An Lsm2-Lsm7 complex in Saccharomyces cerevisiae associates with the small nucleolar RNA snR5

The RNA-binding protein LSM family regulating reproductive development via different RNA metabolism

The LSM (Like-Sm) protein family, characterized by highly conserved LSM domains, is integral to ribonucleic acid (RNA) metabolism. Ubiquitously present in both eukaryotes and select prokaryotes, these proteins bind to RNA molecules with high specificity through their LSM domains. They can also form ring-shaped complexes with other proteins, thereby facilitating various fundamental cellular processes such as mRNA degradation, splicing, and ribosome biogenesis. LSM proteins play crucial roles in gametogenesis, early embryonic development, sex determination, gonadal maturation, and reproductive system formation. In pathological conditions, the absence of LSM14B leads to arrest of oocytes at mid-meiosis, downregulation of LSM4 expression is associated with abnormal spermatogenesis, and aberrant expression of LSM1 protein is linked to the occurrence and progression of breast cancer. This review focuses on the recent advances in the functional research of LSM proteins in reproduction.

Yeast U6 snRNA made by RNA polymerase II is less stable but functional

U6 small nuclear (sn)RNA is the shortest and most conserved snRNA in the spliceosome and forms a substantial portion of its active site. Unlike the other four spliceosomal snRNAs, which are synthesized by RNA polymerase (RNAP) II, U6 is made by RNAP III. To determine if some aspect of U6 function is incompatible with synthesis by RNAP II, we created a U6 snRNA gene with RNAP II promoter and terminator sequences. This "U6-II" gene is functional as the sole source of U6 snRNA in yeast, but its transcript is much less stable than U6 snRNA made by RNAP III. Addition of the U4 snRNA Sm protein binding site to U6-II increased its stability and led to formation of U6-II•Sm complexes. We conclude that synthesis of U6 snRNA by RNAP III is not required for its function and that U6 snRNPs containing the Sm complex can form in vivo. The ability to synthesize U6 snRNA with RNAP II relaxes sequence restraints imposed by intragenic RNAP III promoter and terminator elements and allows facile control of U6 levels via regulators of RNAP II transcription.

Identification of transient intermediates during spliceosome activation by single molecule fluorescence microscopy

Spliceosome activation is the process of creating the catalytic site for RNA splicing and occurs de novo on each intron following spliceosome assembly. Dozens of factors bind to or are released from the activating spliceosome including the Lsm2-8 heteroheptameric ring that binds the U6 small nuclear RNA 3'-end. Lsm2-8 must be released to permit active site stabilization by the Prp19-containing complex (NineTeen Complex, NTC); however, little is known about the temporal order of events and dynamic interactions that lead up to and follow Lsm2-8 release. We have used colocalization single molecule spectroscopy (CoSMoS) to visualize Lsm2-8 dynamics during activation of Saccharomyces cerevisiae spliceosomes in vitro. Lsm2-8 is recruited as a component of the tri-snRNP and is released after integration of the Prp19-containing complex (NTC). Despite Lsm2-8 and the NTC being mutually exclusive in existing cryo-EM structures of yeast B complex spliceosomes, we identify a transient intermediate containing both ([Formula: see text]) and provide a kinetic framework for its formation and transformation during activation. Prior to [Formula: see text] assembly, the NTC rapidly and reversibly samples the spliceosome suggesting a mechanism for preventing NTC sequestration by defective spliceosomes that fail to properly activate. In complementary ensemble assays, we show that a base-pairing-dependent ternary complex can form between Lsm2-8 and U2 and U6 helix II RNAs. We propose that this interaction may play a role in formation of transient spliceosome intermediates formed during activation.

Expression profile and prognostic values of LSM family in skin cutaneous melanoma

Background

The like-Smith (LSM) family is a group of RNA-binding proteins involved in RNA metabolism. However, their involvement in tumors, particularly skin cutaneous melanoma (SKCM), is not fully understood. In this study, we focused on the expression profiles and prognostic values of the LSM family in SKCM.

Methods

Raw data were downloaded from The Cancer Genome Atlas. The expression profile and prognostic value of LSM genes in SKCM were explored using the GEPIA, cBioPortal, and HPA databases. Protein–protein and gene–gene interaction analyses were performed using STRING and GeneMANIA. Enrichment and Cox regression analysis were conducted using R software. The TISIDB database was used to explore the relationship between LSMs and immunomodulators. Receiver operating characteristic curves and nomogram models were constructed to validate prognostic values.

Results

mRNA and protein expression levels of LSM2, LSM4, and LSM12 were significantly elevated in SKCM. The upregulated mRNA expression of LSM2 (p = 0.0013) and LSM4 (p = 0.0043) was significantly correlated with poor overall survival in patients with SKCM, whereas only LSM2 (p = 0.049) overexpression was markedly associated with worse disease-free survival. LSM2 overexpression was an independent risk factor (p = 0.013) and was confirmed to have a high prognostic value in SKCM using the receiver operating characteristic curve (AUC = 0.942) and nomogram models. All LSM genes were identified as genomic mutations, whereas alteration of LSM2 (p = 0.0153) significantly affected the overall survival in patients with SKCM. Significant correlations were observed between LSM family expression, immune cell infiltration, and immunomodulator. Furthermore, function and pathway enrichment analysis showed that the LSM family was mainly RNA binding proteins and involved in RNA splicing and degradation.

Conclusion

Expression profiles and prognostic values of LSM in SKCM were inconsistent. Among the LSM family, only LSM2 may serve as a potential poor prognosticator and immunotherapeutic target of SKCM.

BIONIC: biological network integration using convolutions

Biological networks constructed from varied data can be used to map cellular function, but each data type has limitations. Network integration promises to address these limitations by combining and automatically weighting input information to obtain a more accurate and comprehensive representation of the underlying biology. We developed a deep learning-based network integration algorithm that incorporates a graph convolutional network framework. Our method, BIONIC (Biological Network Integration using Convolutions), learns features that contain substantially more functional information compared to existing approaches. BIONIC has unsupervised and semisupervised learning modes, making use of available gene function annotations. BIONIC is scalable in both size and quantity of the input networks, making it feasible to integrate numerous networks on the scale of the human genome. To demonstrate the use of BIONIC in identifying new biology, we predicted and experimentally validated essential gene chemical-genetic interactions from nonessential gene profiles in yeast.

Core spliceosomal Sm proteins as constituents of cytoplasmic mRNPs in plants

In recent years, research has increasingly focused on the key role of post-transcriptional regulation of messenger ribonucleoprotein (mRNP) function and turnover. As a result of the complexity and dynamic nature of mRNPs, the full composition of a single mRNP complex remains unrevealed and mRNPs are poorly described in plants. Here we identify canonical Sm proteins as part of the cytoplasmic mRNP complex, indicating their function in the post-transcriptional regulation of gene expression in plants. Sm proteins comprise an evolutionarily ancient family of small RNA-binding proteins involved in pre-mRNA splicing. The latest research indicates that Sm could also impact on mRNA at subsequent stages of its life cycle. In this work we show that in the microsporocyte cytoplasm of Larix decidua, the European larch, Sm proteins accumulate within distinct cytoplasmic bodies, also containing polyadenylated RNA. To date, several types of cytoplasmic bodies involved in the post-transcriptional regulation of gene expression have been described, mainly in animal cells. Their role and molecular composition in plants remain less well established, however. A total of 222 mRNA transcripts have been identified as cytoplasmic partners for Sm proteins. The specific colocalization of these mRNAs with Sm proteins within cytoplasmic bodies has been confirmed via microscopic analysis. The results from this work support the hypothesis, that evolutionarily conserved Sm proteins have been adapted to perform a whole repertoire of functions related to the post-transcriptional regulation of gene expression in Eukaryota. This adaptation presumably enabled them to coordinate the interdependent processes of splicing element assembly, mRNA maturation and processing, and mRNA translation regulation, and its degradation.

Identification of eQTLs and sQTLs associated with meat quality in beef

BACKGROUND

Transcription has a substantial genetic control and genetic dissection of gene expression could help us understand the genetic architecture of complex phenotypes such as meat quality in cattle. The objectives of the present research were: 1) to perform eQTL and sQTL mapping analyses for meat quality traits in longissimus dorsi muscle; 2) to uncover genes whose expression is influenced by local or distant genetic variation; 3) to identify expression and splicing hot spots; and 4) to uncover genomic regions affecting the expression of multiple genes.

RESULTS

Eighty steers were selected for phenotyping, genotyping and RNA-seq evaluation. A panel of traits related to meat quality was recorded in longissimus dorsi muscle. Information on 112,042 SNPs and expression data on 8588 autosomal genes and 87,770 exons from 8467 genes were included in an expression and splicing quantitative trait loci (QTL) mapping (eQTL and sQTL, respectively). A gene, exon and isoform differential expression analysis previously carried out in this population identified 1352 genes, referred to as DEG, as explaining part of the variability associated with meat quality traits. The eQTL and sQTL mapping was performed using a linear regression model in the R package Matrix eQTL. Genotype and year of birth were included as fixed effects, and population structure was accounted for by including as a covariate the first PC from a PCA analysis on genotypic data. The identified QTLs were classified as cis or trans using 1 Mb as the maximum distance between the associated SNP and the gene being analyzed. A total of 8377 eQTLs were identified, including 75.6% trans, 10.4% cis, 12.5% DEG trans and 1.5% DEG cis; while 11,929 sQTLs were uncovered: 66.1% trans, 16.9% DEG trans, 14% cis and 3% DEG cis. Twenty-seven expression master regulators and 13 splicing master regulators were identified and were classified as membrane-associated or cytoskeletal proteins, transcription factors or DNA methylases. These genes could control the expression of other genes through cell signaling or by a direct transcriptional activation/repression mechanism.

CONCLUSION

In the present analysis, we show that eQTL and sQTL mapping makes possible positional identification of gene and isoform expression regulators.

Surveillance-ready transcription: nuclear RNA decay as a default fate

Eukaryotic cells synthesize enormous quantities of RNA from diverse classes, most of which are subject to extensive processing. These processes are inherently error-prone, and cells have evolved robust quality control mechanisms to selectively remove aberrant transcripts. These surveillance pathways monitor all aspects of nuclear RNA biogenesis, and in addition remove nonfunctional transcripts arising from spurious transcription and a host of non-protein-coding RNAs (ncRNAs). Surprisingly, this is largely accomplished with only a handful of RNA decay enzymes. It has, therefore, been unclear how these factors efficiently distinguish between functional RNAs and huge numbers of diverse transcripts that must be degraded. Here we describe how bona fide transcripts are specifically protected, particularly by 5' and 3' modifications. Conversely, a plethora of factors associated with the nascent transcripts all act to recruit the RNA quality control, surveillance and degradation machinery. We conclude that initiating RNAPII is 'surveillance ready', with degradation being a default fate for all transcripts that lack specific protective features. We further postulate that this promiscuity is a key feature that allowed the proliferation of vast numbers of ncRNAs in eukaryotes, including humans.

Chance and necessity in the evolution of RNase P

RNase P catalyzes 5'-maturation of tRNAs in all three domains of life. This primary function is accomplished by either a ribozyme-centered ribonucleoprotein (RNP) or a protein-only variant (with one to three polypeptides). The large, multicomponent archaeal and eukaryotic RNase P RNPs appear disproportionate to the simplicity of their role in tRNA 5'-maturation, prompting the question of why the seemingly gratuitously complex RNP forms of RNase P were not replaced with simpler protein counterparts. Here, motivated by growing evidence, we consider the hypothesis that the large RNase P RNP was retained as a direct consequence of multiple roles played by its components in processes that are not related to the canonical RNase P function.

Cytoplasmic LSM-1 protein regulates stress responses through the insulin/IGF-1 signaling pathway in Caenorhabditis elegans

Genes coding for members of the Sm-like (LSm) protein family are conserved through evolution from prokaryotes to humans. These proteins have been described as forming homo- or heterocomplexes implicated in a broad range of RNA-related functions. To date, the nuclear LSm2-8 and the cytoplasmic LSm1-7 heteroheptamers are the best characterized complexes in eukaryotes. Through a comprehensive functional study of the LSm family members, we found that lsm-1 and lsm-3 are not essential for C. elegans viability, but their perturbation, by RNAi or mutations, produces defects in development, reproduction, and motility. We further investigated the function of lsm-1, which encodes the distinctive protein of the cytoplasmic complex. RNA-seq analysis of lsm-1 mutants suggests that they have impaired Insulin/IGF-1 signaling (IIS), which is conserved in metazoans and involved in the response to various types of stress through the action of the FOXO transcription factor DAF-16. Further analysis using a DAF-16::GFP reporter indicated that heat stress-induced translocation of DAF-16 to the nuclei is dependent on lsm-1. Consistent with this, we observed that lsm-1 mutants display heightened sensitivity to thermal stress and starvation, while overexpression of lsm-1 has the opposite effect. We also observed that under stress, cytoplasmic LSm proteins aggregate into granules in an LSM-1-dependent manner. Moreover, we found that lsm-1 and lsm-3 are required for other processes regulated by the IIS pathway, such as aging and pathogen resistance.

Integrity of SRP RNA is ensured by La and the nuclear RNA quality control machinery

The RNA component of signal recognition particle (SRP) is transcribed by RNA polymerase III, and most steps in SRP biogenesis occur in the nucleolus. Here, we examine processing and quality control of the yeast SRP RNA (scR1). In common with other pol III transcripts, scR1 terminates in a U-tract, and mature scR1 retains a U_4–5 sequence at its 3′ end. In cells lacking the exonuclease Rex1, scR1 terminates in a longer U_5–6 tail that presumably represents the primary transcript. The 3′ U-tract of scR1 is protected from aberrant processing by the La homologue, Lhp1 and overexpressed Lhp1 apparently competes with both the RNA surveillance system and SRP assembly factors. Unexpectedly, the TRAMP and exosome nuclear RNA surveillance complexes are also implicated in protecting the 3′ end of scR1, which accumulates in the nucleolus of cells lacking the activities of these complexes. Misassembled scR1 has a primary degradation pathway in which Rrp6 acts early, followed by TRAMP-stimulated exonuclease degradation by the exosome. We conclude that the RNA surveillance machinery has key roles in both SRP biogenesis and quality control of the RNA, potentially facilitating the decision between these alternative fates.

Arabidopsis thaliana LSM proteins function in mRNA splicing and degradation

Sm-like (Lsm) proteins have been identified in all organisms and are related to RNA metabolism. Here, we report that Arabidopsis nuclear AtLSM8 protein, as well as AtLSM5, which localizes to both the cytoplasm and nucleus, function in pre-mRNA splicing, while AtLSM5 and the exclusively cytoplasmic AtLSM1 contribute to 5'-3' mRNA decay. In lsm8 and sad1/lsm5 mutants, U6 small nuclear RNA (snRNA) was reduced and unspliced mRNA precursors accumulated, whereas mRNA stability was mainly affected in plants lacking AtLSM1 and AtLSM5. Some of the mRNAs affected in lsm1a lsm1b and sad1/lsm5 plants were also substrates of the cytoplasmic 5'-3' exonuclease AtXRN4 and of the decapping enzyme AtDCP2. Surprisingly, a subset of substrates was also stabilized in the mutant lacking AtLSM8, which supports the notion that plant mRNAs are actively degraded in the nucleus. Localization of LSM components, purification of LSM-interacting proteins as well as functional analyses strongly suggest that at least two LSM complexes with conserved activities in RNA metabolism, AtLSM1-7 and AtLSM2-8, exist also in plants.

Archaeal and eukaryotic homologs of Hfq: A structural and evolutionary perspective on Sm function

Hfq and other Sm proteins are central in RNA metabolism, forming an evolutionarily conserved family that plays key roles in RNA processing in organisms ranging from archaea to bacteria to human. Sm-based cellular pathways vary in scope from eukaryotic mRNA splicing to bacterial quorum sensing, with at least one step in each of these pathways being mediated by an RNA-associated molecular assembly built upon Sm proteins. Though the first structures of Sm assemblies were from archaeal systems, the functions of Sm-like archaeal proteins (SmAPs) remain murky. Our ignorance about SmAP biology, particularly vis-à-vis the eukaryotic and bacterial Sm homologs, can be partly reduced by leveraging the homology between these lineages to make phylogenetic inferences about Sm functions in archaea. Nevertheless, whether SmAPs are more eukaryotic (RNP scaffold) or bacterial (RNA chaperone) in character remains unclear. Thus, the archaeal domain of life is a missing link, and an opportunity, in Sm-based RNA biology.

Lsm proteins and Hfq: Life at the 3' end

The bacterial Hfq protein is a versatile modulator of RNA function and is particularly important for regulation mediated by small non-coding RNAs. Hfq is a bacterial Sm protein but bears more similarity to the eukaryotic Sm-like (Lsm) family of proteins than the prototypical Sm proteins. Hfq and Lsm proteins share the ability to chaperone RNA-RNA and RNA/protein interactions and an interesting penchant for protecting the 3′ end of a transcript from exonucleolytic decay while encouraging degradation through other pathways. Our view of Lsm function in eukaryotes has historically been informed by studies of Hfq structure and function but mutational analyses and structural studies of Lsm sub-complexes have given important insights as well. Here, we aim to compare and contrast the roles of these evolutionarily related complexes and to highlight areas for future investigation.

Crystal structures of Lsm3, Lsm4 and Lsm5/6/7 from Schizosaccharomyces pombe

Sm-like (Lsm) proteins are ubiquitous and function in many aspects of RNA metabolism, including pre-mRNA splicing, nuclear RNA processing, mRNA decay and miRNA biogenesis. Here three crystal structures including Lsm3, Lsm4 and Lsm5/6/7 sub-complex from S. pombe are reported. These structures show that all the five individual Lsm subunits share a conserved Sm fold, and Lsm3, Lsm4, and Lsm5/6/7 form a heptamer, a trimer and a hexamer within the crystal lattice, respectively. Analytical ultracentrifugation indicates that Lsm3 and Lsm5/6/7 sub-complex exist in solution as a heptamer and a hexamer, respectively while Lsm4 undergoes a dynamic equilibrium between monomer and trimer in solution. RNA binding assays show that Lsm2/3 and Lsm5/6/7 bind to oligo(U) whereas no RNA binding is observed for Lsm3 and Lsm4. Analysis of the inter-subunit interactions in Lsm5/6/7 reveals the organization order among Lsm5, Lsm6 and Lsm7.

The telomere-binding protein Tbf1 demarcates snoRNA gene promoters in Saccharomyces cerevisiae

Small nucleolar RNAs (snoRNAs) play a key role in ribosomal RNA biogenesis, yet factors controlling their expression are unknown. We found that the majority of Saccharomyces snoRNA promoters display an aRCCCTaa sequence motif at the upstream border of a TATA-containing nucleosome-free region. Genome-wide ChIP-seq analysis showed that these motifs are bound by Tbf1, a telomere-binding protein known to recognize mammalian-like T(2)AG(3) repeats at subtelomeric regions. Tbf1 has over 100 additional promoter targets, including several other genes involved in ribosome biogenesis and the TBF1 gene itself. Tbf1 is required for full snoRNA expression, yet it does not influence nucleosome positioning at snoRNA promoters. In contrast, Tbf1 contributes to nucleosome exclusion at non-snoRNA promoters, where it selectively colocalizes with the Tbf1-interacting zinc-finger proteins Vid22 and Ygr071c. Our data show that, besides the ribosomal protein gene regulator Rap1, a second telomere-binding protein also functions as a transcriptional regulator linked to yeast ribosome biogenesis.

A systematic characterization of Cwc21, the yeast ortholog of the human spliceosomal protein SRm300

Cwc21 (complexed with Cef1 protein 21) is a 135 amino acid yeast protein that shares homology with the N-terminal domain of human SRm300/SRRM2, a large serine/arginine-repeat protein shown previously to associate with the splicing coactivator and 3'-end processing stimulatory factor, SRm160. Proteomic analysis of spliceosomal complexes has suggested a role for Cwc21 and SRm300 at the core of the spliceosome. However, specific functions for these proteins have remained elusive. In this report, we employ quantitative genetic interaction mapping, mass spectrometry of tandem affinity-purified complexes, and microarray profiling to investigate genetic, physical, and functional interactions involving Cwc21. Combined data from these assays support multiple roles for Cwc21 in the formation and function of splicing complexes. Consistent with a role for Cwc21 at the core of the spliceosome, we observe strong genetic, physical, and functional interactions with Isy1, a protein previously implicated in the first catalytic step of splicing and splicing fidelity. Together, the results suggest multiple functions for Cwc21/SRm300 in the splicing process, including an important role in the activation of splicing in association with Isy1.

The Drosophila U7 snRNP proteins Lsm10 and Lsm11 are required for histone pre-mRNA processing and play an essential role in development

Metazoan replication-dependent histone mRNAs are not polyadenylated, and instead terminate in a conserved stem-loop structure generated by an endonucleolytic cleavage of the pre-mRNA involving U7 snRNP. U7 snRNP contains two like-Sm proteins, Lsm10 and Lsm11, which replace SmD1 and SmD2 in the canonical heptameric Sm protein ring that binds spliceosomal snRNAs. Here we show that mutations in either the Drosophila Lsm10 or the Lsm11 gene disrupt normal histone pre-mRNA processing, resulting in production of poly(A)+ histone mRNA as a result of transcriptional read-through to cryptic polyadenylation sites present downstream from each histone gene. This molecular phenotype is indistinguishable from that which we previously described for mutations in U7 snRNA. Lsm10 protein fails to accumulate in Lsm11 mutants, suggesting that a pool of Lsm10-Lsm11 dimers provides precursors for U7 snRNP assembly. Unexpectedly, U7 snRNA was detected in Lsm11 and Lsm1 mutants and could be precipitated with anti-trimethylguanosine antibodies, suggesting that it assembles into a snRNP particle in the absence of Lsm10 and Lsm11. However, this U7 snRNA could not be detected at the histone locus body, suggesting that Lsm10 and Lsm11 are necessary for U7 snRNP localization. In contrast to U7 snRNA null mutants, which are viable, Lsm10 and Lsm11 mutants do not survive to adulthood. Because we cannot detect differences in the histone mRNA phenotype between Lsm10 or Lsm11 and U7 mutants, we propose that the different terminal developmental phenotypes result from the participation of Lsm10 and Lsm11 in an essential function that is distinct from histone pre-mRNA processing and that is independent of U7 snRNA.

LSM1 over-expression in Saccharomyces cerevisiae depletes U6 snRNA levels

Lsm1 is a component of the Lsm1-7 complex involved in cytoplasmic mRNA degradation. Lsm1 is over-expressed in multiple tumor types, including over 80% of pancreatic tumors, and increased levels of Lsm1 protein have been shown to induce carcinogenic effects. Therefore, understanding the perturbations in cell process due to increased Lsm1 protein may help to identify possible therapeutics targeting tumors over-expressing Lsm1. Herein, we show that LSM1 over-expression in the yeast Saccharomyces cerevisiae inhibits growth primarily due to U6 snRNA depletion, thereby altering pre-mRNA splicing. The decrease in U6 snRNA levels causes yeast strains over-expressing Lsm1 to be hypersensitive to loss of other proteins required for production or function of the U6 snRNA, supporting a model wherein excess Lsm1 reduces the availability of the Lsm2-7 proteins, which also assemble with Lsm8 to form a complex that binds and stabilizes the U6 snRNA. Yeast strains over-expressing Lsm1 also display minor alterations in mRNA decay and demonstrate increased susceptibility to mutations inhibiting cytoplasmic deadenylation, a process required for both 5′-to-3′ and 3′-to-5′ pathways of exonucleolytic decay. These results suggest that inhibition of splicing and/or deadenylation may be effective therapies for Lsm1-over-expressing tumors.

Analysis of Lsm1p and Lsm8p domains in the cellular localization of Lsm complexes in budding yeast

In eukaryotes, two heteroheptameric Sm-like (Lsm) complexes that differ by a single subunit localize to different cellular compartments and have distinct functions in RNA metabolism. The cytoplasmic Lsm1–7p complex promotes mRNA decapping and localizes to processing bodies, whereas the Lsm2–8p complex takes part in a variety of nuclear RNA processing events. The structural features that determine their different functions and localizations are not known. Here, we analyse a range of mutant and hybrid Lsm1 and Lsm8 proteins, shedding light on the relative importance of their various domains in determining their localization and ability to support growth. Although no single domain is either essential or sufficient for cellular localization, the Lsm1p N-terminus may act as part of a nuclear exclusion signal for Lsm1–7p, and the shorter Lsm8p N-terminus contributes to nuclear accumulation of Lsm2–8p. The C-terminal regions seem to play a secondary role in determining localization, with little or no contribution coming from the central Sm domains. The essential Lsm8 protein is remarkably resistant to mutation in terms of supporting viability, whereas Lsm1p appears more sensitive. These findings contribute to our understanding of how two very similar protein complexes can have different properties.

Evolutionary diversification of the Sm family of RNA-associated proteins

The Sm family of proteins is closely associated with RNA metabolism throughout all life. These proteins form homomorphic and heteromorphic rings consisting of six or seven subunits with a characteristic central pore, the presence of which is critical for binding U-rich regions of single-stranded RNA. Eubacteria and Archaea typically carry one or two forms of Sm proteins and assemble one homomorphic ring per Sm protein. Eukaryotes typically carry 16 or more Sm proteins that assemble to form heteromorphic rings which lie at the center of a number of critical RNA-associated small nuclear ribonucleoproteins (snRNPs). High Sm protein diversity and heteromorphic Sm rings are features stretching back to the origin of eukaryotes; very deep phylogenetic divisions among existing Sm proteins indicate simultaneous evolution across essentially all existing eukaryotic life. Two basic forms of heteromorphic Sm rings are found in eukaryotes. Fixed Sm rings are highly stable and static and are assembled around an RNA cofactor. Flexible Sm rings also stabilize and chaperone RNA but assemble in the absence of an RNA substrate and, more significantly, associate with and dissociate from RNA substrates more freely than fixed rings. This suggests that the conformation of flexible Sm rings might be modified in some specific manner to facilitate association and dissociation with RNA. Diversification of eukaryotic Sm proteins may have been initiated by gene transfers and/or genome clashes that accompanied the origin of the eukaryotic cell itself, with further diversification driven by a greater need for steric specificity within increasingly complex snRNPs.

Identification of the heptameric Lsm complex that binds U6 snRNA in Trypanosoma brucei

Lsm proteins are ubiquitous, multifunctional proteins that are involved in nuclear processing and turnover of many RNAs in eukaryotes. Lsm proteins form two distinct complexes, the Lsm2-8 complex, which binds U6 snRNA, and the Lsm1-7 complex, which governs mRNA degradation. Previously, seven Lsm proteins were identified in Trypanosoma brucei. Two of these proteins were later identified as SSm proteins (specific spliceosomal Sm proteins). In this study, the Lsm proteins (Lsm2 and Lsm5) that bind to U6 snRNA were identified. RNAi silencing and protein purification of TAP-tagged Lsm proteins were used to identify all the components of the trypanosome heptameric Lsm2-8 complex. Localization studies demonstrated that these proteins are found in the nucleus, near the nucleolus. Lsm proteins were not detected in cytoplasmic bodies that were tagged with YFP-Dhh1, which may suggest that in trypanosomes, Lsm-mediated degradation is not confined to such bodies.

Crystal structure of Lsm3 octamer from Saccharomyces cerevisiae: implications for Lsm ring organisation and recruitment

Sm and Sm-like (Lsm) proteins are core components of the ribonucleoprotein complexes essential to key nucleic acid processing events within the eukaryotic cell. They assemble as polyprotein ring scaffolds that have the capacity to bind RNA substrates and other necessary protein factors. The crystal structure of yeast Lsm3 reveals a new organisation of the L/Sm beta-propeller ring, containing eight protein subunits. Little distortion of the characteristic L/Sm fold is required to form the octamer, indicating that the eukaryotic Lsm ring may be more pliable than previously thought. The homomeric Lsm3 octamer is found to successfully recruit Lsm6, Lsm2 and Lsm5 directly from yeast lysate. Our crystal structure shows the C-terminal tail of each Lsm3 subunit to be engaged in connections across rings through specific beta-sheet interactions with elongated loops protruding from neighbouring octamers. While these loops are of distinct length for each Lsm protein and generally comprise low-complexity polar sequences, several Lsm C-termini comprise hydrophobic sequences suitable for beta-sheet interactions. The Lsm3 structure thus provides evidence for protein-protein interactions likely utilised by the highly variable Lsm loops and termini in the recruitment of RNA processing factors to mixed Lsm ring scaffolds. Our coordinates also provide updated homology models for the active Lsm[1-7] and Lsm[2-8] heptameric rings.

Roles of eukaryotic Lsm proteins in the regulation of mRNA function

The eukaryotic Lsm proteins belong to the large family of Sm-like proteins, which includes members from all organisms ranging from archaebacteria to humans. The Sm and Lsm proteins typically exist as hexameric or heptameric complexes in vivo and carry out RNA-related functions. Multiple complexes made up of different combinations of Sm and Lsm proteins are known in eukaryotes and these complexes are involved in a variety of functions such as mRNA decay in the cytoplasm, mRNA and pre-mRNA decay in the nucleus, pre-mRNA splicing, replication dependent histone mRNA 3'-end processing, etc. While most Lsm proteins function in the form of heteromeric complexes that include other Lsm proteins, some Lsm proteins are also known that do not behave in that manner. Abnormal expression of some Lsm proteins has also been implicated in human diseases. The various roles of eukaryotic Lsm complexes impacting mRNA function are discussed in this review.

Requirements for nuclear localization of the Lsm2-8p complex and competition between nuclear and cytoplasmic Lsm complexes

Sm-like (Lsm) proteins are ubiquitous, multifunctional proteins that are involved in the processing and/or turnover of many RNAs. In eukaryotes, a hetero-heptameric complex of seven Lsm proteins (Lsm2-8) affects the processing of small stable RNAs and pre-mRNAs in the nucleus, whereas a different hetero-heptameric complex of Lsm proteins (Lsm1-7) promotes mRNA decapping and decay in the cytoplasm. These two complexes have six constituent proteins in common, yet localize to separate cellular compartments and perform apparently disparate functions. Little is known about the biogenesis of the Lsm complexes, or how they are recruited to different cellular compartments. We show that, in yeast, the nuclear accumulation of Lsm proteins depends on complex formation and that the Lsm8p subunit plays a crucial role. The nuclear localization of Lsm8p is itself most strongly influenced by Lsm2p and Lsm4p, its presumed neighbours in the Lsm2-8p complex. Furthermore, overexpression and depletion experiments imply that Lsm1p and Lsm8p act competitively with respect to the localization of the two complexes, suggesting a potential mechanism for co-regulation of nuclear and cytoplasmic RNA processing. A shift of Lsm proteins from the nucleus to the cytoplasm under stress conditions indicates that this competition is biologically significant.

Structural and biochemical basis for misfolded RNA recognition by the Ro autoantigen

The Ro autoantigen is ring-shaped, binds misfolded noncoding RNAs and is proposed to function in quality control. Here we determine how Ro interacts with misfolded RNAs. Binding of Ro to misfolded precursor (pre)-5S ribosomal RNA requires a single-stranded 3' end and helical elements. As mutating most sequences of the helices and tail results in modest decreases in binding, Ro may be able to associate with a range of RNAs. Ro binds several other RNAs that contain single-stranded tails. A crystal structure of Ro bound to a misfolded pre-5S rRNA fragment reveals that the tail inserts into the cavity, while a helix binds on the surface. Most contacts of Ro with the helix are to the backbone. Mutagenesis reveals that Ro has an extensive RNA-binding surface. We propose that Ro uses this surface to scavenge RNAs that fail to bind their specific RNA-binding proteins.

The La protein functions redundantly with tRNA modification enzymes to ensure tRNA structural stability

Although the La protein stabilizes nascent pre-tRNAs from nucleases, influences the pathway of pre-tRNA maturation, and assists correct folding of certain pre-tRNAs, it is dispensable for growth in both budding and fission yeast. Here we show that the Saccharomyces cerevisiae La shares functional redundancy with both tRNA modification enzymes and other proteins that contact tRNAs during their biogenesis. La is important for growth in the presence of mutations in either the arginyl tRNA synthetase or the tRNA modification enzyme Trm1p. In addition, two pseudouridine synthases, PUS3 and PUS4, are important for growth in strains carrying a mutation in tRNA(Arg)(CCG) and are essential when La is deleted in these strains. Depletion of Pus3p results in accumulation of the aminoacylated mutant tRNA(Arg)(CCG) in nuclei, while depletion of Pus4p results in decreased stability of the mutant tRNA. Interestingly, the degradation of mutant unstable forms of tRNA(Arg)(CCG) does not require the Trf4p poly(A) polymerase, suggesting that yeast cells possess multiple pathways for tRNA decay. These data demonstrate that La functions redundantly with both tRNA modifications and proteins that associate with tRNAs to achieve tRNA structural stability and efficient biogenesis.

Molecular chaperones and quality control in noncoding RNA biogenesis

Although noncoding RNAs have critical roles in all cells, both the mechanisms by which these RNAs fold into functional structures and the quality control pathways that monitor correct folding are only beginning to be elucidated. Here, we discuss several proteins that likely function as molecular chaperones for noncoding RNAs and review the existing knowledge on noncoding RNA quality control. One protein, the La protein, binds many nascent noncoding RNAs in eukaryotes and is required for efficient folding of certain pre-tRNAs. In prokaryotes, the Sm-like protein Hfq is required for the function of many noncoding RNAs. Recent work in bacteria and yeast has revealed the existence of quality control systems involving polyadenylation of unstable noncoding RNAs followed by exonucleolytic degradation. In addition, the Ro protein, which is present in many animal cells and also certain bacteria, binds misfolded noncoding RNAs and is proposed to function in RNA quality control.

Eukaryotic Lsm proteins: lessons from bacteria

Over the last five years Sm-like (Lsm) proteins have emerged as important players in many aspects of RNA metabolism, including splicing, nuclear RNA processing and messenger RNA decay. However, their precise function in these pathways remains somewhat obscure. In contrast, the role of the bacterial Lsm protein Hfq, which bears striking similarities in both structure and function to Lsm proteins, is much better characterized. In this perspective, we have highlighted several functions that Hfq shares with Lsm proteins and put forward hypotheses based on parallels between the two that might further the understanding of Lsm function.

Lsm proteins and RNA processing

Sm and Lsm proteins are ubiquitous in eukaryotes and form complexes that interact with RNAs involved in almost every cellular process. My laboratory has studied the Lsm proteins in the yeast Saccharomyces cerevisiae, identifying in the nucleus and cytoplasm distinct complexes that affect pre-mRNA splicing and degradation, small nucleolar RNA, tRNA processing, rRNA processing and mRNA degradation. These activities suggest RNA chaperone-like roles for Lsm proteins, affecting RNA-RNA and/or RNA-protein interactions. This article reviews the properties of the Sm and Lsm proteins and structurally and functionally related proteins in archaea and eubacteria.

Crosstalk between RNA metabolic pathways: an RNOMICS approach

Eukaryotic cells contain many different RNA species. Nuclear pre-mRNAs and cytoplasmic mRNAs carry genomic information to the protein synthesis machinery, whereas many stable RNA species have important functional roles. The mature, functional forms of these RNA species are generated by post-transcriptional processing, and evidence has been accumulating that there are functional links between the various processing pathways. This indicates that there are regulatory networks that coordinate different stages of RNA metabolism. This article describes the aims and results, to date, of the European RNOMICS project as an example of an integrated approach to investigate these links.

Mutations in the Saccharomyces cerevisiae LSM1 gene that affect mRNA decapping and 3' end protection

The decapping of eukaryotic mRNAs is a key step in their degradation. The heteroheptameric Lsm1p-7p complex is a general activator of decapping and also functions in protecting the 3' ends of deadenylated mRNAs from a 3'-trimming reaction. Lsm1p is the unique member of the Lsm1p-7p complex, distinguishing that complex from the functionally different Lsm2p-8p complex. To understand the function of Lsm1p, we constructed a series of deletion and point mutations of the LSM1 gene and examined their effects on phenotype. These studies revealed the following: (i) Mutations affecting the predicted RNA-binding and inter-subunit interaction residues of Lsm1p led to impairment of mRNA decay, suggesting that the integrity of the Lsm1p-7p complex and the ability of the Lsm1p-7p complex to interact with mRNA are important for mRNA decay function; (ii) mutations affecting the predicted RNA contact residues did not affect the localization of the Lsm1p-7p complex to the P-bodies; (iii) mRNA 3'-end protection could be indicative of the binding of the Lsm1p-7p complex to the mRNA prior to activation of decapping, since all the mutants defective in mRNA 3' end protection were also blocked in mRNA decay; and (iv) in addition to the Sm domain, the C-terminal domain of Lsm1p is also important for mRNA decay function.

Nuclear pre-mRNA decapping and 5' degradation in yeast require the Lsm2-8p complex

Previous analyses have identified related cytoplasmic Lsm1-7p and nuclear Lsm2-8p complexes. Here we report that mature heat shock and MET mRNAs that are trapped in the nucleus due to a block in mRNA export were strongly stabilized in strains lacking Lsm6p or the nucleus-specific Lsm8p protein but not by the absence of the cytoplasmic Lsm1p. These nucleus-restricted mRNAs remain polyadenylated until their degradation, indicating that nuclear mRNA degradation does not involve the incremental deadenylation that is a key feature of cytoplasmic turnover. Lsm8p can be UV cross-linked to nuclear poly(A)(+) RNA, indicating that an Lsm2-8p complex interacts directly with nucleus-restricted mRNA. Analysis of pre-mRNAs that contain intronic snoRNAs indicates that their 5' degradation is specifically inhibited in strains lacking any of the Lsm2-8p proteins but Lsm1p. Nucleus-restricted mRNAs and pre-mRNA degradation intermediates that accumulate in lsm mutants remain 5' capped. We conclude that the Lsm2-8p complex normally targets nuclear RNA substrates for decapping.

Lsm proteins promote regeneration of pre-mRNA splicing activity

Lsm proteins are ubiquitous, multifunctional proteins that affect the processing of most RNAs in eukaryotic cells, but their function is unknown. A complex of seven Lsm proteins, Lsm2-8, associates with the U6 small nuclear RNA (snRNA) that is a component of spliceosome complexes in which pre-mRNA splicing occurs. Spliceosomes contain five snRNAs, U1, U2, U4, U5, and U6, that are packaged as ribonucleoprotein particles (snRNPs). U4 and U6 snRNAs contain extensive sequence complementarity and interact to form U4/U6 di-snRNPs. U4/U6 di-snRNPs associate with U5 snRNPs to form U4/U6.U5 tri-snRNPs prior to spliceosome assembly. Within spliceosomes, disruption of base-paired U4/U6 heterodimer allows U6 snRNA to form part of the catalytic center. Following completion of the splicing reaction, snRNPs must be recycled for subsequent rounds of splicing, although little is known about this process. Here we present evidence that regeneration of splicing activity in vitro is dependent on Lsm proteins. RNP reconstitution experiments with exogenous U6 RNA show that Lsm proteins promote the formation of U6-containing complexes and suggest that Lsm proteins have a chaperone-like function, supporting the assembly or remodeling of RNP complexes involved in splicing. Such a function could explain the involvement of Lsm proteins in a wide variety of RNA processing pathways.