RNA binding by Hfq and ring-forming (L)Sm proteins: a trade-off between optimal sequence readout and RNA backbone conformation

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.

Genome-wide profiling of Hfq-bound RNAs reveals the iron-responsive small RNA RusT in Caulobacter crescentus

The alphaproteobacterium Caulobacter crescentus thrives in oligotrophic environments and is able to optimally exploit minimal resources by entertaining an intricate network of gene expression control mechanisms. Numerous transcriptional activators and repressors have been reported to contribute to these processes, but only few studies have focused on regulation at the post-transcriptional level in C. crescentus. Small RNAs (sRNAs) are a prominent class of regulators of bacterial gene expression, and most sRNAs characterized today engage in direct base-pairing interactions to modulate the translation and/or stability of target mRNAs. In many cases, the ubiquitous RNA chaperone, Hfq, contributes to the establishment of RNA-RNA interactions. Although the deletion of the hfq gene is associated with a severe loss of fitness in C. crescentus, the RNA ligands of the chaperone have remained largely unexplored. Here we report on the identification of coding and non-coding transcripts associated with Hfq in C. crescentus and demonstrate Hfq-dependent post-transcriptional regulation in this organism. We show that the Hfq-bound sRNA RusT is transcriptionally controlled by the NtrYX two-component system and induced in response to iron starvation. By combining RusT pulse expression with whole-genome transcriptome analysis, we determine 16 candidate target transcripts that are deregulated, many of which encode outer membrane transporters. We hence suggest RusT to support remodeling of the C. crescentus cell surface when iron supplies are limited.IMPORTANCEThe conserved RNA-binding protein Hfq contributes significantly to the adaptation of bacteria to different environmental conditions. Hfq not only stabilizes associated sRNAs but also promotes inter-molecular base-pairing interactions with target transcripts. Hfq plays a pivotal role for growth and survival, controlling central metabolism and cell wall synthesis in the oligotroph Caulobacter crescentus. However, direct evidence for Hfq-dependent post-transcriptional regulation and potential oligotrophy in C. crescentus has been lacking. Here, we identified sRNAs and mRNAs associated with Hfq in vivo, and demonstrated the requirement of Hfq for sRNA-mediated regulation, particularly of outer membrane transporters in C. crescentus.

ProQ binding to small RNA RyfA promotes virulence and biofilm formation in avian pathogenic Escherichia coli

Avian pathogenic Escherichia coli (APEC) is a notable subpathotype of the nonhuman extraintestinal pathogenic E. coli (ExPEC). Recognized as an extraintestinal foodborne pathogen, the zoonotic potential of APEC/ExPEC allows for cross-host transmission via APEC-contaminated poultry meat and eggs. ProQ, an RNA binding protein, is evolutionarily conserved in E. coli. However, its regulatory roles in the biofilm formation and virulence of APEC/ExPEC have not been explored. In this study, proQ deletion in the APEC strain FY26 significantly compromised its biofilm-forming ability. Furthermore, animal tests and cellular infection experiments showed that ProQ depletion significantly attenuated APEC virulence, thereby diminishing its capacity for bloodstream infection and effective adherence to and persistence within host cells. Transcriptome analysis revealed a decrease in the transcription level of the small RNA (sRNA) RyfA in the mutant FY26ΔproQ, suggesting a direct interaction between the sRNA RyfA and ProQ. This interaction might indicate that sRNA RyfA is a novel ProQ-associated sRNA. Moreover, the direct binding of ProQ to the sRNA RyfA was crucial for APEC biofilm formation, pathogenicity, adhesion, and intracellular survival. In conclusion, our findings provide detailed insight into the interaction between ProQ and sRNA RyfA and deepen our understanding of the regulatory elements that dictate APEC virulence and biofilm development. Such insights are instrumental in developing strategies to counteract APEC colonization within hosts and impede APEC biofilm establishment on food surfaces.

The mechanism of LSM2 in the progression of live hepatocellular carcinoma was analyzed based on bioinformatics

Comprehensive analysis of the expression and probable function of LSM2 in Live hepatocellular carcinoma (LIHC), and validation via in vitro experiments. Integrated use of database resources to examine the differential expression, survival prognosis, clinicopathological characteristics, and functional enrichment of LSM2 in LIHC. The expression level of LSM2 in LIHC tissues and adjacent tissues was proven via immunohistochemical staining. The biological function of LSM2 in LIHC was detected by cell proliferation, cell cloning, cell scratch, cell migration, and invasion experiments in vitro. TIMER 2.0 and GEPIA indicated that LSM2 was highly expressed in cancers and was strongly associated with survival rates in LIHC, cholangiocarcinoma, breast cancer, and renal clear cell carcinoma. LSM2 was highly expressed in LIHC, which was closely associated to the clinicopathological characteristics of patients, and the overall survival rate and disease-free survival rate of patients with high expression of LSM2 were lower than those with low expression of LSM2. Functional enrichment results revealed that LSM2 was involved to ribosome formation, DNA replication, cell cycle, metabolic processes, JAK-STAT signaling pathways, and FoxO signaling pathways. Knockdown of LSM2 inhibited the proliferation, migration, and invasion of LIHC cells in vitro experiments. LSM2 was highly expressed in LIHC and was related to a poor prognosis. Knockdown of LSM2 could inhibit the proliferation, migration, and invasion of LIHC cells.

FinO/ProQ-family proteins: an evolutionary perspective

RNA-binding proteins are key actors of post-transcriptional networks. Almost exclusively studied in the light of their interactions with RNA ligands and the associated functional events, they are still poorly understood as evolutionary units. In this review, we discuss the FinO/ProQ family of bacterial RNA chaperones, how they evolve and spread across bacterial populations and what properties and opportunities they provide to their host cells. We reflect on major conserved and divergent themes within the family, trying to understand how the same ancestral RNA-binding fold, augmented with additional structural elements, could yield either highly specialised proteins or, on the contrary, globally acting regulatory hubs with a pervasive impact on gene expression. We also consider dominant convergent evolutionary trends that shaped their RNA chaperone activity and recurrently implicated the FinO/ProQ-like proteins in bacterial DNA metabolism, translation and virulence. Finally, we offer a new perspective in which FinO/ProQ-family regulators emerge as active evolutionary players with both negative and positive roles, significantly impacting the evolutionary modes and trajectories of their bacterial hosts.

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.

Dynamic Refolding of OxyS sRNA by the Hfq RNA Chaperone

The Sm protein Hfq chaperones small non-coding RNAs (sRNAs) in bacteria, facilitating sRNA regulation of target mRNAs. Hfq acts in part by remodeling the sRNA and mRNA structures, yet the basis for this remodeling activity is not understood. To understand how Hfq remodels RNA, we used single-molecule Förster resonance energy transfer (smFRET) to monitor conformational changes in OxyS sRNA upon Hfq binding. The results show that E. coli Hfq first compacts OxyS, bringing its 5' and 3 ends together. Next, Hfq destabilizes an internal stem-loop in OxyS, allowing the RNA to adopt a more open conformation that is stabilized by a conserved arginine on the rim of Hfq. The frequency of transitions between compact and open conformations depend on interactions with Hfqs flexible C-terminal domain (CTD), being more rapid when the CTD is deleted, and slower when OxyS is bound to Caulobacter crescentus Hfq, which has a shorter and more stable CTD than E. coli Hfq. We propose that the CTDs gate transitions between OxyS conformations that are stabilized by interaction with one or more arginines. These results suggest a general model for how basic residues and intrinsically disordered regions of RNA chaperones act together to refold RNA.

Computational Insights into the Binding Mechanism of OxyS sRNA with Chaperone Protein Hfq

Under the oxidative stress condition, the small RNA (sRNA) OxyS that acts as essential post-transcriptional regulators of gene expression is produced and plays a regulatory function with the assistance of the RNA chaperone Hfq protein. Interestingly, experimental studies found that the N48A mutation of Hfq protein could enhance the binding affinity with OxyS while resulting in the defection of gene regulation. However, how the Hfq protein interacts with sRNA OxyS and the origin of the stronger affinity of N48A mutation are both unclear. In this paper, molecular dynamics (MD) simulations were performed on the complex structure of Hfq and OxyS to explore their binding mechanism. The molecular mechanics generalized born surface area (MM/GBSA) and interaction entropy (IE) method were combined to calculate the binding free energy between Hfq and OxyS sRNA, and the computational result was correlated with the experimental result. Per-residue decomposition of the binding free energy revealed that the enhanced binding ability of the N48A mutation mainly came from the increased van der Waals interactions (vdW). This research explored the binding mechanism between Oxys and chaperone protein Hfq and revealed the origin of the strong binding affinity of N48A mutation. The results provided important insights into the mechanism of gene expression regulation affected by protein mutations.

Diversity of LSM Family Proteins: Similarities and Differences

Members of the Lsm protein family are found in all three domains of life: bacteria, archaea, and eukarya. They are involved in numerous processes associated with RNA processing and gene expression regulation. A common structural feature of all Lsm family proteins is the presence of the Sm fold consisting of a five-stranded β-sheet and an α-helix at the N-terminus. Heteroheptameric eukaryotic Sm and Lsm proteins participate in the formation of spliceosomes and mRNA decapping. Homohexameric bacterial Lsm protein, Hfq, is involved in the regulation of transcription of different mRNAs by facilitating their interactions with small regulatory RNAs. Furthermore, recently obtained data indicate a new role of Hfq as a ribosome biogenesis factor, as it mediates formation of the productive structure of the 17S rRNA 3'- and 5'-sequences, facilitating their further processing by RNases. Lsm archaeal proteins (SmAPs) form homoheptamers and likely interact with single-stranded uridine-rich RNA elements, although the role of these proteins in archaea is still poorly understood. In this review, we discuss the structural features of the Lsm family proteins from different life domains and their structure-function relationships.

Amplification of 8p11.23 in cancers and the role of amplicon genes

Copy number alterations are widespread in cancer genomes and are part of the genomic instability underlying the pathogenesis of neoplastic diseases. Recurrent copy number alterations of specific chromosomal loci may result in gains of oncogenes or losses of tumor suppressor genes and become entrenched in the genomic framework of certain types of cancers. The locus at chromosome 8p11.23 presents recurrent amplifications most commonly in squamous lung carcinomas, breast cancers, squamous esophageal carcinomas, and urothelial carcinomas. Amplification is rare in other cancers. The amplified segment involves several described oncogenes that may promote cancer cell survival and proliferation, as well as less well characterized genes that could also contribute to neoplastic processes. Genes proposed to be "drivers" in 8p11.23 amplifications include ZNF703, FGFR1 and PLPP5. Additional genes in the locus that could be functionally important in neoplastic networks include co-chaperone BAG4, lysine methyltransferase NSD3, ASH2L, a member of another methyltransferase complex, MLL and the mRNA processing and translation regulators LSM1 and EIF4EBP1. In this paper, genes located in the amplified segment of 8p11.23 will be examined for their role in cancer and data arguing for their importance for cancers with the amplification will be presented.

Stabilize and connect: the role of LARP7 in nuclear non-coding RNA metabolism

La and La-related proteins (LARPs) are characterized by a common RNA interaction platform termed the La module. This structural hallmark allows LARPs to pervade various aspects of RNA biology. The metazoan LARP7 protein binds to the 7SK RNA as part of a 7SK small nuclear ribonucleoprotein (7SK snRNP), which inhibits the transcriptional activity of RNA polymerase II (Pol II). Additionally, recent findings revealed unanticipated roles of LARP7 in the assembly of other RNPs, as well as in the modification, processing and cellular transport of RNA molecules. Reduced levels of functional LARP7 have been linked to cancer and Alazami syndrome, two seemingly unrelated human diseases characterized either by hyperproliferation or growth retardation. Here, we review the intricate regulatory networks centered on LARP7 and assess how malfunction of these networks may relate to the etiology of LARP7-linked diseases.

Crystal structures and RNA-binding properties of Lsm proteins from archaea Sulfolobus acidocaldarius and Methanococcus vannielii: Similarity and difference of the U-binding mode

Sm and Sm-like (Lsm) proteins are considered as an evolutionary conserved family involved in RNA metabolism in organisms from bacteria and archaea to human. Currently, the function of Sm-like archaeal proteins (SmAP) is not well understood. Here, we report the crystal structures of SmAP proteins from Sulfolobus acidocaldarius and Methanococcus vannielii and a comparative analysis of their RNA-binding sites. Our data show that these SmAPs have only a uridine-specific RNA-binding site, unlike their bacterial homolog Hfq, which has three different RNA-binding sites. Moreover, variations in the amino acid composition of the U-binding sites of the two SmAPs lead to a difference in protein affinity for oligo(U) RNA. Surface plasmon resonance data and nucleotide-binding analysis confirm the high affinity of SmAPs for uridine nucleotides and oligo(U) RNA and the reduced affinity for adenines, guanines, cytidines and corresponding oligo-RNAs. In addition, we demonstrate that MvaSmAP1 and SacSmAP2 are capable of melting an RNA hairpin and, apparently, promote its interaction with complementary RNA.

LARP7-Mediated U6 snRNA Modification Ensures Splicing Fidelity and Spermatogenesis in Mice

U6 snRNA, as an essential component of the catalytic core of the pre-mRNA processing spliceosome, is heavily modified post-transcriptionally, with 2'-O-methylation being most common. The role of these modifications in pre-mRNA splicing as well as their physiological function in mammals have remained largely unclear. Here we report that the La-related protein LARP7 functions as a critical cofactor for 2'-O-methylation of U6 in mouse male germ cells. Mechanistically, LARP7 promotes U6 loading onto box C/D snoRNP, facilitating U6 2'-O-methylation by box C/D snoRNP. Importantly, ablation of LARP7 in the male germline causes defective U6 2'-O-methylation, massive alterations in pre-mRNA splicing, and spermatogenic failure in mice, which can be rescued by ectopic expression of wild-type LARP7 but not an U6-loading-deficient mutant LARP7. Our data uncover a novel role of LARP7 in regulating U6 2'-O-methylation and demonstrate the functional requirement of such modification for splicing fidelity and spermatogenesis in mice.

DEKTV and YVG motifs in the Lsm domain are important for the activity of Scd6, a conserved translation repressor protein

Scd6 is a conserved RGG-motif protein which represses translation by binding eIF4G through its RGG-motif. Lsm and FDF are two other conserved domains present in the protein, however the role of both these domains is unclear. We provide evidence in this report that the Lsm domain is important for the role of Scd6 in translation. Mutant of Scd6 lacking the Lsm domain does not cause overexpression growth defect in a manner comparable to the wild type. Similar results were observed with two distinct point mutants of Scd6 wherein putative RNA-binding motifs DxEKxTV and YVG were mutated. Upon overexpression, the three mutants were defective in inducing formation of P-bodies and stress granules which are conserved sites of translation repression. Importantly localization to granules in response to glucose deprivation and sodium azide stress was defective for Lsm domain mutants indicating that the inability to localize to granules could be a reason for their defective role in translation. Deletion of scd6 impairs Lsm1 foci formation upon glucose deprivation stress which could not be rescued by complementation with Lsm-domain deletion mutant of Scd6 when compared to the full-length protein. Put together, our results highlight the role of Lsm domain and its specific motifs in Scd6 activity and provide crucial insight into its function.

Quantitative Analysis of RNA Chaperone Activity by Native Gel Electrophoresis and Fluorescence Spectroscopy

Diverse types of RNA-binding proteins chaperone the interactions of noncoding RNAs by increasing the rate of RNA base pairing and by stabilizing the final RNA duplex. The E. coli protein Hfq facilitates interactions between small noncoding RNAs and their target mRNAs. The chaperone and RNA annealing activity of Hfq and other RNA chaperones can be evaluated by determining the kinetics of RNA base pairing in the presence and absence of the protein. This chapter presents protocols for measuring RNA annealing kinetics using electrophoretic gel mobility shift assays (EMSA), stopped-flow fluorescence, and fluorescence anisotropy. EMSA is low cost and can resolve reaction intermediates of natural small RNAs and mRNA fragments, as long as the complexes are sufficiently long-lived (≥10 s) to be trapped during electrophoresis. Stopped-flow fluorescence can detect annealing reactions between 1 ms and 30 s and is best suited for measuring the rapid annealing of oligoribonucleotides. Fluorescence anisotropy reports the physical size of the complex and is well-suited for monitoring the association and dissociation of RNA from Hfq during the chaperone cycle.

Transcriptional responses of LSm14A after infection of blue eggshell layers with Newcastle disease viruses

LSm14A is a key innate immunity component of processing body (P-body) that mediates interferon-β (IFN-β) signaling by viral RNA. This is the first study to report chicken LSm14A (cLSm14A) cloning from blue eggshell layer, high tibia and frizzle chickens. The cLSm14A gene, encoding 461 amino acids, is highly homologous in the three types of chickens. The cLSm14A was extensively expressed in several tissues. The transcriptional level of cLSm14A was significantly increased in various stages of Newcastle disease virus (NDV) infection. In HEK293 cells, full length cLSm14A from blue eggshell layer was localized in the cytoplasm as dots. The results of this study indicated that cLSm14A is an important sensor that mediates innate immunity in chicken against NDV infections.

Caulobacter crescentus Hfq structure reveals a conserved mechanism of RNA annealing regulation

In many bacteria, the RNA chaperone protein Hfq binds to hundreds of small noncoding RNAs and improves their efficacy by aiding base pairing to target mRNAs. Hfq proteins contain a variable C-terminal domain (CTD), usually structurally disordered, which was recently demonstrated to inhibit Hfq from mediating nonspecific RNA annealing. We obtained a new structure that shows how this inhibition is achieved in Caulobacter crescentus Hfq. The structural data and chaperone assays provide an initial view of the little-known mechanism of small RNA regulation in Caulobacter. In addition, this work demonstrates how the Hfq CTD has evolved to meet the needs for species-specific selectivity in RNA binding and pairing of regulatory RNAs with cognate targets.

We have solved the X-ray crystal structure of the RNA chaperone protein Hfq from the alpha-proteobacterium Caulobacter crescentus to 2.15-Å resolution, resolving the conserved core of the protein and the entire C-terminal domain (CTD). The structure reveals that the CTD of neighboring hexamers pack in crystal contacts, and that the acidic residues at the C-terminal tip of the protein interact with positive residues on the rim of Hfq, as has been recently proposed for a mechanism of modulating RNA binding. De novo computational models predict a similar docking of the acidic tip residues against the core of Hfq. We also show that C. crescentus Hfq has sRNA binding and RNA annealing activities and is capable of facilitating the annealing of certain Escherichia coli sRNA:mRNA pairs in vivo. Finally, we describe how the Hfq CTD and its acidic tip residues provide a mechanism to modulate annealing activity and substrate specificity in various bacteria.

Proteins That Chaperone RNA Regulation

RNA-binding proteins chaperone the biological functions of noncoding RNA by reducing RNA misfolding, improving matchmaking between regulatory RNA and targets, and exerting quality control over RNP biogenesis. Recent studies of Escherichia coli CspA, HIV NCp, and E. coli Hfq are beginning to show how RNA-binding proteins remodel RNA structures. These different protein families use common strategies for disrupting or annealing RNA double helices, which can be used to understand the mechanisms by which proteins chaperone RNA-dependent regulation in bacteria.

Unique Archaeal Small RNAs

Advances in genome-wide sequence technologies allow for detailed insights into the complexity of RNA landscapes of organisms from all three domains of life. Recent analyses of archaeal transcriptomes identified interaction and regulation networks of noncoding RNAs in this understudied domain. Here, we review current knowledge of small, noncoding RNAs with important functions for the archaeal lifestyle, which often requires adaptation to extreme environments. One focus is RNA metabolism at elevated temperatures in hyperthermophilic archaea, which reveals elevated amounts of RNA-guided RNA modification and virus defense strategies. Genome rearrangement events result in unique fragmentation patterns of noncoding RNA genes that require elaborate maturation pathways to yield functional transcripts. RNA-binding proteins, e.g., L7Ae and LSm, are important for many posttranscriptional control functions of RNA molecules in archaeal cells. We also discuss recent insights into the regulatory potential of their noncoding RNA partners.

The Small β-Barrel Domain: A Survey-Based Structural Analysis

The small β-barrel (SBB) is an ancient protein structural domain characterized by extremes: it features a broad range of structural varieties, a deeply intricate evolutionary history, and it is associated with a bewildering array of cellular pathways. Here, we present a thorough, survey-based analysis of the structural properties of SBBs. We first consider the defining properties of the SBB, including various systems of nomenclature used to describe it, and we introduce the unifying concept of an "urfold." To begin elucidating how vast functional diversity can be achieved by a relatively simple domain, we explore the anatomy of the SBB and its representative structural variants. Many SBB proteins assemble into cyclic oligomers as the biologically functional units; these oligomers often bind RNA, and typically exhibit great quaternary structural plasticity (homomeric and heteromeric rings, variable subunit stoichiometries, etc.). We conclude with three themes that emerge from the rich structure ↔ function versatility of the SBB.

Hfq chaperone brings speed dating to bacterial sRNA

Hfq is a ubiquitous, Sm-like RNA binding protein found in most bacteria and some archaea. Hfq binds small regulatory RNAs (sRNAs), facilitates base pairing between sRNAs and their mRNA targets, and directly binds and regulates translation of certain mRNAs. Because sRNAs regulate many stress response pathways in bacteria, Hfq is essential for adaptation to different environments and growth conditions. The chaperone activities of Hfq arise from multipronged RNA binding by three different surfaces of the Hfq hexamer. The manner in which the structured Sm core of Hfq binds RNA has been well studied, but recent work shows that the intrinsically disordered C-terminal domain of Hfq modulates sRNA binding, creating a kinetic hierarchy of RNA competition for Hfq and ensuring the release of double-stranded sRNA-mRNA complexes. A combination of structural, biophysical, and genetic experiments reveals how Hfq recognizes its RNA substrates and plays matchmaker for sRNAs and mRNAs in the cell. The interplay between structured and disordered domains of Hfq optimizes sRNA-mediated post-transcriptional regulation, and is a common theme in RNA chaperones. This article is categorized under: Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Structure and Dynamics > RNA Structure, Dynamics, and Chemistry.

Identification and characterization of two variants of the Hfq-sRNA-chaperone in the fish pathogen Piscirickettsia salmonis

Small RNA and chaperone proteins form synergistic duos that play pivotal roles in controlling gene expression in bacteria. This is the case for Hfq, a highly pleiotropic pretranslational modulator of general protein expression, which responds to harsh environmental conditions and influences fitness and virulence in a wide range of pathogenic Enterobacteria. Given this relevancy, we evaluated the presence and potential role of Hfq in the fish pathogen Piscirickettsia salmonis, a Gram-negative bacterium that threatens the sustainability of Chilean salmon production. Using bioinformatics tools were identified and characterized two variants of Hfq, which share the consensus RNA-binding domains and the active sites described functional Hfq other bacteria. Additionally, we demonstrated that hfq-1 and hfq-2 were transcriptionally active when growing in cell-free media and in infected susceptible fish cell line. Expression of both genes differed under different growth conditions and under stress, suggesting that their roles might be independent and different, depending on the bacterial physiological status. In conclusion, we demonstrate the existence of two different and functional ORF coding for the hfq marker in marine bacteria and a preliminary analysis indicating that these two novel proteins might have relevant roles in the biology and pathogenic potential of P. salmonis.

Ubiquitin carboxyl-terminal hydrolase isozyme L5 inhibits human glioma cell migration and invasion via downregulating SNRPF

Ubiquitin C-terminal Hydrolase-L5 (UCH-L5/UCH37), a member of the deubiquitinases (DUBs), suppresses protein degeneration via removing ubiquitin from the distal subunit of the polyubiquitin chain. The activity of UCH-L5 is enhanced when UCH-L5 combines with proteasome 19S regulatory subunit by Rpn13/Admr1 receptor and inhibited when UCH-L5 interacts with NFRKB. But the role of UCH-L5 in gliomas remains unknown. In this study, analysis of 19 frozen and 51 paraffin-embedded clinic pathological cases showed that UCH-L5 expression in glioma tissues was lower than normal brain tissues. In vitro, we found that UCH-L5 could inhibit migration and invasion of U87MG and U251 cells. It has been reported that the expression of SNRPN, SNRPF, and CKLF was abnormal in gliomas or other tumors. We also found that SNRPF-siRNA, SNRPN-siRNA and CKLF-siRNA could inhibit migration and invasion of U87MG cells. And knockdown of UCH-L5 expression improved both mRNA expression and protein level of SNRPF. The relationship between UCH-L5 and SNRPF was further confirmed in 293T cells. Our study showed that UCH-L5 could inhibit migration and invasion of glioma cells via down regulating expression of SNRPF. And the above findings suggest that UCH-L5 may inhibit occurrence and metastasis of gliomas.

Intermolecular base stacking mediates RNA-RNA interaction in a crystal structure of the RNA chaperone Hfq

The RNA-chaperone Hfq catalyses the annealing of bacterial small RNAs (sRNAs) with target mRNAs to regulate gene expression in response to environmental stimuli. Hfq acts on a diverse set of sRNA-mRNA pairs using a variety of different molecular mechanisms. Here, we present an unusual crystal structure showing two Hfq-RNA complexes interacting via their bound RNA molecules. The structure contains two Hfq₆:A₁₈ RNA assemblies positioned face-to-face, with the RNA molecules turned towards each other and connected via interdigitating base stacking interactions at the center. Biochemical data further confirm the observed interaction, and indicate that RNA-mediated contacts occur between Hfq-RNA complexes with various (ARN)_X motif containing RNA sequences in vitro, including the stress response regulator OxyS and its target, fhlA. A systematic computational survey also shows that phylogenetically conserved (ARN)_X motifs are present in a subset of sRNAs, some of which share similar modular architectures. We hypothesise that Hfq can co-opt RNA-RNA base stacking, an unanticipated structural trick, to promote the interaction of (ARN)_X motif containing sRNAs with target mRNAs on a “speed-dating” fashion, thereby supporting their regulatory function.

Acidic C-terminal domains autoregulate the RNA chaperone Hfq

The RNA chaperone Hfq is an Sm protein that facilitates base pairing between bacterial small RNAs (sRNAs) and mRNAs involved in stress response and pathogenesis. Hfq possesses an intrinsically disordered C-terminal domain (CTD) that may tune the function of the Sm domain in different organisms. In Escherichia coli, the Hfq CTD increases kinetic competition between sRNAs and recycles Hfq from the sRNA-mRNA duplex. Here, de novo Rosetta modeling and competitive binding experiments show that the acidic tip of the E. coli Hfq CTD transiently binds the basic Sm core residues necessary for RNA annealing. The CTD tip competes against non-specific RNA binding, facilitates dsRNA release, and prevents indiscriminate DNA aggregation, suggesting that this acidic peptide mimics nucleic acid to auto-regulate RNA binding to the Sm ring. The mechanism of CTD auto-inhibition predicts the chaperone function of Hfq in bacterial genera and illuminates how Sm proteins may evolve new functions.

Mille viae in eukaryotic mRNA decapping

Cellular mRNA levels are regulated via rates of transcription and decay. Since the removal of the mRNA 5'-cap by the decapping enzyme DCP2 is generally an irreversible step towards decay, it requires regulation. Control of DCP2 activity is likely effected by two interdependent means: by conformational control of the DCP2-DCP1 complex, and by assembly control of the decapping network, an array of mutually interacting effector proteins. Here, we compare three recent and conformationally distinct crystal structures of the DCP2-DCP1 decapping complex in the presence of substrate analogs and decapping enhancers and we discuss alternative substrate recognition modes for the catalytic domain of DCP2. Together with structure-based insight into decapping network assembly, we propose that DCP2-mediated decapping follows more than one path.

Crystal structure and RNA-binding properties of an Hfq homolog from the deep-branching Aquificae: conservation of the lateral RNA-binding mode

The host factor Hfq, as the bacterial branch of the Sm family, is an RNA-binding protein involved in the post-transcriptional regulation of mRNA expression and turnover. Hfq facilitates pairing between small regulatory RNAs (sRNAs) and their corresponding mRNA targets by binding both RNAs and bringing them into close proximity. Hfq homologs self-assemble into homo-hexameric rings with at least two distinct surfaces that bind RNA. Recently, another binding site, dubbed the `lateral rim', has been implicated in sRNA·mRNA annealing; the RNA-binding properties of this site appear to be rather subtle, and its degree of evolutionary conservation is unknown. An Hfq homolog has been identified in the phylogenetically deep-branching thermophile Aquifex aeolicus (Aae), but little is known about the structure and function of Hfq from basal bacterial lineages such as the Aquificae. Therefore, Aae Hfq was cloned, overexpressed, purified, crystallized and biochemically characterized. Structures of Aae Hfq were determined in space groups P1 and P6, both to 1.5 Å resolution, and nanomolar-scale binding affinities for uridine- and adenosine-rich RNAs were discovered. Co-crystallization with U6 RNA reveals that the outer rim of the Aae Hfq hexamer features a well defined binding pocket that is selective for uracil. This Aae Hfq structure, combined with biochemical and biophysical characterization of the homolog, reveals deep evolutionary conservation of the lateral RNA-binding mode, and lays a foundation for further studies of Hfq-associated RNA biology in ancient bacterial phyla.

Coordinating cell cycle-regulated histone gene expression through assembly and function of the Histone Locus Body

Metazoan replication-dependent (RD) histone genes encode the only known cellular mRNAs that are not polyadenylated. These mRNAs end instead in a conserved stem-loop, which is formed by an endonucleolytic cleavage of the pre-mRNA. The genes for all 5 histone proteins are clustered in all metazoans and coordinately regulated with high levels of expression during S phase. Production of histone mRNAs occurs in a nuclear body called the Histone Locus Body (HLB), a subdomain of the nucleus defined by a concentration of factors necessary for histone gene transcription and pre-mRNA processing. These factors include the scaffolding protein NPAT, essential for histone gene transcription, and FLASH and U7 snRNP, both essential for histone pre-mRNA processing. Histone gene expression is activated by Cyclin E/Cdk2-mediated phosphorylation of NPAT at the G1-S transition. The concentration of factors within the HLB couples transcription with pre-mRNA processing, enhancing the efficiency of histone mRNA biosynthesis.

Characterization of RNA-binding properties of the archaeal Hfq-like protein from Methanococcus jannaschii

The Sm and Sm-like proteins are widely distributed among bacteria, archaea and eukarya. They participate in many processes related to RNA-processing and regulation of gene expression. While the function of the bacterial Lsm protein Hfq and eukaryotic Sm/Lsm proteins is rather well studied, the role of Lsm proteins in Archaea is investigated poorly. In this work, the RNA-binding ability of an archaeal Hfq-like protein from Methanococcus jannaschii has been studied by X-ray crystallography, anisotropy fluorescence and surface plasmon resonance. It has been found that MjaHfq preserves the proximal RNA-binding site that usually recognizes uridine-rich sequences. Distal adenine-binding and lateral RNA-binding sites show considerable structural changes as compared to bacterial Hfq. MjaHfq did not bind mononucleotides at these sites and would not recognize single-stranded RNA as its bacterial homologues. Nevertheless, MjaHfq possesses affinity to poly(A) RNA that seems to bind at the unstructured positive-charged N-terminal tail of the protein.

Small RNAs Regulate Primary and Secondary Metabolism in Gram-negative Bacteria

Over the last decade, small (often noncoding) RNA molecules have been discovered as important regulators influencing myriad aspects of bacterial physiology and virulence. In particular, small RNAs (sRNAs) have been implicated in control of both primary and secondary metabolic pathways in many bacterial species. This chapter describes characteristics of the major classes of sRNA regulators, and highlights what is known regarding their mechanisms of action. Specific examples of sRNAs that regulate metabolism in gram-negative bacteria are discussed, with a focus on those that regulate gene expression by base pairing with mRNA targets to control their translation and stability.

Arginine Patch Predicts the RNA Annealing Activity of Hfq from Gram-Negative and Gram-Positive Bacteria

The Sm-protein Hfq facilitates interactions between small non-coding RNA (sRNA) and target mRNAs. In enteric Gram-negative bacteria, Hfq is required for sRNA regulation, and hfq deletion results in stress intolerance and reduced virulence. By contrast, the role of Hfq in Gram-positive is less established and varies among species. The RNA binding and RNA annealing activity of Hfq from Escherichia coli, Pseudomonas aeruginosa, Listeria monocytogenes, Bacillus subtilis, and Staphylococcus aureus were compared using minimal RNAs and fluorescence spectroscopy. The results show that RNA annealing activity increases with the number of arginines in a semi-conserved patch on the rim of the Hfq hexamer and correlates with the previously reported requirement for Hfq in sRNA regulation. Thus, the amino acid sequence of the arginine patch can predict the chaperone function of Hfq in sRNA regulation in different organisms.

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.

Acidic Residues in the Hfq Chaperone Increase the Selectivity of sRNA Binding and Annealing

Hfq facilitates gene regulation by small non-coding RNAs (sRNAs), thereby affecting bacterial attributes such as biofilm formation and virulence. Escherichia coli Hfq recognizes specific U-rich and AAN motifs in sRNAs and target mRNAs, after which an arginine patch on the rim promotes base pairing between their complementary sequences. In the cell, Hfq must discriminate between many similar RNAs. Here, we report that acidic amino acids lining the sRNA binding channel between the inner pore and rim of the Hfq hexamer contribute to the selectivity of Hfq's chaperone activity. RNase footprinting, in vitro binding and stopped-flow fluorescence annealing assays showed that alanine substitution of D9, E18 or E37 strengthened RNA interactions with the rim of Hfq and increased annealing of non-specific or U-tailed RNA oligomers. Although the mutants were less able than wild-type Hfq to anneal sRNAs with wild-type rpoS mRNA, the D9A mutation bypassed recruitment of Hfq to an (AAN)4 motif in rpoS, both in vitro and in vivo. These results suggest that acidic residues normally modulate access of RNAs to the arginine patch. We propose that this selectivity limits indiscriminate target selection by E. coli Hfq and enforces binding modes that favor genuine sRNA and mRNA pairs.

The Heptameric SmAP1 and SmAP2 Proteins of the Crenarchaeon Sulfolobus Solfataricus Bind to Common and Distinct RNA Targets

Sm and Sm-like proteins represent an evolutionarily conserved family with key roles in RNA metabolism. Sm-based regulation is diverse and can range in scope from eukaryotic mRNA splicing to bacterial quorum sensing, with at least one step in these processes being mediated by an RNA-associated molecular assembly built on Sm proteins. Despite the availability of several 3D-structures of Sm-like archaeal proteins (SmAPs), their function has remained elusive. The aim of this study was to shed light on the function of SmAP1 and SmAP2 of the crenarchaeon Sulfolobus solfataricus (Sso). Using co-purification followed by RNA_Seq different classes of non-coding RNAs and mRNAs were identified that co-purified either with both paralogues or solely with Sso-SmAP1 or Sso-SmAP2. The large number of associated intron-containing tRNAs and tRNA/rRNA modifying RNAs may suggest a role of the two Sso-SmAPs in tRNA/rRNA processing. Moreover, the 3D structure of Sso-SmAP2 was elucidated. Like Sso-SmAP1, Sso-SmAP2 forms homoheptamers. The binding of both proteins to distinct RNA substrates is discussed in terms of surface conservation, structural differences in the RNA binding sites and differences in the electrostatic surface potential of the two Sso-SmAP proteins. Taken together, this study may hint to common and different functions of both Sso-SmAPs in Sso RNA metabolism.

Structural insights into the recognition of the internal A-rich linker from OxyS sRNA by Escherichia coli Hfq

Small RNA OxyS is induced during oxidative stress in Escherichia coli and it is an Hfq-dependent negative regulator of mRNA translation. OxyS represses the translation of fhlA and rpoS mRNA, which encode the transcriptional activator and σ^s subunit of RNA polymerase, respectively. However, little is known regarding how Hfq, an RNA chaperone, interacts with OxyS at the atomic level. Here, using fluorescence polarization and tryptophan fluorescence quenching assays, we verified that the A-rich linker region of OxyS sRNA binds Hfq at its distal side. We also report two crystal structures of Hfq in complex with A-rich RNA fragments from this linker region. Both of these RNA fragments bind to the distal side of Hfq and adopt a different conformation compared with those previously reported for the (A-R-N)_n tripartite recognition motif. Furthermore, using fluorescence polarization, electrophoresis mobility shift assays and in vivo translation assays, we found that an Hfq mutant, N48A, increases the binding affinity of OxyS for Hfq in vitro but is defective in the negative regulation of fhlA translation in vivo, suggesting that the normal function of OxyS depends on the details of the interaction with Hfq that may be related to the rapid recycling of Hfq in the cell.

The FinO family of bacterial RNA chaperones

Antisense RNAs have long been known to regulate diverse aspects of plasmid biology. Here we review the FinOP system that modulates F plasmid gene expression through regulation of the F plasmid transcription factor, TraJ. FinOP is a two component system composed of an antisense RNA, FinP, which represses TraJ translation, and a protein, FinO, which is required to stabilize FinP and facilitate its interactions with its traJ mRNA target. We review the evidence that FinO acts as an RNA chaperone to bind and destabilize internal stem-loop structures within the individual RNAs that would otherwise block intermolecular RNA duplexing. Recent structural studies have provided mechanistic insights into how FinO may facilitate interactions between FinP and traJ mRNA. We also review recent findings that two other proteins, Escherichia coli ProQ and Neisseria meningitidis NMB1681, may represent FinO-like RNA chaperones.