shown, in agreement with electron microscopy analysis of Christmas trees, that it progressively condenses to come in close proximity to the 5'-end of the 25S rRNA gene. The SSU-processome is comprised of independent autonomous building blocks that are loaded onto nascent pre-rRNAs and assemble into catalytically active pre-rRNA processing complexes in a stepwise and highly hierarchical process. Failure to assemble SSU-processome subcomplexes with proper kinetics triggers a nucleolar surveillance pathway that targets misassembled pre-rRNAs otherwise destined to mature into small subunit 18S rRNA for polyadenylation, preferentially by TRAMP5, and degradation by the 3' to 5' exoribonucleolytic activity of the Exosome. Trf5 colocalized with nascent pre-rRNPs, indicating that this nucleolar surveillance initiates cotranscriptionally.

The N-terminal PIN domain of the exosome subunit Rrp44 harbors endonuclease activity and tethers Rrp44 to the yeast core exosome

Nuclear and cytoplasmic forms of the yeast exosome share 10 components, of which only Rrp44/Dis3 is believed to possess 3′ exonuclease activity. We report that expression only of Rrp44 lacking 3′-exonuclease activity (Rrp44-exo) supports growth in S288c-related strains (BY4741). In BY4741, rrp44-exo was synthetic-lethal with loss of the cytoplasmic 5′-exonuclease Xrn1, indicating block of mRNA turnover, but not with loss of the nuclear 3′-exonuclease Rrp6. The RNA processing phenotype of rrp44-exo was milder than that seen on Rrp44 depletion, indicating that Rrp44-exo retains important functions. Recombinant Rrp44 was shown to possess manganese-dependent endonuclease activity in vitro that was abolished by four point mutations in the putative metal binding residues of its N-terminal PIN domain. Rrp44 lacking both exonuclease and endonuclease activity failed to support growth in strains depleted of endogenous Rrp44. Strains expressing Rrp44-exo and Rrp44-endo–exo exhibited different RNA processing patterns in vivo suggesting Rrp44-dependent endonucleolytic cleavages in the 5′-ETS and ITS2 regions of the pre-rRNA. Finally, the N-terminal PIN domain was shown to be necessary and sufficient for association with the core exosome, indicating its dual function as a nuclease and structural element.

Nop53p interacts with 5.8S rRNA co-transcriptionally, and regulates processing of pre-rRNA by the exosome

In eukaryotes, pre-rRNA processing depends on a large number of nonribosomal trans-acting factors that form intriguingly organized complexes. One of the early stages of pre-rRNA processing includes formation of the two intermediate complexes pre-40S and pre-60S, which then form the mature ribosome subunits. Each of these complexes contains specific pre-rRNAs, ribosomal proteins and processing factors. The yeast nucleolar protein Nop53p has previously been identified in the pre-60S complex and shown to affect pre-rRNA processing by directly binding to 5.8S rRNA, and to interact with Nop17p and Nip7p, which are also involved in this process. Here we show that Nop53p binds 5.8S rRNA co-transcriptionally through its N-terminal region, and that this protein portion can also partially complement growth of the conditional mutant strain Deltanop53/GAL::NOP53. Nop53p interacts with Rrp6p and activates the exosome in vitro. These results indicate that Nop53p may recruit the exosome to 7S pre-rRNA for processing. Consistent with this observation and similar to the observed in exosome mutants, depletion of Nop53p leads to accumulation of polyadenylated pre-rRNAs.

A yeast exosome cofactor, Mpp6, functions in RNA surveillance and in the degradation of noncoding RNA transcripts

A genome-wide screen for synthetic lethal (SL) interactions with loss of the nuclear exosome cofactors Rrp47/Lrp1 or Air1 identified 3'-->5' exonucleases, the THO complex required for mRNP assembly, and Ynr024w (Mpp6). SL interactions with mpp6Delta were confirmed for rrp47Delta and nuclear exosome component Rrp6. The results of bioinformatic analyses revealed homology between Mpp6 and a human exosome cofactor, underlining the high conservation of the RNA surveillance system. Mpp6 is an RNA binding protein that physically associates with the exosome and was localized throughout the nucleus. The results of functional analyses demonstrated roles for Mpp6 in the surveillance of both pre-rRNA and pre-mRNAs and in the degradation of "cryptic" noncoding RNAs (ncRNAs) derived from intergenic regions and the ribosomal DNA spacer heterochromatin. Strikingly, these ncRNAs are also targeted by other exosome cofactors, including Rrp47, the TRAMP complex (which includes Air1), and the Nrd1/Nab3 complex, and are degraded by both Rrp6 and the core exosome. Heterochromatic transcripts and other ncRNAs are characterized by very rapid degradation, and we predict that functional redundancy is an important feature of ncRNA metabolism.