NSR1 is required for pre-rRNA processing and for the proper maintenance of steady-state levels of ribosomal subunits

Yeast Nucleolin Nsr1 Impedes Replication and Elevates Genome Instability at an Actively Transcribed Guanine-Rich G4 DNA-Forming Sequence

A significant increase in genome instability is associated with the conformational shift of a guanine-run-containing DNA strand into the four-stranded G-quadruplex (G4) DNA. The mechanism underlying the recombination and genome rearrangements following the formation of G4 DNA in vivo has been difficult to elucidate but has become better clarified by the identification and functional characterization of several key G4 DNA-binding proteins. Mammalian nucleolin (NCL) is a highly specific G4 DNA-binding protein with a well-defined role in the transcriptional regulation of genes with associated G4 DNA-forming sequence motifs at their promoters. The consequence of the in vivo interaction between G4 DNA and nucleolin in respect to the genome instability has not been previously investigated. We show here that the yeast nucleolin Nsr1 is enriched at a G4 DNA-forming sequence in vivo and is a major factor in inducing the genome instability associated with the cotranscriptionally formed G4 DNA in the yeast genome. We also show that Nsr1 results in impeding replication past such a G4 DNA-forming sequence. The G4-associated genome instability and the G4 DNA-binding in vivo require the arginine-glycine-glycine (RGG) repeats located at the C-terminus of the Nsr1 protein. Nsr1 with the deletion of RGG domain supports normal cell growth and is sufficient for its pre-rRNA processing function. However, the truncation of the RGG domain of Nsr1 significantly weakens its interaction with G4 DNA in vivo and restores unhindered replication, overall resulting in a sharp reduction in the genome instability associated with a guanine-rich G4 DNA-forming sequence. Our data suggest that the interaction between Nsr1 with the intact RGG repeats and G4 DNA impairs genome stability by precluding the access of G4-resolving proteins and impeding replication.

Rpb4 and Puf3 imprint and post-transcriptionally control the stability of a common set of mRNAs in yeast

Gene expression involving RNA polymerase II is regulated by the concerted interplay between mRNA synthesis and degradation, crosstalk in which mRNA decay machinery and transcription machinery respectively impact transcription and mRNA stability. Rpb4, and likely dimer Rpb4/7, seem the central components of the RNA pol II governing these processes. In this work we unravel the molecular mechanisms participated by Rpb4 that mediate the posttranscriptional events regulating mRNA imprinting and stability. By RIP-Seq, we analysed genome-wide the association of Rpb4 with mRNAs and demonstrated that it targeted a large population of more than 1400 transcripts. A group of these mRNAs was also the target of the RNA binding protein, Puf3. We demonstrated that Rpb4 and Puf3 physically, genetically, and functionally interact and also affect mRNA stability, and likely the imprinting, of a common group of mRNAs. Furthermore, the Rpb4 and Puf3 association with mRNAs depends on one another. We also demonstrated, for the first time, that Puf3 associates with chromatin in an Rpb4-dependent manner. Our data also suggest that Rpb4 could be a key element of the RNA pol II that coordinates mRNA synthesis, imprinting and stability in cooperation with RBPs.

The RNA fold interactome of evolutionary conserved RNA structures in S. cerevisiae

RNA-binding proteins play key roles in regulation of gene expression via recognition of structural features in RNA molecules. Here we apply a quantitative RNA pull-down approach to 186 evolutionary conserved RNA structures and report 162 interacting proteins. Unlike global RNA interactome capture, we associate individual RNA structures within messenger RNA with their interacting proteins. Of our binders 69% are known RNA-binding proteins, whereas some are previously unrelated to RNA binding and do not harbor canonical RNA-binding domains. While current knowledge about RNA-binding proteins relates to their functions at 5′ or 3′-UTRs, we report a significant number of them binding to RNA folds in the coding regions of mRNAs. Using an in vivo reporter screen and pulsed SILAC, we characterize a subset of mRNA-RBP pairs and thus connect structural RNA features to functionality. Ultimately, we here present a generic, scalable approach to interrogate the increasing number of RNA structural motifs.

Previous study identified in vivo structured mRNA regions in Saccharomyces cerevisiae by dimethyl sulfate-sequencing. Here the authors use quantitative proteomics to identify protein interactors of 186 RNA folds in S. cerevisiae, providing functional links between RNA binding proteins and distinct mRNA fold.

Shaping the Nascent Ribosome: AAA-ATPases in Eukaryotic Ribosome Biogenesis

AAA-ATPases are molecular engines evolutionarily optimized for the remodeling of proteins and macromolecular assemblies. Three AAA-ATPases are currently known to be involved in the remodeling of the eukaryotic ribosome, a megadalton range ribonucleoprotein complex responsible for the translation of mRNAs into proteins. The correct assembly of the ribosome is performed by a plethora of additional and transiently acting pre-ribosome maturation factors that act in a timely and spatially orchestrated manner. Minimal disorder of the assembly cascade prohibits the formation of functional ribosomes and results in defects in proliferation and growth. Rix7, Rea1, and Drg1, which are well conserved across eukaryotes, are involved in different maturation steps of pre-60S ribosomal particles. These AAA-ATPases provide energy for the efficient removal of specific assembly factors from pre-60S particles after they have fulfilled their function in the maturation cascade. Recent structural and functional insights have provided the first glimpse into the molecular mechanism of target recognition and remodeling by Rix7, Rea1, and Drg1. Here we summarize current knowledge on the AAA-ATPases involved in eukaryotic ribosome biogenesis. We highlight the latest insights into their mechanism of mechano-chemical complex remodeling driven by advanced cryo-EM structures and the use of highly specific AAA inhibitors.

Meiotic cellular rejuvenation is coupled to nuclear remodeling in budding yeast

Production of healthy gametes in meiosis relies on the quality control and proper distribution of both nuclear and cytoplasmic contents. Meiotic differentiation naturally eliminates age-induced cellular damage by an unknown mechanism. Using time-lapse fluorescence microscopy in budding yeast, we found that nuclear senescence factors - including protein aggregates, extrachromosomal ribosomal DNA circles, and abnormal nucleolar material - are sequestered away from chromosomes during meiosis II and subsequently eliminated. A similar sequestration and elimination process occurs for the core subunits of the nuclear pore complex in both young and aged cells. Nuclear envelope remodeling drives the formation of a membranous compartment containing the sequestered material. Importantly, de novo generation of plasma membrane is required for the sequestration event, preventing the inheritance of long-lived nucleoporins and senescence factors into the newly formed gametes. Our study uncovers a new mechanism of nuclear quality control and provides insight into its function in meiotic cellular rejuvenation.

Nop9 binds the central pseudoknot region of 18S rRNA

The assembly of eukaryotic ribosomes requires numerous factors that transiently associate with evolving pre-ribosomal particles. The Pumilio repeat-containing protein Nop9 briefly associates with the 90S pre-ribosome during its co-transcriptional assembly. Here, we show that Nop9 specifically binds an 11-nucleotide sequence of 18S rRNA that forms the 3΄ side of the central pseudoknot and helix 28 in the mature subunit. Crystal structures of Nop9 in the free and RNA-bound states reveal a new type of Pumilio repeat protein with a distinct structure, target sequence and RNA-binding mode. Nop9 contains 10 Pumilio repeats arranged into a U-shaped scaffold. The target RNA is recognized by two stretches of repeats in a bipartite manner, and three central bases are unrecognized as a result of the degeneracy of repeats 6 and 7. Our data suggest that Nop9 regulates the folding of 18S rRNA at early assembly stages of 90S.

OsNucleolin1-L Expression in Arabidopsis Enhances Photosynthesis via Transcriptome Modification under Salt Stress Conditions

OsNUC1 encodes rice nucleolin, which has been shown to be involved in salt stress responses. Expression of the full-length OsNUC1 gene in Arabidopsis resulted in hypersensitivity to ABA during germination. Transcriptome analysis of the transgenic lines, in comparison with the wild type, revealed that the RNA abundance of >1,900 genes was significantly changed under normal growth conditions, while under salt stress conditions the RNAs of 999 genes were found to be significantly regulated. Gene enrichment analysis showed that under normal conditions OsNUC1 resulted in repression of genes involved in photosynthesis, while in salt stress conditions OsNUC1 increased expression of the genes involved in the light-harvesting complex. Correspondingly, the net rate of photosynthesis of the transgenic lines was increased under salt stress. Transgenic rice lines with overexpression of the OsNUC1-L gene were generated and tested for photosynthetic performance under salt stress conditions. The transgenic rice lines treated with salt stress at the booting stage had a higher photosynthetic rate and stomatal conductance in flag leaves and second leaves than the wild type. Moreover, higher contents of Chl a and carotenoids were found in flag leaves of the transgenic rice. These results suggest a role for OsNUC1 in the modification of the transcriptome, especially the gene transcripts responsible for photosynthesis, leading to stabilization of photosynthesis under salt stress conditions.

The new world of inorganic polyphosphates

Post-translational modifications (PTMs) add regulatory features to proteins that help establish the complex functional networks that make up higher organisms. Advances in analytical detection methods have led to the identification of more than 200 types of PTMs. However, some modifications are unstable under the present detection methods, anticipating the existence of further modifications and a much more complex map of PTMs. An example is the recently discovered protein modification polyphosphorylation. Polyphosphorylation is mediated by inorganic polyphosphate (polyP) and represents the covalent attachment of this linear polymer of orthophosphate to lysine residues in target proteins. This modification has eluded MS analysis as both polyP itself and the phosphoramidate bonds created upon its reaction with lysine residues are highly unstable in acidic conditions. Polyphosphorylation detection was only possible through extensive biochemical characterization. Two targets have been identified: nuclear signal recognition 1 (Nsr1) and its interacting partner, topoisomerase 1 (Top1). Polyphosphorylation occurs within a conserved N-terminal polyacidic serine (S) and lysine (K) rich (PASK) cluster. It negatively regulates Nsr1-Top1 interaction and impairs Top1 enzymatic activity, namely relaxing supercoiled DNA. Modulation of cellular levels of polyP regulates Top1 activity by modifying its polyphosphorylation status. Here we discuss the significance of the recently identified new role of inorganic polyP.

Stepwise and dynamic assembly of the earliest precursors of small ribosomal subunits in yeast

In this study, Zhang et al. researched how the 90S preribosomal particle is cotranscriptionally assembled in yeast using a novel approach. They determined the assembly point of 65 proteins and the U3, U14, and snR30 snoRNAs, revealing a stepwise and dynamic assembly map, thereby advancing our understanding of small subunit biogenesis.

The eukaryotic ribosomal RNA (rRNA) is associated cotranscriptionally with numerous factors into an enormous 90S preribosomal particle that conducts early processing of small ribosomal subunits. The assembly pathway and structure of the 90S particle is poorly understood. Here, we affinity-purified and analyzed the constituents of yeast 90S particles that were assembled on a series of plasmid-encoded 3′-truncated pre-18S RNAs. We determined the assembly point of 65 proteins and the U3, U14, and snR30 small nucleolar RNAs (snoRNAs), revealing a stepwise and dynamic assembly map. The 5′ external transcribed spacer (ETS) alone can nucleate a large complex. When the 18S rRNA is nearly complete, the 90S structure undergoes a dramatic reorganization, releasing U14, snR30, and 14 protein factors that bind earlier. We also identified a reference state of 90S that is fully assembled yet has not undergone 5′ETS processing. The assembly map present here provides a new framework to understand small subunit biogenesis.

The eukaryote-specific N-terminal extension of ribosomal protein S31 contributes to the assembly and function of 40S ribosomal subunits

The archaea-/eukaryote-specific 40S-ribosomal-subunit protein S31 is expressed as an ubiquitin fusion protein in eukaryotes and consists of a conserved body and a eukaryote-specific N-terminal extension. In yeast, S31 is a practically essential protein, which is required for cytoplasmic 20S pre-rRNA maturation. Here, we have studied the role of the N-terminal extension of the yeast S31 protein. We show that deletion of this extension partially impairs cell growth and 40S subunit biogenesis and confers hypersensitivity to aminoglycoside antibiotics. Moreover, the extension harbours a nuclear localization signal that promotes active nuclear import of S31, which associates with pre-ribosomal particles in the nucleus. In the absence of the extension, truncated S31 inefficiently assembles into pre-40S particles and two subpopulations of mature small subunits, one lacking and another one containing truncated S31, can be identified. Plasmid-driven overexpression of truncated S31 partially suppresses the growth and ribosome biogenesis defects but, conversely, slightly enhances the hypersensitivity to aminoglycosides. Altogether, these results indicate that the N-terminal extension facilitates the assembly of S31 into pre-40S particles and contributes to the optimal translational activity of mature 40S subunits but has only a minor role in cytoplasmic cleavage of 20S pre-rRNA at site D.

Nucleolin: dual roles in rDNA chromatin transcription

Nucleolin is a major nucleolar protein conserved in all eukaryotic organisms. It is a multifunctional protein involved in different cellular aspects like chromatin organization and stability, DNA and RNA metabolism, assembly of ribonucleoprotein complexes, cytokinesis, cell proliferation and stress response. The multifunctionality of nucleolin is linked to its tripartite structure, post-translational modifications and its ability of shuttling from and to the nucleolus/nucleoplasm and cytoplasm. Nucleolin has been now studied for many years and its activities and properties have been described in a number of excellent reviews. Here, we overview the role of nucleolin in RNA polymerase I (RNAPI) transcription and describe recent results concerning its functional interaction with rDNA chromatin organization. For a long time, nucleolin has been associated with rRNA gene expression and pre-rRNA processing. However, the functional connection between nucleolin and active versus inactive rRNA genes is still not fully understood. Novel evidence indicates that the nucleolin protein might be required for controlling the transcriptional ON/OFF states of rDNA chromatin in both mammals and plants.

The nucleoplasmin homolog NLP mediates centromere clustering and anchoring to the nucleolus

Centromere clustering during interphase is a phenomenon known to occur in many different organisms and cell types, yet neither the factors involved nor their physiological relevance is well understood. Using Drosophila tissue culture cells and flies, we identified a network of proteins, including the nucleoplasmin-like protein (NLP), the insulator protein CTCF, and the nucleolus protein Modulo, to be essential for the positioning of centromeres. Artificial targeting further demonstrated that NLP and CTCF are sufficient for clustering, while Modulo serves as the anchor to the nucleolus. Centromere clustering was found to depend on centric chromatin rather than specific DNA sequences. Moreover, unclustering of centromeres results in the spatial destabilization of pericentric heterochromatin organization, leading to partial defects in the silencing of repetitive elements, defects during chromosome segregation, and genome instability.

Localization of lipid raft proteins to the plasma membrane is a major function of the phospholipid transfer protein Sec14

The Sec14 protein domain is a conserved tertiary structure that binds hydrophobic ligands. The Sec14 protein from Saccharomyces cerevisiae is essential with studies of S. cerevisiae Sec14 cellular function facilitated by a sole temperature sensitive allele, sec14^ts. The sec14^ts allele encodes a protein with a point mutation resulting in a single amino acid change, Sec14^G266D. In this study results from a genome-wide genetic screen, and pharmacological data, provide evidence that the Sec14^G266D protein is present at a reduced level compared to wild type Sec14 due to its being targeted to the proteosome. Increased expression of the sec14^ts allele ameliorated growth arrest, but did not restore the defects in membrane accumulation or vesicular transport known to be defective in sec14^ts cells. We determined that trafficking and localization of two well characterized lipid raft resident proteins, Pma1 and Fus-Mid-GFP, were aberrant in sec14^ts cells. Localization of both lipid raft proteins was restored upon increased expression of the sec14^ts allele. We suggest that a major function provided by Sec14 is trafficking and localization of lipid raft proteins.

The small subunit processome in ribosome biogenesis—progress and prospects

The small subunit (SSU) processome is a 2.2-MDa ribonucleoprotein complex involved in the processing, assembly, and maturation of the SSU of eukaryotic ribosomes. The identities of many of the factors involved in SSU biogenesis have been elucidated over the past 40 years. However, as our understanding increases, so do the number of questions about the nature of this complicated process. Cataloging the components is the first step toward understanding the molecular workings of a system. This review will focus on how identifying components of ribosome biogenesis has led to the knowledge of how these factors, protein and RNA alike, associate with one another into subcomplexes, with a concentration on the small ribosomal subunit. We will also explore how this knowledge of subcomplex assembly has informed our understanding of the workings of the ribosome synthesis system as a whole.

In vivo approaches to dissecting the function of RNA helicases in eukaryotic ribosome assembly

In eukaryotes, ribosome biogenesis involves the nucleolar transcription and processing of pre-ribosomal RNA molecules (pre-rRNA) in a complex pathway requiring the participation of myriad protein and ribonucleoprotein factors. Through efforts aimed at categorizing and characterizing these factors, at least 20 RNA helicases have been shown to interact with or participate in the activities of the major ribosome biogenesis complexes. Unfortunately, little is known about the enzymatic properties of most of these helicases, and less is known about their roles in ribosome biogenesis and pre-rRNA maturation. This chapter presents approaches for characterizing RNA helicases involved in ribosome biogenesis. Included are methods for depletion of specific protein targets, with standard protocols for assaying the typical ribosome biogenesis defects that may result. Procedures and rationales for mutagenic studies of target proteins are discussed, as well as several approaches for identifying protein-protein interactions in order to determine functional context and potential cofactors of RNA helicases.

Deletion of a subgroup of ribosome-related genes minimizes hypoxia-induced changes and confers hypoxia tolerance

Hypoxia is a widely occurring condition experienced by diverse organisms under numerous physiological and disease conditions. To probe the molecular mechanisms underlying hypoxia responses and tolerance, we performed a genome-wide screen to identify mutants with enhanced hypoxia tolerance in the model eukaryote, the yeast Saccharomyces cerevisiae. Yeast provides an excellent model for genomic and proteomic studies of hypoxia. We identified five genes whose deletion significantly enhanced hypoxia tolerance. They are RAI1, NSR1, BUD21, RPL20A, and RSM22, all of which encode functions involved in ribosome biogenesis. Further analysis of the deletion mutants showed that they minimized hypoxia-induced changes in polyribosome profiles and protein synthesis. Strikingly, proteomic analysis by using the iTRAQ profiling technology showed that a substantially fewer number of proteins were changed in response to hypoxia in the deletion mutants, compared with the parent strain. Computational analysis of the iTRAQ data indicated that the activities of a group of regulators were regulated by hypoxia in the wild-type parent cells, but such regulation appeared to be diminished in the deletion strains. These results show that the deletion of one of the genes involved in ribosome biogenesis leads to the reversal of hypoxia-induced changes in gene expression and related regulators. They suggest that modifying ribosomal function is an effective mechanism to minimize hypoxia-induced specific protein changes and to confer hypoxia tolerance. These results may have broad implications in understanding hypoxia responses and tolerance in diverse eukaryotes ranging from yeast to humans.

Contribution of the glycolytic flux and hypoxia adaptation to efficient biofilm formation by Candida albicans

The fungal pathogen Candida albicans forms therapeutically challenging biofilms on biomedical implants. Using a transcript profiling approach genes whose expression is favoured upon biofilm growth compared with planktonic growth have been previously identified. Knock-out mutants for 38 of these genes were constructed, six of which showed a specific defect in biofilm formation. Among these genes, TYE7 that encodes a transcriptional activator of glycolytic genes in planktonic and biofilm growth conditions was identified as being required for the cohesiveness of biofilms. Biofilms formed by the tye7Δ knock-out mutant showed a hyperfilamentous morphology, and growth of this mutant on solid medium under hypoxia was also associated with the production of hyphae. Similar to TYE7 inactivation, inhibition of glycolysis or ATP synthesis using oxalate or an uncoupler, respectively, triggered morphogenesis when a wild-type strain was grown under hypoxia. These treatments also induced the formation of weakly cohesive, hyper-filamentous biofilms by a wild-type strain. Our data indicate that a hypoxic environment is generated within C. albicans biofilms and that continued biofilm development requires a Tye7p-dependent upregulation of glycolytic genes necessary to adapt to hypoxia and prevent uncontrolled hyphal formation. Thus, adaptation to hypoxia is an integral component of biofilm formation in C. albicans.

Identification of arginine- and lysine-methylation in the proteome of Saccharomyces cerevisiae and its functional implications

Background

The methylation of eukaryotic proteins has been proposed to be widespread, but this has not been conclusively shown to date. In this study, we examined 36,854 previously generated peptide mass spectra from 2,607 Saccharomyces cerevisiae proteins for the presence of arginine and lysine methylation. This was done using the FindMod tool and 5 filters that took advantage of the high number of replicate analysis per protein and the presence of overlapping peptides.

Results

A total of 83 high-confidence lysine and arginine methylation sites were found in 66 proteins. Motif analysis revealed many methylated sites were associated with MK, RGG/RXG/RGX or WXXXR motifs. Functionally, methylated proteins were significantly enriched for protein translation, ribosomal biogenesis and assembly and organellar organisation and were predominantly found in the cytoplasm and ribosome. Intriguingly, methylated proteins were seen to have significantly longer half-life than proteins for which no methylation was found. Some 43% of methylated lysine sites were predicted to be amenable to ubiquitination, suggesting methyl-lysine might block the action of ubiquitin ligase.

Conclusions

This study suggests protein methylation to be quite widespread, albeit associated with specific functions. Large-scale tandem mass spectroscopy analyses will help to further confirm the modifications reported here.

Inositol pyrophosphates: structure, enzymology and function

The stereochemistry of the inositol backbone provides a platform on which to generate a vast array of distinct molecular motifs that are used to convey information both in signal transduction and many other critical areas of cell biology. Diphosphoinositol phosphates, or inositol pyrophosphates, are the most recently characterized members of the inositide family. They represent a new frontier with both novel targets within the cell and novel modes of action. This includes the proposed pyrophosphorylation of a unique subset of proteins. We review recent insights into the structures of these molecules and the properties of the enzymes which regulate their concentration. These enzymes also act independently of their catalytic activity via protein-protein interactions. This unique combination of enzymes and products has an important role in diverse cellular processes including vesicle trafficking, endo- and exocytosis, apoptosis, telomere length regulation, chromatin hyperrecombination, the response to osmotic stress, and elements of nucleolar function.

Acclimation of Saccharomyces cerevisiae to low temperature: a chemostat-based transcriptome analysis

Effects of suboptimal temperatures on transcriptional regulation in yeast have been extensively studied in batch cultures. To eliminate indirect effects of specific growth rates that are inherent to batch-cultivation studies, genome-wide transcriptional responses to low temperatures were analyzed in steady-state chemostats, grown at a fixed specific growth rate (0.03 h(-1)). Although in vivo metabolic fluxes were essentially the same in cultures grown at 12 and at 30 degrees C, concentrations of the growth-limiting nutrients (glucose or ammonia) were higher at 12 degrees C. This difference was reflected by transcript levels of genes that encode transporters for the growth-limiting nutrients. Several transcriptional responses to low temperature occurred under both nutrient-limitation regimes. Increased transcription of ribosome-biogenesis genes emphasized the importance of adapting protein-synthesis capacity to low temperature. In contrast to observations in cold-shock and batch-culture studies, transcript levels of environmental stress response genes were reduced at 12 degrees C. Transcription of trehalose-biosynthesis genes and intracellular trehalose levels indicated that, in contrast to its role in cold-shock adaptation, trehalose is not involved in steady-state low-temperature adaptation. Comparison of the chemostat-based transcriptome data with literature data revealed large differences between transcriptional reprogramming during long-term low-temperature acclimation and the transcriptional responses to a rapid transition to low temperature.

Yeast responses to stresses associated with industrial brewery handling: Figure 1

During brewery handling, production strains of yeast must respond to fluctuations in dissolved oxygen concentration, pH, osmolarity, ethanol concentration, nutrient supply and temperature. Fermentation performance of brewing yeast strains is dependent on their ability to adapt to these changes, particularly during batch brewery fermentation which involves the recycling (repitching) of a single yeast culture (slurry) over a number of fermentations (generations). Modern practices, such as the use of high-gravity worts and preparation of dried yeast for use as an inoculum, have increased the magnitude of the stresses to which the cell is subjected. The ability of yeast to respond effectively to these conditions is essential not only for beer production but also for maintaining the fermentation fitness of yeast for use in subsequent fermentations. During brewery handling, cells inhabit a complex environment and our understanding of stress responses under such conditions is limited. The advent of techniques capable of determining genomic and proteomic changes within the cell is likely vastly to improve our knowledge of yeast stress responses during industrial brewery handling.

Arabidopsis nucleolin affects plant development and patterning

Nucleolin is a major nucleolar protein implicated in many aspects of ribosomal biogenesis, including early events such as processing of the large 35S preribosomal RNA. We found that the Arabidopsis (Arabidopsis thaliana) parallel1 (parl1) mutant, originally identified by its aberrant leaf venation, corresponds to the Arabidopsis nucleolin gene. parl1 mutants display parallel leaf venation, aberrant localization of the provascular marker Athb8:beta-glucuronidase, the auxin-sensitive reporter DR5:beta-glucuronidase, and auxin-dependent growth defects. PARL1 is highly similar to the yeast (Saccharomyces cerevisiae) nucleolin NUCLEAR SIGNAL RECOGNITION 1 (NSR1) multifunctional protein; the Arabidopsis PARL1 gene can rescue growth defects of yeast nsr1 null mutants. This suggests that PARL1 protein may have roles similar to those of the yeast nucleolin in nuclear signal recognition, ribosomal processing, and ribosomal subunit accumulation. Based on the range of auxin-related defects in parl1 mutants, we propose that auxin-dependent organ growth and patterning is highly sensitive to the efficiency of nucleolin-dependent ribosomal processing.

Characterization of AtNUC-L1 reveals a central role of nucleolin in nucleolus organization and silencing of AtNUC-L2 gene in Arabidopsis

Nucleolin is one of the most abundant protein in the nucleolus and is a multifunctional protein involved in different steps of ribosome biogenesis. In contrast to animals and yeast, the genome of the model plant Arabidopsis thaliana encodes two nucleolin-like proteins, AtNUC-L1 and AtNUC-L2. However, only the AtNUC-L1 gene is ubiquitously expressed in normal growth conditions. Disruption of this AtNUC-L1 gene leads to severe plant growth and development defects. AtNUC-L1 is localized in the nucleolus, mainly in the dense fibrillar component. Absence of this protein in Atnuc-L1 plants induces nucleolar disorganization, nucleolus organizer region decondensation, and affects the accumulation levels of pre-rRNA precursors. Remarkably, in Atnuc-L1 plants the AtNUC-L2 gene is activated, suggesting that AtNUC-L2 might rescue, at least partially, the loss of AtNUC-L1. This work is the first description of a higher eukaryotic organism with a disrupted nucleolin-like gene and defines a new role for nucleolin in nucleolus structure and rDNA chromatin organization.

Nucleolin inhibits Hdm2 by multiple pathways leading to p53 stabilization

Nucleolin is a c-Myc-induced gene product with defined roles in ribosomal RNA processing and the inhibition of chromosomal DNA replication following stress. Here we find that changes in nucleolin protein levels in unstressed cells cause parallel changes in the amount of p53 protein. Alterations in p53 levels arise from nucleolin binding to the p53 antagonist Hdm2, resulting in the inhibition of both p53 ubiquitination and Hdm2 auto-ubiquitination. Nucleolin does not alter p53 ubiquitination by human papillomavirus E6, indicating that the effect is specific for Hdm2. Although the inhibition of ligase activity would be expected to stabilize Hdm2, we instead find that nucleolin also reduces Hdm2 protein levels, demonstrating that nucleolin inhibits Hdm2 using multiple mechanisms. Increases in nucleolin levels in unstressed cells led to higher expression of p21(cip1/waf1), a reduced rate of cellular proliferation, and an increase in apoptosis. Thus, nucleolin has a number of properties in common with the tumor suppressor ARF (alternate reading frame). We propose that nucleolin, like ARF, responds to hyperproliferative signals by upregulation of p53 through Hdm2 inhibition.

A novel interaction [corrected] of nucleolin with Rad51

Nucleolin associates with various DNA repair, recombination, and replication proteins, and possesses DNA helicase, strand annealing, and strand pairing activities. Examination of nuclear protein extracts from human somatic cells revealed that nucleolin and Rad51 co-immunoprecipitate. Furthermore, purified recombinant Rad51 associates with in vitro transcribed and translated nucleolin. Electroporation-mediated introduction of anti-nucleolin antibody resulted in a 10- to 20-fold reduction in intra-plasmid homologous recombination activity in human fibrosarcoma cells. Additionally, introduction of anti-nucleolin antibody sensitized cells to death induced by the topoisomerase II inhibitor, amsacrine. Introduction of anti-Rad51 antibody also reduced intra-plasmid homologous recombination activity and induced hypersensitivity to amsacrine-induced cell death. Co-introduction of anti-nucleolin and anti-Rad51 antibodies did not produce additive effects on homologous recombination or on cellular sensitivity to amsacrine. The association of the two proteins raises the intriguing possibility that nucleolin binding to Rad51 may function to regulate homologous recombinational repair of chromosomal DNA.

The Gly-Arg-rich C-terminal domain of pea nucleolin is a DNA helicase that catalytically translocates in the 5'- to 3'-direction

Nucleolin is a major nucleolar phosphoprotein of exponentially growing eukaryotic cells. Here we report the cloning, purification, and characterization of the C-terminal glycine/arginine-rich (GAR) domain of pea nucleolin. The purified recombinant protein (17 kDa) shows ATP-/Mg(2+)-dependent DNA helicase and ssDNA-/Mg(2+)-dependent ATPase activities. The enzyme unwinds DNA in the 5'- to 3'-direction, which is the first report in plant for this directional activity. It unwinds forked/non-forked DNA with equal efficiency. The anti-nucleolin antibodies immunodepleted the activities of the enzyme. The DNA interacting ligands nogalamycin, daunorubicin, actinomycin C1, and ethidium bromide were inhibitory to DNA unwinding (with K(i) values of 0.40, 2.21, 8.0, and 9.0 microM, respectively) and ATPase (with K(i) values of 0.43, 1.65, 4.6, and 7.0 microM, respectively) activities of the enzyme. This study confirms that the unwinding and ATPase activities of pea nucleolin resided in the GAR domain. This study should make important contribution to our better understanding of DNA transaction in plants, mechanism of DNA unwinding, and the mechanism by which these ligands can disturb genome integrity.

Silencing of RNA helicase II/Gualpha inhibits mammalian ribosomal RNA production

The intricate production of ribosomal RNA is well defined in yeast, but its complexity in higher organisms is barely understood. We recently showed that down-regulation of nucleolar protein RNA helicase II/Gualpha (RH-II/Gualpha or DDX21) in Xenopus oocytes inhibited processing of 20 S rRNA to 18 S and contributed to degradation of 28 S rRNA (Yang, H., Zhou, J., Ochs, R. L., Henning, D., Jin, R., and Valdez, B. C. (2003) J. Biol. Chem. 278, 38847-38859). Since no nucleolar RNA helicase has been functionally characterized in mammalian cells, we used short interfering RNA to search for functions for RH-II/Gualpha and its paralogue RH-II/Gubeta in rRNA production. Silencing of RH-II/Gualpha by more than 80% in HeLa cells resulted in an almost 80% inhibition of 18 and 28 S rRNA production. This inhibition could be reversed by exogenous expression of wild type RH-II/Gualpha. A helicase-deficient mutant form having ATPase activity was able to rescue the production of 28 S but not 18 S rRNA. A phenotype exhibiting inhibition of 18 S and 28 S rRNA production was also observed when the paralogue RH-II/Gubeta was overexpressed. Both down-regulation of RH-II/Gualpha and overexpression of RH-II/Gubeta slowed cell proliferation. The opposite effects of the two paralogues suggest antagonistic functions.

Down-regulation of RNA helicase II/Gu results in the depletion of 18 and 28 S rRNAs in Xenopus oocyte

Genetic manipulations have revealed the functions of RNA helicases in ribosomal RNA (rRNA) biogenesis in yeast. However, no report shows the role of an RNA helicase in rRNA formation in higher eukaryotes. This study reports the functional characterization of the frog homologue of nucleolar RNA helicase II/Gu (xGu or DDX21). Down-regulation of xGu in Xenopus laevis oocyte using an antisense oligodeoxynucleotide results in the depletion of 18 and 28 S rRNAs. The disappearance of 18 S rRNA is accompanied by an accumulation of 20 S, indicating that xGu is critical in the processing of 20 to 18 S rRNA. The degradation of 28 S rRNA into fragments smaller than 18 S is also associated with a specific decrease in the level of xGu protein. These effects are reversed in the presence of in vitro synthesized wild type xGu mRNA but not its helicase-deficient mutant form. Similar aberrant rRNA processing is observed when antibody against xGu is microinjected. The involvement of xGu in processing of rRNA is consistent with the localization of Gu protein to the granular and dense fibrillar components of PtK2 cell nucleoli by immunoelectron microscopy. Our results show that xGu is involved in the processing of 20 to 18 S rRNA and contributes to the stability of 28 S rRNA in Xenopus oocytes.

Ribosome assembly in eukaryotes

Ribosome synthesis is a highly complex and coordinated process that occurs not only in the nucleolus but also in the nucleoplasm and the cytoplasm of eukaryotic cells. Based on the protein composition of several ribosomal subunit precursors recently characterized in yeast, a total of more than 170 factors are predicted to participate in ribosome biogenesis and the list is still growing. So far the majority of ribosomal factors have been implicated in RNA maturation (nucleotide modification and processing). Recent advances gave insight into the process of ribosome export and assembly. Proteomic approaches have provided the first indications for a ribosome assembly pathway in eukaryotes and confirmed the dynamic character of the whole process.

A panoramic view of yeast noncoding RNA processing

Predictive analysis using publicly available yeast functional genomics and proteomics data suggests that many more proteins may be involved in biogenesis of ribonucleoproteins than are currently known. Using a microarray that monitors abundance and processing of noncoding RNAs, we analyzed 468 yeast strains carrying mutations in protein-coding genes, most of which have not previously been associated with RNA or RNP synthesis. Many strains mutated in uncharacterized genes displayed aberrant noncoding RNA profiles. Ten factors involved in noncoding RNA biogenesis were verified by further experimentation, including a protein required for 20S pre-rRNA processing (Tsr2p), a protein associated with the nuclear exosome (Lrp1p), and a factor required for box C/D snoRNA accumulation (Bcd1p). These data present a global view of yeast noncoding RNA processing and confirm that many currently uncharacterized yeast proteins are involved in biogenesis of noncoding RNA.

In vivo analysis of nucleolar proteins modified by the yeast arginine methyltransferase Hmt1/Rmt1p

In this report, we have investigated the impact of arginine methylation on the Gar1, Nop1, and Nsr1 nucleolar proteins in Saccharomyces cerevisiae. Although previous reports have established that protein arginine methylation is important for nucleocytoplasmic shuttling, they have focused on the examination of heterogeneous nuclear ribonucleoproteins (hnRNPs). We have extended this analysis to several nucleolar proteins that represent a distinct functional class of arginine-methylated proteins. We first developed an in vivo assay to identify proteins methylated by the Hmt1 arginine methyltransferase. This assay is based on the fact that the Hmt1 enzyme utilizes S-Adenosyl-L-methionine as the methyl donor for protein arginine methylation. Following SDS polyacrylamide electrophoresis, 11 distinct proteins were identified as substrates for the Hmt1 methyltransferase. Hmt1p overexpression did not increase the methylation level on these proteins, suggesting they are fully methylated under the conditions examined. Three of the radiolabeled proteins were confirmed to be Gar1p, Nop1p, and Nsr1p. To monitor the cellular localization of these proteins, functional GFP fusion proteins were generated and found to be localized to the nucleolus. This localization was independent of arginine methylation. Furthermore, all three proteins examined did not export to the cytoplasm. In contrast, arginine methylation is required for the export of the nuclear RNA-binding proteins Npl3p, Hrp1p, and Nab2p. The observation that three nucleolar proteins are modified by Hmt1p but are not exported from the nucleolus implies an alternate role for arginine methylation.

Comprehensive expression analysis of time-dependent genetic responses in yeast cells to low temperature

We performed genome-wide expression analysis to determine genetic responses in Saccharomyces cerevisiae to a low temperature environment using a cDNA microarray. Approximately 25% of the genes in the yeast genome were found to be involved in the response of yeast to low temperature. This finding of a large number of genes being involved in the response to low temperature enabled us to give a functional interpretation to the genetic responses to the stimulus. Functional and clustering analyses of temporal changes in gene expression revealed that global states of the expressions of up-regulated genes could be characterized as having three phases (the early, middle, and late phases). In each phase, genes related to rRNA synthesis, ribosomal proteins, or several stress responses are time-dependently up-regulated, respectively. Through these phases, yeast cells may improve reduced efficiency of translation and enhance cell protection mechanisms to survive under a low temperature condition. Furthermore, these time-dependent regulations of these genes would be controlled by the cAMP-protein kinase A pathway. The results of our study provide a global description of transcriptional response for adaptation to low temperature in yeast cells.

Dual function of eIF3j/Hcr1p in processing 20 S pre-rRNA and translation initiation

eIF3j/Hcr1p, a protein associated with eIF3, was shown to bind to, and stabilize, the multifactor complex containing eIFs 1, 2, 3, and 5 and Met-tRNA(i)(Met), whose formation is required for an optimal rate of translation initiation. Here we present evidence that eIF3j/Hcr1p is an RNA binding protein that enhances a late step in 40 S ribosome maturation involving cleavage of the 20 S precursor of 18 S rRNA in the cytoplasm. Immunofluorescence staining shows that eIF3j/Hcr1p is localized predominantly in the cytoplasm. The hcr1Delta mutant exhibits a decreased amount of 40 S subunits, hypersensitivity to paromomycin, and increased levels of 20 S pre-rRNA. Combining the hcr1Delta mutation with drs2Delta or rps0aDelta, deletions of two other genes involved in the same step of 40 S subunit biogenesis, produced a synthetic growth defect. p35, the human ortholog of eIF3j/Hcr1p, partially complemented the slow growth phenotype conferred by hcr1Delta when overexpressed in yeast. heIF3j/p35 was found physically associated with yeast eIF3 and 43 S initiation complexes in vitro and in vivo. Because it did not complement the 40 S biogenesis defect of hcr1Delta, it appears that heIF3j can substitute for eIF3j/Hcr1p only in translation initiation. We conclude that eIF3j/Hcr1p is required for rapid processing of 20 S to 18 S rRNA besides its role in translation initiation, providing an intriguing link between ribosome biogenesis and translation.

Trypanosoma brucei 5'ETS A'-cleavage is directed by 3'-adjacent sequences, but not two U3 snoRNA-binding elements, which are all required for subsequent pre-small subunit rRNA processing events

Trypanosoma brucei pre-rRNA processing commences by cleavage near the 5' end of 5.8 S sequences. The 5' external transcribed spacer (5'ETS) is removed from pre-small subunit (SSU) rRNAs by sequential cleavages at internal A' and A0 sites, and A1 at the 5' end of SSU rRNA. The A' and A0 sites positionally resemble the U3 small nucleolar RNA-dependent, primary pre-rRNA cleavages of vertebrates and yeast, respectively. Uniquely in T. brucei, two U3-crosslinkable 5'ETS sites are essential for SSU rRNA production: site1b is novel in its 3' location to the A' site, and site3 lies upstream of A0 in a position analogous to the yeast U3-binding site. Here, in vivo analysis of mutated 5'ETS sequences shows that sequences 5' to the A' site are not needed for A' cleavage or SSU rRNA production. A' cleavage is linked to, but is not sufficient to trigger, downstream pre-SSU rRNA processing events. These events require an intact 11 nt sequence, 3'-adjacent to A', which directs efficient and accurate A' cleavage. Neither the A' nearby site1b nor the site3 U3-binding elements affect A' processing, yet each is required for A0 and A1 cleavage, and SSU rRNA production. The same U3 3' hinge bases evidently bind a core element, UGUu/gGGU, within site1a and site3; the U3-site1b interaction is less reliant on base-pairing than the U3-site3 interaction. As yeast U3 5' hinge bases pair to 5'ETS sequences, it is clear that distinct U3 hinge regions can interact at both novel and related 5'ETS sites to promote 3'-proximal 5'ETS processing events in diverse organisms. The T. brucei data fit a model wherein processing factors assemble at the 5'ETS site1a to affect A' cleavage and stabilize a U3-site1b complex, which may work in concert with the downstream U3-site3 complex to assist processing events leading to ribosomal SSU production.

Novel stress-responsive genes EMG1 and NOP14 encode conserved, interacting proteins required for 40S ribosome biogenesis

Under stressful conditions organisms adjust the synthesis, processing, and trafficking of molecules to allow survival from and recovery after stress. In baker's yeast Saccharomyces cerevisiae, the cellular production of ribosomes is tightly matched with environmental conditions and nutrient availability through coordinate transcriptional regulation of genes involved in ribosome biogenesis. On the basis of stress-responsive gene expression and functional studies, we have identified a novel, evolutionarily conserved gene, EMG1, that has similar stress-responsive gene expression patterns as ribosomal protein genes and is required for the biogenesis of the 40S ribosomal subunit. The Emg1 protein is distributed throughout the cell; however, its nuclear localization depends on physical interaction with a newly characterized nucleolar protein, Nop14. Yeast depleted of Nop14 or harboring a temperature-sensitive allele of emg1 have selectively reduced levels of the 20S pre-rRNA and mature18S rRNA and diminished cellular levels of the 40S ribosomal subunit. Neither Emg1 nor Nop14 contain any characterized functional motifs; however, isolation and functional analyses of mammalian orthologues of Emg1 and Nop14 suggest that these proteins are functionally conserved among eukaryotes. We conclude that Emg1 and Nop14 are novel proteins whose interaction is required for the maturation of the 18S rRNA and for 40S ribosome production.

Light differentially regulates cell division and the mRNA abundance of pea nucleolin during de-etiolation

The abundance of plant nucleolin mRNA is regulated during de-etiolation by phytochrome. A close correlation between the mRNA abundance of nucleolin and mitosis has also been previously reported. These results raised the question of whether the effects of light on nucleolin mRNA expression were a consequence of light effects on mitosis. To test this we compared the kinetics of light-mediated increases in cell proliferation with that of light-mediated changes in the abundance of nucleolin mRNA using plumules of dark-grown pea (Pisum sativum) seedlings. These experiments show that S-phase increases 9 h after a red light pulse, followed by M-phase increases in the plumule leaves at 12 h post-irradiation, a time course consistent with separately measured kinetics of red light-induced increases in the expression of cell cycle-regulated genes. These increases in cell cycle-regulated genes are photoreversible, implying that the light-induced increases in cell proliferation are, like nucleolin mRNA expression, regulated via phytochrome. Red light stimulates increases in the mRNA for nucleolin at 6 h post-irradiation, prior to any cell proliferation changes and concurrent with the reported timing of phytochrome-mediated increases of rRNA abundance. After a green light pulse, nucleolin mRNA levels increase without increasing S-phase or M-phase. Studies in animals and yeast indicate that nucleolin plays a significant role in ribosome biosynthesis. Consistent with this function, pea nucleolin can rescue nucleolin deletion mutants of yeast that are defective in rRNA synthesis. Our data show that during de-etiolation, the increased expression of nucleolin mRNA is more directly regulated by light than by mitosis.

Role for nucleolin/Nsr1 in the cellular localization of topoisomerase I

Nucleolin functions in ribosome biogenesis and contains an acidic N terminus that binds nuclear localization sequences. In previous work we showed that human nucleolin associates with the N-terminal region of human topoisomerase I (Top1). We have now mapped the topoisomerase I interaction domain of nucleolin to the N-terminal 225 amino acids. We also show that the Saccharomyces cerevisiae nucleolin ortholog, Nsr1p, physically interacts with yeast topoisomerase I, yTop1p. Studies of isogenic NSR1(+) and Deltansr1 strains indicate that NSR1 is important in determining the cellular localization of yTop1p. Moreover, deletion of NSR1 reduces sensitivity to camptothecin, an antineoplastic topoisomerase I inhibitor. By contrast, Deltansr1 cells are hypersensitive to the topoisomerase II-targeting drug amsacrine. These findings indicate that nucleolin/Nsr1 is involved in the cellular localization of Top1 and that this localization may be important in determining sensitivity to drugs that target topoisomerases.

Factors affecting nuclear export of the 60S ribosomal subunit in vivo

In Saccharomyces cerevisiae, the 60S ribosomal subunit assembles in the nucleolus and then is exported to the cytoplasm, where it joins the 40S subunit for translation. Export of the 60S subunit from the nucleus is known to be an energy-dependent and factor-mediated process, but very little is known about the specifics of its transport. To begin to address this problem, an assay was developed to follow the localization of the 60S ribosomal subunit in S. cerevisiae. Ribosomal protein L11b (Rpl11b), one of the approximately 45 ribosomal proteins of the 60S subunit, was tagged at its carboxyl terminus with the green fluorescent protein (GFP) to enable visualization of the 60S subunit in living cells. A panel of mutant yeast strains was screened for their accumulation of Rpl11b-GFP in the nucleus as an indicator of their involvement in ribosome synthesis and/or transport. This panel included conditional alleles of several rRNA-processing factors, nucleoporins, general transport factors, and karyopherins. As predicted, conditional alleles of rRNA-processing factors that affect 60S ribosomal subunit assembly accumulated Rpl11b-GFP in the nucleus. In addition, several of the nucleoporin mutants as well as a few of the karyopherin and transport factor mutants also mislocalized Rpl11b-GFP. In particular, deletion of the previously uncharacterized karyopherin KAP120 caused accumulation of Rpl11b-GFP in the nucleus, whereas ribosomal protein import was not impaired. Together, these data further define the requirements for ribosomal subunit export and suggest a biological function for KAP120.

Novel RING finger proteins, Air1p and Air2p, interact with Hmt1p and inhibit the arginine methylation of Npl3p

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are involved in the mRNA processing and export and are post-translationally modified by methylation at arginine residues in their arginine-glycine-rich (RGG) domains. We screened the factors that can interact with the RGG domain of Npl3p only in the presence of Hmt1p with the two-hybrid system in Saccharomyces cerevisiae. An isolated clone, YIL079, encodes a novel RING finger protein that was not directly bound to Npl3p but associated with the N terminus of Hmt1p. Thus, we designated the gene product Air1p (arginine methyltransferase-interacting RING finger protein). Air1p inhibited the Hmt1p-mediated methylation of Npl3p in vitro. Overexpression of Air1p repressed the Hmt1p-dependent growth of cells. Since homology searches indicate that the YDL175 gene product has significant identity (45%) with Air1p, we designated the gene AIR2. Air2p also has a RING finger domain and was bound to Hmt1p. Although single disruption of either gene gave no effect on the cell growth, cells lacking Air1p and Air2p grew at an extremely slow rate with accumulated poly(A)(+) RNA in the nucleus. Thus, Air1p and Air2p may affect mRNA transport by regulating the arginine methylation state of heterogeneous nuclear ribonucleoproteins.

Inheritance of suppressors of the drug sensitivity of a NSR1 deleted yeast strain

The NSR1 gene product is involved in ribosomal RNA production and ribosome assembly in Saccharomyces cerevisiae. Yeast strains carrying a deletion of the NSR1 gene have a defect in rRNA processing, an aberrant ribosome profile and are sensitive to the drug paromomycin. This paper reports the isolation and characterization of spontaneous suppressors of the paromomycin sensitivity. Such suppressors could be isolated at very high frequency and do not exhibit straightforward single-gene inheritance patterns. The suppressors are not influenced by non-Mendelian factors such as psi or rho. Through a replacement of chromosomal rDNA with a plasmid rDNA system, I show that suppression of paromomycin sensitivity is mediated by rDNA. Swapping wild-type plasmid rDNA for chromosomal rDNA can reverse the suppression, but the effect does not appear to be due to amplification of rDNA or amplification of a pre-existing mutant rDNA copy.

Formation of a complex between nucleolin and replication protein A after cell stress prevents initiation of DNA replication

We used a biochemical screen to identify nucleolin, a key factor in ribosome biogenesis, as a high-affinity binding partner for the heterotrimeric human replication protein A (hRPA). Binding studies in vitro demonstrated that the two proteins physically interact, with nucleolin using an unusual contact with the small hRPA subunit. Nucleolin significantly inhibited both simian virus 40 (SV-40) origin unwinding and SV-40 DNA replication in vitro, likely by nucleolin preventing hRPA from productive interaction with the SV-40 initiation complex. In vivo, use of epifluorescence and confocal microscopy showed that heat shock caused a dramatic redistribution of nucleolin from the nucleolus to the nucleoplasm. Nucleolin relocalization was concomitant with a tenfold increase in nucleolin-hRPA complex formation. The relocalized nucleolin significantly overlapped with the position of hRPA, but only poorly with sites of ongoing DNA synthesis. We suggest that the induced nucleolin-hRPA interaction signifies a novel mechanism that represses chromosomal replication after cell stress.

Ribosome synthesis in Saccharomyces cerevisiae

The synthesis of ribosomes is one of the major metabolic pathways in all cells. In addition to around 75 individual ribosomal proteins and 4 ribosomal RNAs, synthesis of a functional eukaryotic ribosome requires a remarkable number of trans-acting factors. Here, we will discuss the recent, and often surprising, advances in our understanding of ribosome synthesis in the yeast Saccharomyces cerevisiae. These will underscore the unexpected complexity of eukaryotic ribosome synthesis.

Structure and functions of nucleolin

Nucleolin is an abundant protein of the nucleolus. Nucleolar proteins structurally related to nucleolin are found in organisms ranging from yeast to plants and mammals. The association of several structural domains in nucleolin allows the interaction of nucleolin with different proteins and RNA sequences. Nucleolin has been implicated in chromatin structure, rDNA transcription, rRNA maturation, ribosome assembly and nucleo-cytoplasmic transport. Studies of nucleolin over the last 25 years have revealed a fascinating role for nucleolin in ribosome biogenesis. The involvement of nucleolin at multiple steps of this biosynthetic pathway suggests that it could play a key role in this highly integrated process.

Yeast ribosomal proteins L4, L17, L20, and L25 exhibit different binding characteristics for the yeast 35S precursor rRNA

In vitro synthesized radioactive yeast 35S precursor rRNA (35S pre-rRNA) molecules were used to determine the binding characteristics of 13 proteins from the yeast 60S ribosome subunit. L4, L17, L20 and L25 were found to bind the 35S pre-rRNA molecule in vitro in the absence of any other cellular components as determined by a modified membrane filtration assay and an agarose gel mobility shift assay. In all cases, RNA-protein complex formation was proportional to the amount of protein added to the binding reaction mixture. Binding to the pre-rRNA could be saturated yielding a molar RNA/protein ratio approaching one. Non-radioactive 35S pre-rRNA transcript competed for the binding in a dosage-dependent manner. Presence of 18S rRNA species and poly(A) did not affect their binding to the 35S RNA. However, in the presence of the 25S rRNA species, the four proteins exhibited distinct binding characteristics for the pre-rRNA molecule. L4 did not bind the 25S rRNA but interacted specifically with the 35S pre-rRNA molecule with a binding constant of 4.4x10(6)/M. L17 bound the pre-rRNA molecule preferentially (Ka=17x10(6)/M) but also bound the mature 25S rRNA species (Ka=10x10(6)/M). L20 bound both the pre-rRNA molecule and the 25S rRNA species equally well (Ka=11-12x10(6)/M). L25 also bound both the 35S pre-rRNA and the mature 25S rRNA with slightly different affinities, with Ka=3.1 vs. 2.5x10(6)M, respectively. We speculate that L4, L17, and L25 are among the early assembled ribosomal proteins but L4 may be one of the first ribosomal proteins that bind to the 35S pre-rRNA molecule during ribosome biogenesis.

Synthetic lethal interactions with conditional poly(A) polymerase alleles identify LCP5, a gene involved in 18S rRNA maturation

To identify new genes involved in 3'-end formation of mRNAs in Saccharomyces cerevisiae, we carried out a screen for synthetic lethal mutants with the conditional poly(A) polymerase allele, pap1-7. Five independent temperature-sensitive mutations called Icp1 to Icp5 (for lethal with conditional pap1 allele) were isolated. Here, we describe the characterization of the essential gene LCP5 which codes for a protein with a calculated molecular mass of 40.8 kD. Unexpectedly, we found that mutations in LCP5 caused defects in pre-ribosomal RNA (pre-rRNA) processing, whereas mRNA 3'-end formation in vitro was comparable to wild-type. Early cleavage steps (denoted A0 to A2) that lead to the production of mature 18S rRNA were impaired. In vivo depletion of Lcp5p also inhibited pre-rRNA processing. As a consequence, mutant and depleted cells showed decreased levels of polysomes compared to wild-type cells. Indirect immunofluorescence indicated a predominant localization of Lcp5p in the nucleolus. In addition, antibodies directed against Lcp5p specifically immunoprecipitated the yeast U3 snoRNA snR17, suggesting that the protein is directly involved in pre-rRNA processing.

NCL1, a novel gene for a non-essential nuclear protein in Saccharomyces cerevisiae

The nucleolar protein Nop2p is an essential gene product that is required for pre-rRNA processing and ribosome biogenesis in Saccharomyces cerevisiae (Hong, B. et al., 1997, Mol. Cell. Biol., 17, 378-388). A search for proteins similar to Nop2p identified a novel yeast gene product that also shares significant homology with the human proliferation associated nucleolar protein p120. The gene encoding this 78kDa protein was termed NCL1 (for nuclear protein 1; corresponding to YBL024w). Ncl1p and Nop2p contain an evolutionarily conserved motif that has been termed the 'NOL1/NOP2/fmu family signature' (NOL1 encodes p120). Epitope tagged Ncl1p was found to be localized to the nucleus, including the nucleolus, and was concentrated at the nuclear periphery. NCL1 is not essential. Strains containing a disruption of NCL1, or strains overexpressing NCL1, grow essentially identically to wildtype NCL1 strains on a number of different media and at different temperatures. Disruption of NCL1 does not affect steady-state levels of large and small ribosome subunits, monoribosomes, and polyribosomes. However, disruption of NCL1 leads to increased sensitivity to the antibiotic paromomycin.

RNA and protein interactions modulated by protein arginine methylation

This review summarizes the current status of protein arginine N-methylation reactions. These covalent modifications of proteins are now recognized in a number of eukaryotic proteins and their functional significance is beginning to be understood. Genes that encode those methyltransferases specific for catalyzing the formation of asymmetric dimethylarginine have been identified. The enzyme modifies a number of generally nuclear or nucleolar proteins that interact with nucleic acids, particularly RNA. Postulated roles for these reactions include signal transduction, nuclear transport, or a direct modulation of nucleic acid interactions. A second methyltransferase activity that symmetrically dimethylates an arginine residue in myelin basic protein, a major component of the axon sheath, has also been characterized. However, a gene encoding this activity has not been identified to date and the cellular function for this methylation reaction has not been clearly established. From the analysis of the sequences surrounding known arginine methylation sites, we have determined consensus methyl-accepting sequences that may be useful in identifying novel substrates for these enzymes and may shed further light on their physiological role.

The role of the Schizosaccharomyces pombe gar2 protein in nucleolar structure and function depends on the concerted action of its highly charged N terminus and its RNA-binding domains

Nonribosomal nucleolar protein gar2 is required for 18S rRNA and 40S ribosomal subunit production in Schizosaccharomyces pombe. We have investigated the consequences of the absence of each structural domain of gar2 on cell growth, 18S rRNA production, and nucleolar structure. Deletion of gar2 RNA-binding domains (RBDs) causes stronger inhibition of growth and 18S rRNA accumulation than the absence of the whole protein, suggesting that other factors may be titrated by its remaining N-terminal basic/acidic serine-rich domain. These drastic functional defects correlate with striking nucleolar hypertrophy. Point mutations in the conserved RNP1 motifs of gar2 RBDs supposed to inhibit RNA-protein interactions are sufficient to induce severe nucleolar modifications but only in the presence of the N-terminal domain of the protein. Gar2 and its mutants also distribute differently in glycerol gradients: gar2 lacking its RBDs is found either free or assembled into significantly larger complexes than the wild-type protein. We propose that gar2 helps the assembly on rRNA of factors necessary for 40S subunit synthesis by providing a physical link between them. These factors may be recruited by the N-terminal domain of gar2 and may not be released if interaction of gar2 with rRNA is impaired.