SURF2 is a MDM2 antagonist in triggering the nucleolar stress response.

Cancer cells rely on high ribosome production to sustain their proliferation rate. Many chemotherapies impede ribosome production which is perceived by cells as "nucleolar stress" (NS), triggering p53-dependent and independent pathways leading to cell cycle arrest and/or apoptosis. The 5S ribonucleoprotein (RNP) particle, a sub-ribosomal particle, is instrumental to NS response. Upon ribosome assembly defects, the 5S RNP accumulates as free form. This free form is able to sequester and inhibit MDM2, thus promoting p53 stabilization. To investigate how cancer cells can resist to NS, here we purify free 5S RNP and uncover an interaction partner, SURF2. Functional characterization of SURF2 shows that its depletion increases cellular sensitivity to NS, while its overexpression promotes their resistance to it. Consistently, SURF2 is overexpressed in many cancers and its expression level is an independent marker of prognosis for adrenocortical cancer. Our data demonstrate that SURF2 buffers free 5S RNP particles, and can modulate their activity, paving the way for the research of new molecules that can finely tune the response to nucleolar stress in the framework of cancer therapies.

Deciphering cellular and molecular determinants of human DPCD protein in complex with RUVBL1/RUVBL2 AAA-ATPases.

DPCD is a protein that may play a role in cilia formation and whose absence leads to primary ciliary dyskinesia (PCD), a rare disease caused by impairment of ciliated cells. Except for high-throughput studies that identified DPCD as a possible RUVBL1 (R1) and RUVBL2 (R2) partner, no in-depth cellular, biochemical, and structural investigation involving DPCD have been reported so far. R1 and R2 proteins are ubiquitous highly conserved AAA + family ATPases that assemble and mature a plethora of macromolecular complexes and are pivotal in numerous cellular processes, especially by guaranteeing a co-chaperoning function within R2TP or R2TP-like machineries. In the present study, we identified DPCD as a new R1R2 partner in vivo. We show that DPCD interacts directly with R1 and R2 in vitro and in cells. We characterized the physico-chemical properties of DPCD in solution and built a 3D model of DPCD. In addition, we used a variety of orthogonal biophysical techniques including small-angle X-ray scattering, structural mass spectrometry and electron microscopy to assess the molecular determinants of DPCD interaction with R1R2. Interestingly, DPCD disrupts the dodecameric state of R1R2 complex upon binding and this interaction occurs mainly via the DII domains of R1R2.

Probing small ribosomal subunit RNA helix 45 acetylation across eukaryotic evolution.

NAT10 is an essential enzyme that catalyzes N4-acetylcytidine (ac4C) in eukaryotic transfer RNA and 18S ribosomal RNA. Recent studies suggested that rRNA acetylation is dependent on SNORD13, a box C/D small nucleolar RNA predicted to base-pair with 18S rRNA via two antisense elements. However, the selectivity of SNORD13-dependent cytidine acetylation and its relationship to NAT10's essential function remain to be defined. Here, we demonstrate that SNORD13 is required for acetylation of a single cytidine of human and zebrafish 18S rRNA. In-depth characterization revealed that SNORD13-dependent ac4C is dispensable for human cell growth, ribosome biogenesis, translation and development. This loss of function analysis inspired a cross-evolutionary survey of the eukaryotic rRNA acetylation 'machinery' that led to the characterization of many novel metazoan SNORD13 genes. This includes an atypical SNORD13-like RNA in Drosophila melanogaster which guides ac4C to 18S rRNA helix 45 despite lacking one of the two rRNA antisense elements. Finally, we discover that Caenorhabditis elegans 18S rRNA is not acetylated despite the presence of an essential NAT10 homolog. Our findings shed light on the molecular mechanisms underlying SNORD13-mediated rRNA acetylation across eukaryotic evolution and raise new questions regarding the biological and evolutionary relevance of this highly conserved rRNA modification.

The final step of 40S ribosomal subunit maturation is controlled by a dual key lock.

Preventing premature interaction of pre-ribosomes with the translation apparatus is essential for translational accuracy. Hence, the final maturation step releasing functional 40S ribosomal subunits, namely processing of the 18S ribosomal RNA 3' end, is safeguarded by the protein DIM2, which both interacts with the endoribonuclease NOB1 and masks the rRNA cleavage site. To elucidate the control mechanism that unlocks NOB1 activity, we performed cryo-electron microscopy analysis of late human pre-40S particles purified using a catalytically inactive form of the ATPase RIO1. These structures, together with in vivo and in vitro functional analyses, support a model in which ATP-loaded RIO1 cooperates with ribosomal protein RPS26/eS26 to displace DIM2 from the 18S rRNA 3' end, thereby triggering final cleavage by NOB1; release of ADP then leads to RIO1 dissociation from the 40S subunit. This dual key lock mechanism requiring RIO1 and RPS26 guarantees the precise timing of pre-40S particle conversion into translation-competent ribosomal subunits.

Good Vibrations: Structural Remodeling of Maturing Yeast Pre-40S Ribosomal Particles Followed by Cryo-Electron Microscopy.

Assembly of eukaryotic ribosomal subunits is a very complex and sequential process that starts in the nucleolus and finishes in the cytoplasm with the formation of functional ribosomes. Over the past few years, characterization of the many molecular events underlying eukaryotic ribosome biogenesis has been drastically improved by the "resolution revolution" of cryo-electron microscopy (cryo-EM). However, if very early maturation events have been well characterized for both yeast ribosomal subunits, little is known regarding the final maturation steps occurring to the small (40S) ribosomal subunit. To try to bridge this gap, we have used proteomics together with cryo-EM and single particle analysis to characterize yeast pre-40S particles containing the ribosome biogenesis factor Tsr1. Our analyses lead us to refine the timing of the early pre-40S particle maturation steps. Furthermore, we suggest that after an early and structurally stable stage, the beak and platform domains of pre-40S particles enter a "vibrating" or "wriggling" stage, that might be involved in the final maturation of 18S rRNA as well as the fitting of late ribosomal proteins into their mature position.

Conformational proofreading of distant 40S ribosomal subunit maturation events by a long-range communication mechanism.

Eukaryotic ribosomes are synthesized in a hierarchical process driven by a plethora of assembly factors, but how maturation events at physically distant sites on pre-ribosomes are coordinated is poorly understood. Using functional analyses and cryo-EM, we show that ribosomal protein Rps20 orchestrates communication between two multi-step maturation events across the pre-40S subunit. Our study reveals that during pre-40S maturation, formation of essential contacts between Rps20 and Rps3 permits assembly factor Ltv1 to recruit the Hrr25 kinase, thereby promoting Ltv1 phosphorylation. In parallel, a deeply buried Rps20 loop reaches to the opposite pre-40S side, where it stimulates Rio2 ATPase activity. Both cascades converge to the final maturation steps releasing Rio2 and phosphorylated Ltv1. We propose that conformational proofreading exerted via Rps20 constitutes a checkpoint permitting assembly factor release and progression of pre-40S maturation only after completion of all earlier maturation steps.

Maturation of pre-40S particles in yeast and humans.

The synthesis of ribosomal subunits in eukaryotes requires the interplay of numerous maturation and assembly factors (AFs) that intervene in the insertion of ribosomal proteins within pre-ribosomal particles, the ribosomal subunit precursors, as well as in pre-ribosomal RNA (rRNA) processing and folding. Here, we review the intricate nuclear and cytoplasmic maturation steps of pre-40S particles, the precursors to the small ribosomal subunits, in both yeast and human cells, with particular emphasis on the timing and mechanisms of AF association with and dissociation from pre-40S particles and the roles of these AFs in the maturation process. We highlight the particularly complex pre-rRNA processing pathway in human cells, compared to yeast, to generate the mature 18S rRNA. We discuss the information gained from the recently published cryo-electron microscopy atomic models of yeast and human pre-40S particles, as well as the checkpoint/quality control systems that seem to operate to probe functional sites within yeast cytoplasmic pre-40S particles. This article is categorized under: RNA Processing > rRNA Processing Translation > Ribosome Biogenesis.

The Npa1p complex chaperones the assembly of the earliest eukaryotic large ribosomal subunit precursor.

The early steps of the production of the large ribosomal subunit are probably the least understood stages of eukaryotic ribosome biogenesis. The first specific precursor to the yeast large ribosomal subunit, the first pre-60S particle, contains 30 assembly factors (AFs), including 8 RNA helicases. These helicases, presumed to drive conformational rearrangements, usually lack substrate specificity in vitro. The mechanisms by which they are targeted to their correct substrate within pre-ribosomal particles and their precise molecular roles remain largely unknown. We demonstrate that the Dbp6p helicase, essential for the normal accumulation of the first pre-60S pre-ribosomal particle in S. cerevisiae, associates with a complex of four AFs, namely Npa1p, Npa2p, Nop8p and Rsa3p, prior to their incorporation into the 90S pre-ribosomal particles. By tandem affinity purifications using yeast extracts depleted of one component of the complex, we show that Npa1p forms the backbone of the complex. We provide evidence that Npa1p and Npa2p directly bind Dbp6p and we demonstrate that Npa1p is essential for the insertion of the Dbp6p helicase within 90S pre-ribosomal particles. In addition, by an in vivo cross-linking analysis (CRAC), we map Npa1p rRNA binding sites on 25S rRNA adjacent to the root helices of the first and last secondary structure domains of 25S rRNA. This finding supports the notion that Npa1p and Dbp6p function in the formation and/or clustering of root helices of large subunit rRNAs which creates the core of the large ribosomal subunit RNA structure. Npa1p also crosslinks to snoRNAs involved in decoding center and peptidyl transferase center modifications and in the immediate vicinity of the binding sites of these snoRNAs on 25S rRNA. Our data suggest that the Dbp6p helicase and the Npa1p complex play key roles in the compaction of the central core of 25S rRNA and the control of snoRNA-pre-rRNA interactions.

The Rio1p ATPase hinders premature entry into translation of late pre-40S pre-ribosomal particles.

Cytoplasmic maturation of precursors to the small ribosomal subunit in yeast requires the intervention of a dozen assembly factors (AFs), the precise roles of which remain elusive. One of these is Rio1p that seems to intervene at a late step of pre-40S particle maturation. We have investigated the role played by Rio1p in the dynamic association and dissociation of AFs with and from pre-40S particles. Our results indicate that Rio1p depletion leads to the stalling of at least 4 AFs (Nob1p, Tsr1p, Pno1p/Dim2p and Fap7p) in 80S-like particles. We conclude that Rio1p is important for the timely release of these factors from 80S-like particles. In addition, we present immunoprecipitation and electron microscopy evidence suggesting that when Rio1p is depleted, a subset of Nob1p-containing pre-40S particles associate with translating polysomes. Using Nob1p as bait, we purified pre-40S particles from cells lacking Rio1p and performed ribosome profiling experiments which suggest that immature 40S subunits can carry out translation elongation. We conclude that lack of Rio1p allows premature entry of pre-40S particles in the translation process and that the presence of Nob1p and of the 18S rRNA 3' extension in the 20S pre-rRNA is not incompatible with translation elongation.

Synthesis, Function, and Heterogeneity of snoRNA-Guided Posttranscriptional Nucleoside Modifications in Eukaryotic Ribosomal RNAs.

Ribosomal RNAs contain numerous 2'-O-methylated nucleosides and pseudouridines. Methylation of the 2' oxygen of ribose moieties and isomerization of uridines into pseudouridines are catalyzed by C/D and H/ACA small nucleolar ribonucleoprotein particles, respectively. We review the composition, structure, and mode of action of archaeal and eukaryotic C/D and H/ACA particles. Most rRNA modifications cluster in functionally crucial regions of the rRNAs, suggesting they play important roles in translation. Some of these modifications promote global translation efficiency or modulate translation fidelity. Strikingly, recent quantitative nucleoside modification profiling methods have revealed that a subset of modification sites is not always fully modified. The finding of such ribosome heterogeneity is in line with the concept of specialized ribosomes that could preferentially translate specific mRNAs. This emerging concept is supported by findings that some human diseases are caused by defects in the rRNA modification machinery correlated with a significant alteration of IRES-dependent translation.

Characterization of the lipid envelope of exosome encapsulated HEV particles protected from the immune response.

The hepatitis E virus (HEV) is the most common cause of acute hepatitis worldwide. Although HEV is a small, naked RNA virus, HEV particles become associated with lipids in the blood of infected patients and in the supernatant of culture systems. The egress of these particles from cells implies the exocytosis pathway but the question of the role of the resulting HEV RNA containing exosomes and the nature of the lipids they contain has not been fully addressed. We determined the lipid proportions of exosomes from uninfected and HEV-infected cells and their role in HEV spreading. We cultured a suitable HEV strain on HepG2/C3A cells and analyzed the population of exosomes containing HEV RNA using lipidomics methods and electron microscopy. We also quantified HEV infectivity using an infectivity endpoint method based on HEV RNA quantification to calculate the tissue culture infectious dose 50. Exosomes produced by HEV-infected HepG2/C3A cells contained encapsidated HEV RNA. These HEV RNA-containing exosomes were infectious but ten times less than stools. HEV from stools, but not exosome-associated HEV from culture supernatant, was neutralized by anti-HEV antibodies in a dose-dependent manner. HEV infection did not influence the morphology or lipid proportions of the bulk of exosomes. These exosomes contained significantly more cholesterol, phosphatidylserine, sphingomyelin and ceramides than the parent cells, but less phosphoinositides and polyunsaturated fatty acids. Exosomes play a major role in HEV egress but HEV infection does not modify the characteristics of the bulk of exosomes produced by infected cells. PS and cholesterol enriched in these vesicles could then be critical for HEV entry. HEV particles in exosomes are protected from the immune response which could lead to the wide circulation of HEV in its host.

Structure of a human pre-40S particle points to a role for RACK1 in the final steps of 18S rRNA processing.

Synthesis of ribosomal subunits in eukaryotes is a complex and tightly regulated process that has been mostly characterized in yeast. The discovery of a growing number of diseases linked to defects in ribosome biogenesis calls for a deeper understanding of these mechanisms and of the specificities of human ribosome maturation. We present the 19 Å resolution cryo-EM reconstruction of a cytoplasmic precursor to the human small ribosomal subunit, purified by using the tagged ribosome biogenesis factor LTV1 as bait. Compared to yeast pre-40S particles, this first three-dimensional structure of a human 40S subunit precursor shows noticeable differences with respect to the position of ribosome biogenesis factors and uncovers the early deposition of the ribosomal protein RACK1 during subunit maturation. Consistently, RACK1 is required for efficient processing of the 18S rRNA 3'-end, which might be related to its role in translation initiation. This first structural analysis of a human pre-ribosomal particle sets the grounds for high-resolution studies of conformational transitions accompanying ribosomal subunit maturation.

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.

Post-mitotic dynamics of pre-nucleolar bodies is driven by pre-rRNA processing.

Understanding the relationship between the topological dynamics of nuclear subdomains and their molecular function is a central issue in nucleus biology. Pre-nucleolar bodies (PNBs) are transient nuclear subdomains, which form at telophase and contain nucleolar proteins, snoRNPs and pre-ribosomal RNAs (pre-rRNAs). These structures gradually disappear in early G1 phase and are currently regarded as reservoirs of nucleolar factors that participate to post-mitotic reassembly of the nucleolus. Here, we provide evidence from fluorescence in situ hybridization and loss-of-function experiments in HeLa cells that PNBs are in fact active ribosome factories in which maturation of the pre-rRNAs transiting through mitosis resumes at telophase. We show that the pre-rRNA spacers are sequentially removed in PNBs when cells enter G1 phase, indicating regular pre-rRNA processing as in the nucleolus. Accordingly, blocking pre-rRNA maturation induces accumulation in PNBs of stalled pre-ribosomes characterised by specific pre-rRNAs and pre-ribosomal factors. The presence of pre-ribosomal particles in PNBs is corroborated by observation of these domains by correlative electron tomography. Most importantly, blocking pre-rRNA maturation also prevents the gradual disappearance of PNBs, which persist for several hours in the nucleoplasm. In a revised model, we propose that PNBs are autonomous extra-nucleolar ribosome maturation sites, whose orderly disassembly in G1 phase is driven by the maturation and release of their pre-ribosome content.

Structural insights into the molecular mechanisms of cauliflower mosaic virus transmission by its insect vector.

Cauliflower mosaic virus (CaMV) is transmitted from plant to plant through a seemingly simple interaction with insect vectors. This process involves an aphid receptor and two viral proteins, P2 and P3. P2 binds to both the aphid receptor and P3, itself tightly associated with the virus particle, with the ensemble forming a transmissible viral complex. Here, we describe the conformations of both unliganded CaMV P3 protein and its virion-associated form. X-ray crystallography revealed that the N-terminal domain of unliganded P3 is a tetrameric parallel coiled coil with a unique organization showing two successive four-stranded subdomains with opposite supercoiling handedness stabilized by a ring of interchain disulfide bridges. A structural model of virus-liganded P3 proteins, folding as an antiparallel coiled-coil network coating the virus surface, was derived from molecular modeling. Our results highlight the structural and biological versatility of this coiled-coil structure and provide new insights into the molecular mechanisms involved in CaMV acquisition and transmission by the insect vector.