PUB1: a major yeast poly(A)+ RNA-binding protein

Protein Phase Separation during Stress Adaptation and Cellular Memory

Cells need to organise and regulate their biochemical processes both in space and time in order to adapt to their surrounding environment. Spatial organisation of cellular components is facilitated by a complex network of membrane bound organelles. Both the membrane composition and the intra-organellar content of these organelles can be specifically and temporally controlled by imposing gates, much like bouncers controlling entry into night-clubs. In addition, a new level of compartmentalisation has recently emerged as a fundamental principle of cellular organisation, the formation of membrane-less organelles. Many of these structures are dynamic, rapidly condensing or dissolving and are therefore ideally suited to be involved in emergency cellular adaptation to stresses. Remarkably, the same proteins have also the propensity to adopt self-perpetuating assemblies which properties fit the needs to encode cellular memory. Here, we review some of the principles of phase separation and the function of membrane-less organelles focusing particularly on their roles during stress response and cellular memory.

Deep conservation of prion-like composition in the eukaryotic prion-former Pub1/Tia1 family and its relatives

Pub1 protein is an important RNA-binding protein functional in stress granule assembly in budding yeast Saccharomyces cerevisiae and, as its co-ortholog Tia1, in humans. It is unique among proteins in evidencing prion-like aggregation in both its yeast and human forms. Previously, we noted that Pub1/Tia1 was the only protein linked to human disease that has prion-like character and and has demonstrated such aggregation in both species. Thus, we were motivated to probe further into the evolution of the Pub1/Tia1 family (and its close relative Nam8 and its orthologs) to gain a picture of how such a protein has evolved over deep evolutionary time since the last common ancestor of eukaryotes. Here, we discover that the prion-like composition of this protein family is deeply conserved across eukaryotes, as is the prion-like composition of its close relative Nam8/Ngr1. A sizeable minority of protein orthologs have multiple prion-like domains within their sequences (6-20% depending on criteria). The number of RNA-binding RRM domains is conserved at three copies over >86% of the Pub1 family (>71% of the Nam8 family), but proteins with just one or two RRM domains occur frequently in some clades, indicating that these are not due to annotation errors. Overall, our results indicate that a basic scaffold comprising three RNA-binding domains and at least one prion-like region has been largely conserved since the last common ancestor of eukaryotes, providing further evidence that prion-like aggregation may be a very ancient and conserved phenomenon for certain specific proteins.

A New Method for Determining Structure Ensemble: Application to a RNA Binding Di-Domain Protein

Structure ensemble determination is the basis of understanding the structure-function relationship of a multidomain protein with weak domain-domain interactions. Paramagnetic relaxation enhancement has been proven a powerful tool in the study of structure ensembles, but there exist a number of challenges such as spin-label flexibility, domain dynamics, and overfitting. Here we propose a new (to our knowledge) method to describe structure ensembles using a minimal number of conformers. In this method, individual domains are considered rigid; the position of each spin-label conformer and the structure of each protein conformer are defined by three and six orthogonal parameters, respectively. First, the spin-label ensemble is determined by optimizing the positions and populations of spin-label conformers against intradomain paramagnetic relaxation enhancements with a genetic algorithm. Subsequently, the protein structure ensemble is optimized using a more efficient genetic algorithm-based approach and an overfitting indicator, both of which were established in this work. The method was validated using a reference ensemble with a set of conformers whose populations and structures are known. This method was also applied to study the structure ensemble of the tandem di-domain of a poly (U) binding protein. The determined ensemble was supported by small-angle x-ray scattering and nuclear magnetic resonance relaxation data. The ensemble obtained suggests an induced fit mechanism for recognition of target RNA by the protein.

Harnessing natural sequence variation to dissect posttranscriptional regulatory networks in yeast

Understanding how genomic variation influences phenotypic variation through the molecular networks of the cell is one of the central challenges of biology. Transcriptional regulation has received much attention, but equally important is the posttranscriptional regulation of mRNA stability. Here we applied a systems genetics approach to dissect posttranscriptional regulatory networks in the budding yeast Saccharomyces cerevisiae. Quantitative sequence-to-affinity models were built from high-throughput in vivo RNA binding protein (RBP) binding data for 15 yeast RBPs. Integration of these models with genome-wide mRNA expression data allowed us to estimate protein-level RBP regulatory activity for individual segregants from a genetic cross between two yeast strains. Treating these activities as a quantitative trait, we mapped trans-acting loci (activity quantitative trait loci, or aQTLs) that act via posttranscriptional regulation of transcript stability. We predicted and experimentally confirmed that a coding polymorphism at the IRA2 locus modulates Puf4p activity. Our results also indicate that Puf3p activity is modulated by distinct loci, depending on whether it acts via the 5′ or the 3′ untranslated region of its target mRNAs. Together, our results validate a general strategy for dissecting the connectivity between posttranscriptional regulators and their upstream signaling pathways.

Global analysis of yeast mRNPs

Proteins regulate gene expression by controlling mRNA biogenesis, localization, translation and decay. Identifying the composition, diversity and function of mRNA-protein complexes (mRNPs) is essential to understanding these processes. In a global survey of Saccharomyces cerevisiae mRNA-binding proteins, we identified 120 proteins that cross-link to mRNA, including 66 new mRNA-binding proteins. These include kinases, RNA-modification enzymes, metabolic enzymes and tRNA- and rRNA-metabolism factors. These proteins show dynamic subcellular localization during stress, including assembly into stress granules and processing bodies (P bodies). Cross-linking and immunoprecipitation (CLIP) analyses of the P-body components Pat1, Lsm1, Dhh1 and Sbp1 identified sites of interaction on specific mRNAs, revealing positional binding preferences and co-assembly preferences. When taken together, this work defines the major yeast mRNP proteins, reveals widespread changes in their subcellular location during stress and begins to define assembly rules for P-body mRNPs.

Pub1p C-terminal RRM domain interacts with Tif4631p through a conserved region neighbouring the Pab1p binding site

Pub1p, a highly abundant poly(A)+ mRNA binding protein in Saccharomyces cerevisiae, influences the stability and translational control of many cellular transcripts, particularly under some types of environmental stresses. We have studied the structure, RNA and protein recognition modes of different Pub1p constructs by NMR spectroscopy. The structure of the C-terminal RRM domain (RRM3) shows a non-canonical N-terminal helix that packs against the canonical RRM fold in an original fashion. This structural trait is conserved in Pub1p metazoan homologues, the TIA-1 family, defining a new class of RRM-type domains that we propose to name TRRM (TIA-1 C-terminal domain-like RRM). Pub1p TRRM and the N-terminal RRM1-RRM2 tandem bind RNA with high selectivity for U-rich sequences, with TRRM showing additional preference for UA-rich ones. RNA-mediated chemical shift changes map to β-sheet and protein loops in the three RRMs. Additionally, NMR titration and biochemical in vitro cross-linking experiments determined that Pub1p TRRM interacts specifically with the N-terminal region (1-402) of yeast eIF4G1 (Tif4631p), very likely through the conserved Box1, a short sequence motif neighbouring the Pab1p binding site in Tif4631p. The interaction involves conserved residues of Pub1p TRRM, which define a protein interface that mirrors the Pab1p-Tif4631p binding mode. Neither protein nor RNA recognition involves the novel N-terminal helix, whose functional role remains unclear. By integrating these new results with the current knowledge about Pub1p, we proposed different mechanisms of Pub1p recruitment to the mRNPs and Pub1p-mediated mRNA stabilization in which the Pub1p/Tif4631p interaction would play an important role.

Dissecting the expression dynamics of RNA-binding proteins in posttranscriptional regulatory networks

In eukaryotic organisms, gene expression requires an additional level of coordination that links transcriptional and posttranslational processes. Messenger RNAs have traditionally been viewed as passive molecules in the pathway from transcription to translation. However, it is now clear that RNA-binding proteins (RBPs) play an important role in cellular homeostasis by controlling gene expression at the posttranscriptional level. Here, we show that RBPs, as a class of proteins, show distinct gene expression dynamics compared to other protein coding genes in the eukaryote Sacchoromyces cerevisiae. We find that RBPs generally exhibit high protein stability, translational efficiency, and protein abundance but their encoding transcripts tend to have a low half-life. We show that RBPs are also most often posttranslationally modified, indicating their potential for regulation at the protein level to control diverse cellular processes. Further analysis of the RBP-RNA interaction network showed that the number of distinct targets bound by an RBP (connectivity) is strongly correlated with its protein stability, translational efficiency, and abundance. We also note that RBPs show less noise in their expression in a population of cells, with highly connected RBPs showing significantly lower noise. Our results indicate that highly connected RBPs are likely to be tightly regulated at the protein level as significant changes in their expression may bring about large-scale changes in global expression levels by affecting their targets. These observations might explain the molecular basis behind the cause of a number of disorders associated with misexpression or mutation in RBPs. Future studies uncovering the posttranscriptional networks in higher eukaryotes can help our understanding of the link between different levels of regulation and their role in pathological conditions.

The pivotal roles of TIA proteins in 5' splice-site selection of alu exons and across evolution

More than 5% of alternatively spliced internal exons in the human genome are derived from Alu elements in a process termed exonization. Alus are comprised of two homologous arms separated by an internal polypyrimidine tract (PPT). In most exonizations, splice sites are selected from within the same arm. We hypothesized that the internal PPT may prevent selection of a splice site further downstream. Here, we demonstrate that this PPT enhanced the selection of an upstream 5′ splice site (5′ss), even in the presence of a stronger 5′ss downstream. Deletion of this PPT shifted selection to the stronger downstream 5′ss. This enhancing effect depended on the strength of the downstream 5′ss, on the efficiency of base-pairing to U1 snRNA, and on the length of the PPT. This effect of the PPT was mediated by the binding of TIA proteins and was dependent on the distance between the PPT and the upstream 5′ss. A wide-scale evolutionary analysis of introns across 22 eukaryotes revealed an enrichment in PPTs within ∼20 nt downstream of the 5′ss. For most metazoans, the strength of the 5′ss inversely correlated with the presence of a downstream PPT, indicative of the functional role of the PPT. Finally, we found that the proteins that mediate this effect, TIA and U1C, and in particular their functional domains, are highly conserved across evolution. Overall, these findings expand our understanding of the role of TIA1/TIAR proteins in enhancing recognition of exons, in general, and Alu exons, in particular.

Human genes are composed of functional regions, termed exons, separated by non-functional regions, termed introns. Intronic sequences may gradually accumulate mutations and subsequently become recognized by the splicing machinery as exons, a process termed exonization. Alu elements are prone to undergo exonization: more than 5% of alternatively spliced internal exons in the human genome originate from Alu elements. A typical Alu element is ∼300 nucleotides long, consisting of two arms separated by a polypyrimdine tract (PPT). Interestingly, in most cases, exonization occurs almost exclusively within either the right arm or the left, not both. Here we found that the PPT between the two arms serves as a binding site for TIA proteins and prevents the exon selection process from expanding into downstream regions. To obtain a wider overview of TIA function, we performed a cross-evolutionary analysis within 22 eukaryotes of this protein and of U1C, a protein known to interact with it, and found that functional regions of both these proteins were highly conserved. These findings highlight the pivotal role of TIA proteins in 5′ splice-site selection of Alu exons and exon recognition in general.

A novel class of mRNA-containing cytoplasmic granules are produced in response to UV-irradiation

Nucleic acids are substrates for different types of damage, but little is known about the fate of damaged RNAs. We addressed the existence of an RNA-damage response in yeast. The decay kinetics of GAL1p-driven mRNAs revealed a dose-dependent mRNA stabilization upon UV-irradiation that was not observed after heat or saline shocks, or during nitrogen starvation. UV-induced mRNA stabilization did not depend on DNA repair, damage checkpoint or mRNA degradation machineries. Notably, fluorescent in situ hybridization revealed that after UV-irradiation, polyadenylated mRNA accumulated in cytoplasmic foci that increased in size with time. In situ colocalization showed that these foci are not processing-bodies, eIF4E-, eIF4G-, and Pab1-containing bodies, stress granules, autophagy vesicles, or part of the secretory or endocytic pathways. These results point to the existence of a specific eukaryotic RNA-damage response, which leads to new polyadenylated mRNA-containing granules (UV-induced mRNA granules; UVGs). We propose that potentially damaged mRNAs, which may be deleterious to the cell, are temporarily stored in UVG granules to safeguard cell viability.

Imp associates with squid and Hrp48 and contributes to localized expression of gurken in the oocyte

Localization and translational control of Drosophila melanogaster gurken and oskar mRNAs rely on the hnRNP proteins Squid and Hrp48, which are complexed with one another in the ovary. Imp, the Drosophila homolog of proteins acting in localization of mRNAs in other species, is also associated with Squid and Hrp48. Notably, Imp is concentrated at sites of gurken and oskar mRNA localization in the oocyte, and alteration of gurken localization also alters Imp distribution. Imp binds gurken mRNA with high affinity in vitro; thus, the colocalization with gurken mRNA in vivo is likely to be the result of direct binding. Imp mutants support apparently normal regulation of gurken and oskar mRNAs. However, loss of Imp activity partially suppresses a gurken misexpression phenotype, indicating that Imp does act in control of gurken expression but has a largely redundant role that is only revealed when normal gurken expression is perturbed. Overexpression of Imp disrupts localization of gurken mRNA as well as localization and translational regulation of oskar mRNA. The opposing effects of reduced and elevated Imp activity on gurken mRNA expression indicate a role in gurken mRNA regulation.

Global analysis of Pub1p targets reveals a coordinate control of gene expression through modulation of binding and stability

Regulation of mRNA turnover is an important cellular strategy for posttranscriptional control of gene expression, mediated by the interplay of cis-acting sequences and associated trans-acting factors. Pub1p, an ELAV-like yeast RNA-binding protein with homology to T-cell internal antigen 1 (TIA-1)/TIA-1-related protein (TIAR), is an important modulator of the decay of two known classes of mRNA. Our goal in this study was to determine the range of mRNAs whose stability is dependent on Pub1p, as well as to identify specific transcripts that directly bind to this protein. We have examined global mRNA turnover in isogenic PUB1 and pub1delta strains through gene expression analysis and demonstrate that 573 genes exhibit a significant reduction in half-life in a pub1delta strain. We also examine the binding specificity of Pub1p using affinity purification followed by microarray analysis to comprehensively distinguish between direct and indirect targets and find that Pub1p significantly binds to 368 cellular transcripts. Among the Pub1p-associated mRNAs, 53 transcripts encoding proteins involved in ribosomal biogenesis and cellular metabolism are selectively destabilized in the pub1delta strain. In contrast, genes involved in transporter activity demonstrate association with Pub1p but display no measurable changes in transcript stability. Characterization of two candidate genes, SEC53 and RPS16B, demonstrate that both Pub1p-dependent regulation of stability and Pub1p binding require 3' untranslated regions, which harbor distinct sequence motifs. These results suggest that Pub1p binds to discrete subsets of cellular transcripts and posttranscriptionally regulates their expression at multiple levels.

Scp160p, a multiple KH-domain protein, is a component of mRNP complexes in yeast

Scp160p is a 160 kDa protein in the yeast Saccharomyces cerevisiae that contains 14 repeats of the hnRNP K-homology (KH) domain, and demonstrates significant sequence homology to a family of proteins collectively known as vigilins. As a first step towards defining the function of Scp160p, we have characterized the subcellular distribution and in vivo interactions of this protein. Using sucrose gradient fractionation studies we have demonstrated that Scp160p in cytoplasmic lysates is predominantly associated with polyribosomes. Furthermore, we have found that Scp160p is released from polyribosomes by EDTA in the form of a large complex of> or =1300 kDa that is sensitive both to RNase and NaCl. Using affinity-chromatography to isolate these complexes, we have identified two protein components other than Scp160p: poly(A) binding protein, Pab1p, and Bfr1p. The presence of Pab1p confirms these complexes to be mRNPs. The presence of Bfr1p is intriguing because the null phenotype for this gene is essentially the same as that reported for scp160 -null cells: increased cell size and aberrant DNA content. These results demonstrate that Scp160p associates with polyribosome-bound mRNP complexes in vivo, implicating a role for this protein in one or more levels of mRNA metabolism in yeast.

RNA-binding proteins TIA-1 and TIAR link the phosphorylation of eIF-2 alpha to the assembly of mammalian stress granules

In response to environmental stress, the related RNA-binding proteins TIA-1 and TIAR colocalize with poly(A)(+) RNA at cytoplasmic foci that resemble the stress granules (SGs) that harbor untranslated mRNAs in heat shocked plant cells (Nover et al. 1989; Nover et al. 1983; Scharf et al. 1998). The accumulation of untranslated mRNA at SGs is reversible in cells that recover from a sublethal stress, but irreversible in cells subjected to a lethal stress. We have found that the assembly of TIA-1/R(+) SGs is initiated by the phosphorylation of eIF-2alpha. A phosphomimetic eIF-2alpha mutant (S51D) induces the assembly of SGs, whereas a nonphosphorylatable eIF-2alpha mutant (S51A) prevents the assembly of SGs. The ability of a TIA-1 mutant lacking its RNA-binding domains to function as a transdominant inhibitor of SG formation suggests that this RNA-binding protein acts downstream of the phosphorylation of eIF-2alpha to promote the sequestration of untranslated mRNAs at SGs. The assembly and disassembly of SGs could regulate the duration of stress- induced translational arrest in cells recovering from environmental stress.

The Mex67p-mediated nuclear mRNA export pathway is conserved from yeast to human

Human TAP is an orthologue of the yeast mRNA export factor Mex67p. In mammalian cells, TAP has a preferential intranuclear localization, but can also be detected at the nuclear pores and shuttles between the nucleus and the cytoplasm. TAP directly associates with mRNA in vivo, as it can be UV-crosslinked to poly(A)+ RNA in HeLa cells. Both the FG-repeat domain of nucleoporin CAN/Nup214 and a novel human 15 kDa protein (p15) with homology to NTF2 (a nuclear transport factor which associates with RanGDP), directly bind to TAP. When green fluorescent protein (GFP)-tagged TAP and p15 are expressed in yeast, they localize to the nuclear pores. Strikingly, co-expression of human TAP and p15 restores growth of the otherwise lethal mex67::HIS3/mtr2::HIS3 double knockout strain. Thus, the human TAP-p15 complex can functionally replace the Mex67p-Mtr2p complex in yeast and thus performs a conserved role in nuclear mRNA export.

Repression of gene expression by an exogenous sequence element acting in concert with a heterogeneous nuclear ribonucleoprotein-like protein, Nrd1, and the putative helicase Sen1

We have fortuitously identified a nucleotide sequence that decreases expression of a reporter gene in the yeast Saccharomyces cerevisiae 20-fold when inserted into an intron. The primary effect of the insertion is a decrease in pre-mRNA abundance accompanied by the appearance of 3'-truncated transcripts, consistent with premature transcriptional termination and/or pre-mRNA degradation. Point mutations in the cis element relieve the negative effect, demonstrating its sequence specificity. A novel yeast protein, named Nrd1, and a previously identified putative helicase, Sen1, help mediate the negative effect of the cis element. Sen1 is an essential nuclear protein that has been implicated in a variety of nuclear functions. Nrd1 has hallmarks of a heterogeneous nuclear ribonucleoprotein, including an RNA recognition motif, a region rich in RE and RS dipeptides, and a proline- and glutamine-rich domain. An N-terminal domain of Nrd1 may facilitate direct interaction with RNA polymerase II. Disruption of the NRD1 gene is lethal, yet C-terminal truncations that delete the RNA recognition motif and abrogate the negative effect of the cis element nevertheless support cell growth. Thus, expression of a gene containing the cis element could be regulated through modulation of the activity of Nrd1. The recent identification of Nrd1-related proteins in mammalian cells suggests that this potential regulatory pathway is widespread among eukaryotes.

Cap-dependent and cap-independent translation by internal initiation of mRNAs in cell extracts prepared from Saccharomyces cerevisiae

Translation extracts were prepared from various strains of Saccharomyces cerevisiae. The translation of mRNA molecules in these extracts were cooperatively enhanced by the presence of 5'-terminal cap structures and 3'-terminal poly(A) sequences. These cooperative effects could not be observed in other translation systems such as those prepared from rabbit reticulocytes, wheat germ, and human HeLa cells. Because the yeast translation system mimicked the effects of the cap structure and poly(A) tail on translational efficiency seen in vivo, this system was used to study cap-dependent and cap-independent translation of viral and cellular mRNA molecules. Both the 5' noncoding regions of hepatitis C virus and those of coxsackievirus B1 conferred cap-independent translation to a reporter coding region during translation in the yeast extracts; thus, the yeast translational apparatus is capable of initiating cap-independent translation. Although the translation of most yeast mRNAs was cap dependent, the unusually long 5' noncoding regions of mRNAs encoding cellular transcription factors TFIID and HAP4 were shown to mediate cap-independent translation in these extracts. Furthermore, both TFIID and HAP4 5' noncoding regions mediated translation of a second cistron when placed into the intercistronic spacer region of a dicistronic mRNA, indicating that these leader sequences can initiate translation by an internal ribosome binding mechanism in this in vitro translation system. This finding raises the possibility that an internal translation initiation mechanism exists in yeast cells for regulated translation of endogenous mRNAs.

Cap-dependent and cap-independent translation by internal initiation of mRNAs in cell extracts prepared from Saccharomyces cerevisiae

Translation extracts were prepared from various strains of Saccharomyces cerevisiae. The translation of mRNA molecules in these extracts were cooperatively enhanced by the presence of 5'-terminal cap structures and 3'-terminal poly(A) sequences. These cooperative effects could not be observed in other translation systems such as those prepared from rabbit reticulocytes, wheat germ, and human HeLa cells. Because the yeast translation system mimicked the effects of the cap structure and poly(A) tail on translational efficiency seen in vivo, this system was used to study cap-dependent and cap-independent translation of viral and cellular mRNA molecules. Both the 5' noncoding regions of hepatitis C virus and those of coxsackievirus B1 conferred cap-independent translation to a reporter coding region during translation in the yeast extracts; thus, the yeast translational apparatus is capable of initiating cap-independent translation. Although the translation of most yeast mRNAs was cap dependent, the unusually long 5' noncoding regions of mRNAs encoding cellular transcription factors TFIID and HAP4 were shown to mediate cap-independent translation in these extracts. Furthermore, both TFIID and HAP4 5' noncoding regions mediated translation of a second cistron when placed into the intercistronic spacer region of a dicistronic mRNA, indicating that these leader sequences can initiate translation by an internal ribosome binding mechanism in this in vitro translation system. This finding raises the possibility that an internal translation initiation mechanism exists in yeast cells for regulated translation of endogenous mRNAs.

GRSF-1: a poly(A)+ mRNA binding protein which interacts with a conserved G-rich element

Computer predictions identified similarities to a 14-base G-rich element in numerous mRNAs at a variety of locations. A Northwestern screening strategy was used to obtain a cDNA clone from a HeLa cell library using the G-rich RNA element as a probe. A cellular protein (called GRSF-1), which was encoded by this cDNA, binds RNAs containing the G-rich element. GRSF-1 was distinct from DSEF-1, a nuclear protein we have previously identified that interacts with the G-rich element, based on differences in molecular weight and partial peptide maps, as well as the lack of cross-reactivity with GRSF-1 specific monoclonal antibodies. Using indirect immunofluorescence microscopy, we localized GRSF-1 to the cytoplasm. In vivo UV cross-linking further demonstrated that GRSF-1 was bound to poly(A)+ mRNA in living human cells. Western blot analysis revealed four cytoplasmic proteins which expressed GRSF-1 specific epitopes. GRSF-1 contains three potential RNA recognition motifs and two auxiliary domains. Curiously, the domain organization of GRSF-1 is similar to the RNA binding proteins PUB1, ELAV, HuD, Hel-N1, mcs94-1 and RBP9.

Two functional domains of the influenza virus NS1 protein are required for regulation of nuclear export of mRNA

The influenza virus NS1 protein is the only known example of a protein that inhibits the nuclear export of mRNA. To identify the functional domains of this protein, we introduced 18 2- or 3-amino-acid substitutions at approximately equally spaced locations along the entire length of the protein. Two functional domains were identified. The domain near the amino end (amino acids 19 through 38) was shown to be the RNA-binding domain, by using a gel shift assay with purified NS1 protein and spliced viral NS2 mRNA as the RNA target. The second domain, which is in the carboxy half of the molecule, was presumed to be the effector domain that interacts with host nuclear proteins to carry out the nuclear RNA export function, by analogy with the effector domain of the Rev proteins of human immunodeficiency virus (HIV) and other lentiviruses which facilitate rather than inhibit nuclear RNA export. The NS1 protein has a 10-amino-acid sequence that is similar to the consensus sequence in the effector domains of lentivirus Rev proteins, specifically including two crucial leucines at positions 7 and 9 of this sequence. However, the effector domains of the NS1 and Rev (HIV type 1 [HIV-1]) proteins differed in several significant ways including the following: (i) unlike the HIV-1 Rev protein, NS1 effector domain mutants were negative recessive rather than negative dominant, (ii) the NS1 effector domain is about three times larger than the effector domain of the HIV-1 Rev protein, and (iii) unlike the HIV-1 protein, NS1 effector domain mutants exhibited a surprising property, a changed intracellular/intranuclear distribution, compared with the wild-type protein. These differences strongly suggest that the effector domains of the NS1 and Rev proteins interact with different nuclear protein targets, which likely explains the opposite effects of these two proteins on nuclear mRNA export.

RNP1, a new ribonucleoprotein gene of the yeast Saccharomyces cerevisiae

A previously unidentified ribonucleoprotein (RNP) gene of yeast has been cloned and sequenced. The gene, named RNP1, was found adjacent to a previously sequenced gene encoding the second gene for ribosomal protein L4. RNP1 contains two RNA Recognition Motifs (RRM), [alternatively known as RNA binding Domains (RBD)], but unlike most RNP genes does not contain any auxiliary simple sequence domains. The first RRM (RRM1) most resembles RRM domains found in the hnRNP A/B class of RNP proteins. The second RRM (RRM2) most resembles a RRM so far seen only in the single RRM of the yeast SSB1 gene. Two null mutants of RNP1 that were created, a frameshift disruption and a complete deletion of the gene, were viable, demonstrating that the gene is not essential for cell growth. Two double null mutants of yeast RNP genes that were created (delta RNP1/delta SSB1 and delta SSB1/delta NPL3) were also viable. A fragment identical in size to the RRM1 domain could be amplified by PCR from the DNA of fungi, plants, and animals, using primers matching the ends of this domain, indicating that the structure of RRM1 is conserved. Another potential open reading frame on the same cloned fragment of DNA encodes a gene product whose structure resembles that of a seven-transmembrane-segment membrane receptor protein.

The yeast protein encoded by PUB1 binds T-rich single stranded DNA

We have characterized binding activities in yeast which recognise the T-rich strand of the yeast ARS consensus element and have purified two of these to homogeneity. One (ACBP-60) is detectable in both nuclear and whole cell extracts, while the other (ACBP-67) is apparent only after fractionation of extracts by heparin-sepharose chromatography. The major binding activity detected in nuclear extracts was purified on a sequence-specific DNA affinity column as a single polypeptide with apparent mobility of 60kDa (ACBP-60). This protein co-fractionates with nuclei, is present at several thousand copies per cell and has a Kd for the T-rich single strand of the ARS consensus between 10(-9) and 10(-10) M. Competition studies with simple nucleic acid polymers show that ACBP-60 has marginally higher affinity for poly dT30 than for a 30 nt oligomer containing the T-rich strand of ARS 307, and approximately 10 fold higher affinity for poly rU. Internal sequence information of purified p60 reveals identity with the open reading frames of genes PUB1 and RNP1 which encode polyuridylate binding protein(s). The second binding activity, ACBP-67, also binds specifically to the T-rich single strand of the ARS consensus, but with considerably lower affinity than ACBP-60. Peptide sequence reveals that the 67kDa protein is identical to the major polyA binding protein in yeast, PAB1.