hnRNP H/F drive RNA G-quadruplex-mediated translation linked to genomic instability and therapy resistance in glioblastoma

The shaping of mRNA translation plasticity by RNA G-quadruplexes in cancer progression and therapy resistance

Translational reprogramming in response to oncogenic signaling or microenvironmental stress factors shapes the proteome of cancer cells, enabling adaptation and phenotypic changes underlying cell plasticity, tumor progression and response to cancer therapy. Among the mechanisms regulating translation are RNA G-quadruplexes (RG4s), non-canonical four-stranded structures whose conformational modulation by small molecule ligands and RNA-binding proteins affects the expression of cancer proteins. Here, we discuss the role of RG4s in the regulation of mRNA translation by focusing on paradigmatic examples showing their contribution to adaptive mechanisms of mRNA translation in cancer.

Exploring the biological roles of DHX36, a DNA/RNA G-quadruplex helicase, highlights functions in male infertility: A comprehensive review

It is estimated that 15 % of couples at reproductive age worldwide suffer from infertility, approximately 50 % of cases are caused by male factors. Significant progress has been made in the diagnosis and treatment of male infertility through assisted reproductive technology and molecular genetics methods. However, there is still inadequate research on the underlying mechanisms of gene regulation in the process of spermatogenesis. Guanine-quadruplexes (G4s) are a class of non-canonical secondary structures of nucleic acid commonly found in genomes and RNAs that play important roles in various biological processes. Interestingly, the DEAH-box helicase 36 (DHX36) displays high specificity for the G4s which can unwind both DNA G4s and RNA G4s enzymatically and is highly expressed in testis, thereby regulating multiple cellular functions including transcription, pre-mRNA splicing, translation, telomere maintenance, genomic stability, and RNA metabolism in development and male infertility. This review provides an overview of the roles of G4s and DHX36 in reproduction and development. We mainly focus on the potential role of DHX36 in male infertility. We also discuss possible future research directions regarding the mechanism of spermatogenesis mediated by DHX36 through G4s in spermatogenesis-related genes and provide new targets for gene therapy of male infertility.

SERBP1 interacts with PARP1 and is present in PARylation-dependent protein complexes regulating splicing, cell division, and ribosome biogenesis

RNA binding proteins (RBPs) containing intrinsically disordered regions (IDRs) are present in diverse molecular complexes where they function as dynamic regulators. Their characteristics promote liquid-liquid phase separation (LLPS) and the formation of membraneless organelles such as stress granules and nucleoli. IDR-RBPs are particularly relevant in the nervous system and their dysfunction is associated with neurodegenerative diseases and brain tumor development. SERBP1 is a unique member of this group, being mostly disordered and lacking canonical RNA-binding domains. Using a proteomics approach followed by functional analysis, we defined SERBP1's interactome. We uncovered novel SERBP1 roles in splicing, cell division, and ribosomal biogenesis and showed its participation in pathological stress granules and Tau aggregates in Alzheimer's disease brains. SERBP1 preferentially interacts with other G-quadruplex (G4) binders, implicated in different stages of gene expression, suggesting that G4 binding is a critical component of SERBP1 function in different settings. Similarly, we identified important associations between SERBP1 and PARP1/polyADP-ribosylation (PARylation). SERBP1 interacts with PARP1 and its associated factors and influences PARylation. Moreover, protein complexes in which SERBP1 participates contain mostly PARylated proteins and PAR binders. Based on these results, we propose a feedback regulatory model in which SERBP1 influences PARP1 function and PARylation, while PARylation modulates SERBP1 functions and participation in regulatory complexes.

Predicting nuclear G-quadruplex RNA-binding proteins with roles in transcription and phase separation

RNA-binding proteins are central for many biological processes and their characterization has demonstrated a broad range of functions as well as a wide spectrum of target structures. RNA G-quadruplexes are important regulatory elements occurring in both coding and non-coding transcripts, yet our knowledge of their structure-based interactions is at present limited. Here, using theoretical predictions and experimental approaches, we show that many chromatin-binding proteins bind to RNA G-quadruplexes, and we classify them based on their RNA G-quadruplex-binding potential. Combining experimental identification of nuclear RNA G-quadruplex-binding proteins with computational approaches, we build a prediction tool that assigns probability score for a nuclear protein to bind RNA G-quadruplexes. We show that predicted G-quadruplex RNA-binding proteins exhibit a high degree of protein disorder and hydrophilicity and suggest involvement in both transcription and phase-separation into membrane-less organelles. Finally, we present the G4-Folded/UNfolded Nuclear Interaction Explorer System (G4-FUNNIES) for estimating RNA G4-binding propensities at http://service.tartaglialab.com/new_submission/G4FUNNIES .

RNA G-quadruplexes and stress: emerging mechanisms and functions

RNA G-quadruplexes (rG4s) are noncanonical secondary structures formed by guanine-rich sequences that are found in different regions of RNA molecules. These structures have been implicated in diverse biological processes, including translation, splicing, and RNA stability. Recent studies have suggested that rG4s play a role in the cellular response to stress. This review summarizes the current knowledge on rG4s under stress, focusing on their formation, regulation, and potential functions in stress response pathways. We discuss the molecular mechanisms that regulate the formation of rG4 under different stress conditions and the impact of these structures on RNA metabolism, gene expression, and cell survival. Finally, we highlight the potential therapeutic implications of targeting rG4s for the treatment of stress-related diseases through modulating cell survival.

Effect of ionic liquids as mobile phase additives on retention behaviors of G-quadruplexes in reversed-phase high performance liquid chromatography

G-quadruplexes (G4s) play an important role in a variety of biological processes and have extensive application prospects. Due to the significance of G4s in physiology and biosensing, studies on G4s have attracted much attention, stimulating the development or improvement of methods for G4 structures and polymorphism analysis. In this work, ionic liquids (ILs) were involved as mobile phase additives in reversed-phase high performance liquid chromatography (RP-HPLC) to analyse G4s with various conformations for the first time. How ILs affected the retention behaviors of G4s was investigated comprehensively. It was found that the addition of ILs markedly enhanced G4 retention, along with obvious amelioration on chromatographic peak shapes and separation. The influence of pH of mobile phase and types of ILs were also included in order to acquire an in-depth understanding. It appeared that the effect of ILs on G4 retention behaviors was the result of a combination of various interactions between G4s with the hydrophobic stationary phase and with the IL-containing mobile phase, where ion pair mechanism and enhanced hydrophobic interaction dominated. The findings of this work revealed that ILs could effectively improve the separation of G4s in RP-HPLC, which was conducive to G4 structural analysis, especially for G4s polymorphism elucidation.

Identification of bona fide RNA G-quadruplex binding proteins

RNAs often accomplish their diverse functions through direct interactions with RNA-binding proteins (RBPs) in a sequence- and/or structure-dependent manner. RNA G-quadruplexes (rG4s) are unique secondary structures formed by guanine-rich RNA sequences which impact RNA function independently and in combination with RBPs. Efforts from several labs have identified dozens of rG4 specific RBPs (rG4BPs), although the research is still in the growing phase. Here we present methods for the systematic identification of rG4BPs using a pull-down approach that takes advantage of the chemical modification of guanine bases. This allows abolishing the rG4 structures while still maintaining the base composition intact, and hence helps in recognizing true rG4BPS (in contrast to G-rich motif binders). In combination with other biochemical assays, such an approach can be efficiently used for the identification and characterization of bona fide rG4BPs.

The VAX2-LINC01189-hnRNPF signaling axis regulates cell invasion and migration in gastric cancer

Transcription factors (TFs) and long noncoding RNAs (lncRNAs) contribute to gastric cancer (GC). However, the roles of TFs and lncRNAs in the invasion and metastasis of GC remain largely unknown. Here, we observed that the transcription factor VAX2 is significantly upregulated in GC cells and tissues and acts as an oncogene. Moreover, high VAX2 expression is associated with the advancement of tumors in GC. In terms of functionality, the enforced expression of VAX2 promotes the proliferation and metastasis of GC cells. Mechanistically, VAX2 specifically interacts with the LINC01189 promoter and represses LINC01189 transcription. Furthermore, LINC01189 exhibits significant downregulation in GC and functions as a suppressor gene. Functionally, it inhibits migratory and invasive abilities in GC cells. In the context of GC metastasis, VAX2 plays a role in modulating it by trans-repressing the expression of LINC01189. Additionally, LINC01189 binds to hnRNPF to enhance hnRNPF degradation through ubiquitination. The cooperation between LINC01189 and hnRNPF regulates GC cell invasion and migration. In addition, both VAX2 and hnRNPF are highly expressed, while LINC01189 is expressed in at low levels in GC tissues compared to normal gastric tissues. Our study suggests that VAX2 expression facilitates, while LINC01189 expression suppresses, metastasis and that the VAX2-LINC01189-hnRNPF axis plays a contributory role in GC development.

Emerging roles for G-quadruplexes in proteostasis

How nucleic acids interact with proteins, and how they affect protein folding, aggregation, and misfolding is a still-evolving area of research. Considerable effort is now focusing on a particular structure of RNA and DNA, G-quadruplexes, and their role in protein homeostasis and disease. In this state-of-the-art review, we track recent reports on how G-quadruplexes influence protein aggregation, proteolysis, phase separation, and protein misfolding diseases, and pose currently unanswered questions in the advance of this scientific field.

The translational repressor Glorund uses interchangeable RNA recognition domains to recognize Drosophila nanos

The Drosophila melanogaster protein Glorund (Glo) represses nanos (nos) translation and uses its quasi-RNA recognition motifs (qRRMs) to recognize both G-tract and structured UA-rich motifs within the nos translational control element (TCE). We showed previously that each of the three qRRMs is multifunctional, capable of binding to G-tract and UA-rich motifs, yet if and how the qRRMs combine to recognize the nos TCE remained unclear. Here we determined solution structures of a nos TCEI_III RNA containing the G-tract and UA-rich motifs. The RNA structure demonstrated that a single qRRM is physically incapable of recognizing both RNA elements simultaneously. In vivo experiments further indicated that any two qRRMs are sufficient to repress nos translation. We probed interactions of Glo qRRMs with TCEI_III RNA using NMR paramagnetic relaxation experiments. Our in vitro and in vivo data support a model whereby tandem Glo qRRMs are indeed multifunctional and interchangeable for recognition of TCE G-tract or UA-rich motifs. This study illustrates how multiple RNA recognition modules within an RNA-binding protein may combine to diversify the RNAs that are recognized and regulated.

The HNRNPF/H RNA binding proteins and disease

The members of the HNRNPF/H family of heterogeneous nuclear RNA proteins-HNRNPF, HNRNPH1, HNRNPH2, HNRNPH3, and GRSF1, are critical regulators of RNA maturation. Documented functions of these proteins include regulating splicing, particularly alternative splicing, 5' capping and 3' polyadenylation of RNAs, and RNA export. The assignment of these proteins to the HNRNPF/H protein family members relates to differences in the amino acid composition of their RNA recognition motifs, which differ from those of other RNA binding proteins (RBPs). HNRNPF/H proteins typically bind RNA sequences enriched with guanine (G) residues, including sequences that, in the presence of a cation, have the potential to form higher-order G-quadruplex structures. The need to further investigate members of the HNRNPF/H family of RBPs has intensified with the recent descriptions of their involvement in several disease states, including the pediatric tumor Ewing sarcoma and the hematological malignancy mantle cell lymphoma; newly described groups of developmental syndromes; and neuronal-related disorders, including addictive behavior. Here, to foster the study of the HNRNPF/H family of RBPs, we discuss features of the genes encoding these proteins, their structures and functions, and emerging contributions to disease. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Processing > Splicing Regulation/Alternative Splicing RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

The RNA-binding proteins hnRNP H and F regulate splicing of a MYC-dependent HRAS exon in prostate cancer cells

The MYC proto-oncogene contributes to the pathogenesis of more than half of human cancers. Malignant transformation by MYC transcriptionally up-regulates the core pre-mRNA splicing machinery and causes misregulation of alternative splicing. However, our understanding of how splicing changes are directed by MYC is limited. We performed a signaling pathway-guided splicing analysis to identify MYC-dependent splicing events. These included an HRAS cassette exon repressed by MYC across multiple tumor types. To molecularly dissect the regulation of this HRAS exon, we used antisense oligonucleotide tiling to identify splicing enhancers and silencers in its flanking introns. RNA-binding motif prediction indicated multiple binding sites for hnRNP H and hnRNP F within these cis-regulatory elements. Using siRNA knockdown and cDNA expression, we found that both hnRNP H and F activate the HRAS cassette exon. Mutagenesis and targeted RNA immunoprecipitation implicate two downstream G-rich elements in this splicing activation. Analyses of ENCODE RNA-seq datasets confirmed hnRNP H regulation of HRAS splicing. Analyses of RNA-seq datasets across multiple cancers showed a negative correlation of HNRNPH gene expression with MYC hallmark enrichment, consistent with the effect of hnRNP H on HRAS splicing. Interestingly, HNRNPF expression showed a positive correlation with MYC hallmarks and thus was not consistent with the observed effects of hnRNP F. Loss of hnRNP H/F altered cell cycle progression and induced apoptosis in the PC3 prostate cancer cell line. Collectively, our results reveal mechanisms for MYC-dependent regulation of splicing and point to possible therapeutic targets in prostate cancers.

G-quadruplexes and associated proteins in aging and Alzheimer's disease

Aging is a prominent risk factor for many neurodegenerative disorders, such as Alzheimer's disease (AD). Alzheimer's disease is characterized by progressive cognitive decline, memory loss, and neuropsychiatric and behavioral symptoms, accounting for most of the reported dementia cases. This disease is now becoming a major challenge and burden on modern society, especially with the aging population. Over the last few decades, a significant understanding of the pathophysiology of AD has been gained by studying amyloid deposition, hyperphosphorylated tau, synaptic dysfunction, oxidative stress, calcium dysregulation, and neuroinflammation. This review focuses on the role of non-canonical secondary structures of DNA/RNA G-quadruplexes (G4s, G4-DNA, and G4-RNA), G4-binding proteins (G4BPs), and helicases, and their roles in aging and AD. Being critically important for cellular function, G4s are involved in the regulation of DNA and RNA processes, such as replication, transcription, translation, RNA localization, and degradation. Recent studies have also highlighted G4-DNA's roles in inducing DNA double-strand breaks that cause genomic instability and G4-RNA's participation in regulating stress granule formation. This review emphasizes the significance of G4s in aging processes and how their homeostatic imbalance may contribute to the pathophysiology of AD.

G-quadruplexes from non-coding RNAs

Non-coding RNAs (ncRNAs) are significant regulators of gene expression in a wide range of biological processes, such as transcription, RNA maturation, or translation. ncRNAs interplay with proteins or other RNAs through not only classical sequence-based mechanisms but also unique higher-order structures such as RNA G-quadruplexes (rG4s). rG4s are predictably formed in guanine-rich sequences and are closely related to various human diseases, such as tumors, neurodegenerative diseases, and infections. This review focuses on the vital role of rG4s in ncRNAs, particularly lncRNAs and miRNAs. We outline the dynamic balance between rG4s and RNA stem-loop/hairpin structures and the interplay between ncRNAs and interactors, thereby modulating gene expression and disease progression. A complete understanding of the biological regulatory role and mechanism of rG4s in ncRNAs affirms the critical importance of folding into the appropriate three-dimensional structure in maintaining or modulating the functions of ncRNAs. It makes them novel therapeutic targets for adjusting potential-G4-containing-ncRNAs-associated diseases.

The RNA-binding protein hnRNP F is required for the germinal center B cell response

The T cell-dependent (TD) antibody response involves the generation of high affinity, immunoglobulin heavy chain class-switched antibodies that are generated through germinal center (GC) response. This process is controlled by coordinated transcriptional and post-transcriptional gene regulatory mechanisms. RNA-binding proteins (RBPs) have emerged as critical players in post-transcriptional gene regulation. Here we demonstrate that B cell-specific deletion of RBP hnRNP F leads to diminished production of class-switched antibodies with high affinities in response to a TD antigen challenge. B cells deficient in hnRNP F are characterized by defective proliferation and c-Myc upregulation upon antigenic stimulation. Mechanistically, hnRNP F directly binds to the G-tracts of Cd40 pre-mRNA to promote the inclusion of Cd40 exon 6 that encodes its transmembrane domain, thus enabling appropriate CD40 cell surface expression. Furthermore, we find that hnRNP A1 and A2B1 can bind to the same region of Cd40 pre-mRNA but suppress exon 6 inclusion, suggesting that these hnRNPs and hnRNP F might antagonize each-other's effects on Cd40 splicing. In summary, our study uncovers an important posttranscriptional mechanism regulating the GC response.

In Vivo Acute Toxicity Studies of Novel Anti-Melanoma Compounds Downregulators of hnRNPH1/H2

Despite the recent advances in melanoma therapy, the need for new targets and novel approaches to therapy is urgent. We previously reported melanoma actives that work via binding and downregulating spliceosomal proteins hnRNPH1 and H2. Given the lack of knowledge about the side effects of using spliceosomal binders in humans, an acute toxicity study was conducted to evaluate these compounds in mice. Male and female mice were treated with compounds 2155-14 and 2155-18 at 50 mg/kg/day via subcutaneous injections, and the clinical signs of distress were monitored for 21 days and compared with control mice. Additionally, the effect of the leads on blood chemistry, blood cell counts, and organs was evaluated. No significant changes were observed in the body weight, blood cell count, blood chemistry, or organs of the mice following the compound treatment. The results show that our compounds, 2155-14 and 2155-18, are not toxic for the study period of three weeks.

Emerging roles for heterogeneous ribonuclear proteins in normal and malignant B cells

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are among the most abundantly expressed RNA binding proteins in the cell and play major roles in all facets of RNA metabolism. hnRNPs are increasingly appreciated as essential for mammalian B cell development by regulating the carefully ordered expression of specific genes. Due to this tight regulation of the hnRNP-RNA network, it is no surprise that a growing number of genes encoding hnRNPs have been causally associated with the onset or progression of many cancers, including B cell neoplasms. Here we discuss our current understanding of hnRNP-driven regulation in normal, perturbed, and malignant B cells, and the most recent and emerging therapeutic innovations aimed at targeting the hnRNP-RNA network in lymphoma.

Structurally diverse G-quadruplexes as the noncanonical nucleic acid drug target for live cell imaging and antibacterial study

The formation of G-quadruplex structures (G4s) in vitro from guanine (G)-rich nucleic acid sequences of DNA and RNA stabilized with monovalent cations, typically K⁺ and Na⁺, under physiological conditions, has been verified experimentally and some of them have high-resolution NMR or X-ray crystal structures; however, the biofunction of these special noncanonical secondary structures of nucleic acids has not been fully understood and their existence in vivo is still controversial at present. It is generally believed that the folding and unfolding of G4s in vivo is a transient process. Accumulating evidence has shown that G4s may play a role in the regulation of certain important cellular functions including telomere maintenance, replication, transcription and translation. Therefore, both DNA and RNA G4s of human cancer hallmark genes are recognized as the potential anticancer drug target for the investigation in cancer biology, chemical biology and drug discovery. The relationship between the sequence, structure and stability of G4s, the interaction of G4s with small molecules, and insights into the rational design of G4-selective binding ligands have been intensively studied over the decade. At present, some G4-ligands have achieved a new milestone and successfully entered the human clinical trials for anticancer therapy. Over the past few decades, numerous efforts have been devoted to anticancer therapy; however, G4s for molecular recognition and live cell imaging and for application as antibacterial agents and antibiofilms against antibiotic resistance have been obviously underexplored. The recent advances in G4-ligands in these areas are thus selected and discussed concentratedly in this article in order to shed light on the emerging role of G4s in chemical biology and therapeutic prospects against bacterial infections. In addition, the recently published molecular scaffolds for designing small ligands selectively targeting G4s in live cell imaging, bacterial biofilm imaging, and antibacterial studies are discussed. Furthermore, a number of underexplored G4-targets from the cytoplasmic membrane-associated DNA, the conserved promoter region of K. pneumoniae genomes, the RNA G4-sites in the transcriptome of E. coli and P. aeruginosa, and the mRNA G4-sites in the sequence for coding the vital bacterial FtsZ protein are highlighted to further explore in G4-drug development against human diseases.

Study on the Interaction of a Peptide Targeting Specific G-Quadruplex Structures Based on Chromatographic Retention Behavior

G-quadruplexes (G4s) are of vital biological significance and G4-specific ligands with conformational selectivity show great application potential in disease treatment and biosensing. RHAU, a RNA helicase associated with AU-rich element, exerts biological functions through the mediation of G4s and has been identified to be a G4 binder. Here, we investigated the interactions between the RHAU peptide and G4s with different secondary structures using size exclusion chromatography (SEC) in association with circular dichroism (CD), ultraviolet-visible (UV-Vis) absorption, and native polyacrylamide gel electrophoresis (Native-PAGE). Spectral results demonstrated that the RHAU peptide did not break the main structure of G4s, making it more reliable for G4 structural analysis. The RHAU peptide was found to display a structural selectivity for a preferential binding to parallel G4s as reflected by the distinct chromatographic retention behaviors. In addition, the RHAU peptide exhibited different interactions with intermolecular parallel G4s and intramolecular parallel G4s, providing a novel recognition approach to G4 structures. The findings of this study enriched the insight into the binding of RHAU to G4s with various conformations. It is noteworthy that SEC technology can be easy and reliable for elucidating G4-peptide interactions, especially for a multiple G4 coexisting system, which supplied an alternative strategy to screen novel specific ligands for G4s.

RNA binding proteins (RBPs) and their role in DNA damage and radiation response in cancer

Traditionally majority of eukaryotic gene expression is influenced by transcriptional and post-transcriptional events. Alterations in the expression of proteins that act post-transcriptionally can affect cellular signaling and homeostasis. RNA binding proteins (RBPs) are a family of proteins that specifically bind to RNAs and are involved in post-transcriptional regulation of gene expression and important cellular processes such as cell differentiation and metabolism. Deregulation of RNA-RBP interactions and any changes in RBP expression or function can lead to various diseases including cancer. In cancer cells, RBPs play an important role in regulating the expression of tumor suppressors and oncoproteins involved in various cell-signaling pathways. Several RBPs such as HuR, AUF1, RBM38, LIN28, RBM24, tristetrapolin family and Musashi play critical roles in various types of cancers and their aberrant expression in cancer cells makes them an attractive therapeutic target for cancer treatment. In this review we provide an overview of i). RBPs involved in cancer progression and their mechanism of action ii). the role of RBPs, including HuR, in breast cancer progression and DNA damage response and iii). explore RBPs with emphasis on HuR as therapeutic target for breast cancer therapy.

QUADRatlas: the RNA G-quadruplex and RG4-binding proteins database

RNA G-quadruplexes (RG4s) are non-canonical, disease-associated post-transcriptional regulators of gene expression whose functions are driven by RNA-binding proteins (RBPs). Being able to explore transcriptome-wide RG4 formation and interaction with RBPs is thus paramount to understanding how they are regulated and exploiting them as potential therapeutic targets. Towards this goal, we present QUADRatlas (https://rg4db.cibio.unitn.it), a database of experimentally-derived and computationally predicted RG4s in the human transcriptome, enriched with biological function and disease associations. As RBPs are key to their function, we mined known interactions of RG4s with such proteins, complemented with an extensive RBP binding sites dataset. Users can thus intersect RG4s with their potential regulators and effectors, enabling the formulation of novel hypotheses on RG4 regulation, function and pathogenicity. To support this capability, we provide analysis tools for predicting whether an RBP can bind RG4s, RG4 enrichment in a gene set, and de novo RG4 prediction. Genome-browser and table views allow exploring, filtering, and downloading the data quickly for individual genes and in batch. QUADRatlas is a significant step forward in our ability to understand the biology of RG4s, offering unmatched data content and enabling the integrated analysis of RG4s and their interactions with RBPs.

The nascent polypeptide-associated complex (NAC) controls translation initiation in cis by recruiting nucleolin to the encoding mRNA

Protein aggregates and abnormal proteins are toxic and associated with neurodegenerative diseases. There are several mechanisms to help cells get rid of aggregates but little is known on how cells prevent aggregate-prone proteins from being synthesised. The EBNA1 of the Epstein-Barr virus (EBV) evades the immune system by suppressing its own mRNA translation initiation in order to minimize the production of antigenic peptides for the major histocompatibility (MHC) class I pathway. Here we show that the emerging peptide of the disordered glycine–alanine repeat (GAr) within EBNA1 dislodges the nascent polypeptide-associated complex (NAC) from the ribosome. This results in the recruitment of nucleolin to the GAr-encoding mRNA and suppression of mRNA translation initiation in cis. Suppressing NAC alpha (NACA) expression prevents nucleolin from binding to the GAr mRNA and overcomes GAr-mediated translation inhibition. Taken together, these observations suggest that EBNA1 exploits a nascent protein quality control pathway to regulate its own rate of synthesis that is based on sensing the nascent GAr peptide by NAC followed by the recruitment of nucleolin to the GAr-encoding RNA sequence.

Kinetics measurements of G-quadruplex binding and unfolding by helicases

G-quadruplex structures (G4s) form readily in DNA and RNA and play diverse roles in gene expression and other processes, and their inappropriate formation and stabilization are linked to human diseases. G4s are inherently long-lived, such that their timely unfolding depends on a suite of DNA and RNA helicase proteins. Biochemical analysis of G4 binding and unfolding by individual helicase proteins is important for establishing their levels of activity, affinity, and specificity for G4s, including individual G4s of varying sequence and structure. Here we describe a set of simple, accessible methods in which electrophoretic mobility shift assays (EMSA) are used to measure the kinetics of G4 binding, dissociation, and unfolding by helicase proteins. We focus on practical considerations and the pitfalls that are most likely to arise when these methods are used to study the activities of helicases on G4s.

Recent advances in RNA structurome

RNA structures are essential to support RNA functions and regulation in various biological processes. Recently, a range of novel technologies have been developed to decode genome-wide RNA structures and novel modes of functionality across a wide range of species. In this review, we summarize key strategies for probing the RNA structurome and discuss the pros and cons of representative technologies. In particular, these new technologies have been applied to dissect the structural landscape of the SARS-CoV-2 RNA genome. We also summarize the functionalities of RNA structures discovered in different regulatory layers-including RNA processing, transport, localization, and mRNA translation-across viruses, bacteria, animals, and plants. We review many versatile RNA structural elements in the context of different physiological and pathological processes (e.g., cell differentiation, stress response, and viral replication). Finally, we discuss future prospects for RNA structural studies to map the RNA structurome at higher resolution and at the single-molecule and single-cell level, and to decipher novel modes of RNA structures and functions for innovative applications.

Insights into the structural dynamics and helicase-catalyzed unfolding of plant RNA G-quadruplexes

RNA G-quadruplexes (rG4s) are noncanonical RNA secondary structures formed by guanine (G)-rich sequences. These complexes play important regulatory roles in both animals and plants through their structural dynamics and are closely related to human diseases and plant growth, development, and adaption. Thus, studying the structural dynamics of rG4s is fundamentally important; however, their folding pathways and their unfolding by specialized helicases are not well understood. In addition, no plant rG4-specialized helicases have been identified. Here, using single-molecule FRET, we experimentally elucidated for the first time the folding pathway and intermediates, including a G-hairpin and G-triplex. In addition, using proteomics screening and microscale thermophoresis, we identified and validated five rG4-specialized helicases in Arabidopsis thaliana. Furthermore, DExH1, the ortholog of the famous human rG4 helicase RHAU/DHX36, stood out for its robust rG4 unwinding ability. Taken together, these results shed light on the structural dynamics of plant rG4s.

Interaction of HnRNP F with the guanine-rich segments in viral antigenomic RNA enhances porcine reproductive and respiratory syndrome virus-2 replication

Background

Heterogeneous nuclear ribonucleoprotein (HnRNP) F is a member of HnRNP family proteins that participate in splicing of cellular newly synthesized mRNAs by specifically recognizing tandem guanine-tracts (G-tracts) RNA sequences. Whether HnRNP F could recognize viral-derived tandem G-tracts and affect virus replication remain poorly defined.

Methods

The effect of HnRNP F on porcine reproductive and respiratory syndrome virus (PRRSV) propagation was evaluated by real-time PCR, western blotting, and plaque-forming unit assay. The association between HnRNP F and PRRSV guanine-rich segments (GRS) were analyzed by RNA pulldown and RNA immunoprecipitation. The expression pattern of HnRNP F was investigated by western blotting and nuclear and cytoplasmic fractionation.

Results

Knockdown of endogenous HnRNP F effectively blocks the synthesis of viral RNA and nucleocapsid (N) protein. Conversely, overexpression of porcine HnRNP F has the opposite effect. Moreover, RNA pulldown and RNA immunoprecipitation assays reveal that the qRMM1 and qRRM2 domains of HnRNP F recognize the GRS in PRRSV antigenomic RNA. Finally, HnRNP F is redistributed into the cytoplasm and forms a complex with guanine-quadruplex (G4) helicase DHX36 during PRRSV infection.

Conclusions

These findings elucidate the potential functions of HnRNP F in regulating the proliferation of PRRSV and contribute to a better molecular understanding of host-PRRSV interactions.

Secondary structures in RNA synthesis, splicing and translation

Even though the functional role of mRNA molecules is primarily decided by the nucleotide sequence, several properties are determined by secondary structure conformations. Examples of secondary structures include long range interactions, hairpins, R-loops and G-quadruplexes and they are formed through interactions of non-adjacent nucleotides. Here, we discuss advances in our understanding of how secondary structures can impact RNA synthesis, splicing, translation and mRNA half-life. During RNA synthesis, secondary structures determine RNA polymerase II (RNAPII) speed, thereby influencing splicing. Splicing is also determined by RNA binding proteins and their binding rates are modulated by secondary structures. For the initiation of translation, secondary structures can control the choice of translation start site. Here, we highlight the mechanisms by which secondary structures modulate these processes, discuss advances in technologies to detect and study them systematically, and consider the roles of RNA secondary structures in disease.

G-Quadruplex-Binding Proteins: Promising Targets for Drug Design

G-quadruplexes (G4s) are non-canonical secondary nucleic acid structures. Sequences with the potential to form G4s are abundant in regulatory regions of the genome including telomeres, promoters and 5′ non-coding regions, indicating they fulfill important genome regulatory functions. Generally, G4s perform various biological functions by interacting with proteins. In recent years, an increasing number of G-quadruplex-binding proteins have been identified with biochemical experiments. G4-binding proteins are involved in vital cellular processes such as telomere maintenance, DNA replication, gene transcription, mRNA processing. Therefore, G4-binding proteins are also associated with various human diseases. An intensive study of G4-protein interactions provides an attractive approach for potential therapeutics and these proteins can be considered as drug targets for novel medical treatment. In this review, we present biological functions and structural properties of G4-binding proteins, and discuss how to exploit G4-protein interactions to develop new therapeutic targets.

RNA G-quadruplex in TMPRSS2 reduces SARS-CoV-2 infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection continues to have devastating consequences worldwide. Recently, great efforts have been made to identify SARS-CoV-2 host factors, but the regulatory mechanisms of these host molecules, as well as the virus per se, remain elusive. Here we report a role of RNA G-quadruplex (RG4) in SARS-CoV-2 infection. Combining bioinformatics, biochemical and biophysical assays, we demonstrate the presence of RG4s in both SARS-CoV-2 genome and host factors. The biological and pathological importance of these RG4s is then exemplified by a canonical 3-quartet RG4 within Tmprss2, which can inhibit Tmprss2 translation and prevent SARS-CoV-2 entry. Intriguingly, G-quadruplex (G4)-specific stabilizers attenuate SARS-CoV-2 infection in pseudovirus cell systems and mouse models. Consistently, the protein level of TMPRSS2 is increased in lungs of COVID-19 patients. Our findings reveal a previously unknown mechanism underlying SARS-CoV-2 infection and suggest RG4 as a potential target for COVID-19 prevention and treatment.

Understanding the mechanisms of SARS-CoV-2 infection is important to control the pandemic. Here the authors show the biological and pathological role of RNA G-quadruplex structure in both SARS-CoV-2 genome and host factors, particularly TMPRSS2.

Translational Regulation by hnRNP H/F Is Essential for the Proliferation and Survival of Glioblastoma

Deregulation of mRNA translation is a widespread characteristic of glioblastoma (GBM), aggressive malignant brain tumors that are resistant to conventional therapies. RNA-binding proteins (RBPs) play a critical role in translational regulation, yet the mechanisms and impact of these regulations on cancer development, progression and response to therapy remain to be fully understood. Here, we showed that hnRNP H/F RBPs are potent regulators of translation through several mechanisms that converge to modulate the expression and/or the activity of translation initiation factors. Among these, hnRNP H/F regulate the phosphorylation of eIF4E and its translational targets by controlling RNA splicing of the A-Raf kinase mRNA, which in turn modulates the MEK-ERK/MAPK signaling pathway. The underlying mechanism involves RNA G-quadruplex (RG4s), RNA structures whose modulation phenocopies hnRNP H/F translation regulation in GBM cells. Our results highlighted that hnRNP H/F are essential for key functional pathways regulating proliferation and survival of GBM, highlighting its targeting as a promising strategy for improving therapeutic outcomes.

Identification and targeting of G-quadruplex structures in MALAT1 long non-coding RNA

RNA G-quadruplexes (rG4s) have functional roles in many cellular processes in diverse organisms. While a number of rG4 examples have been reported in coding messenger RNAs (mRNA), so far only limited works have studied rG4s in non-coding RNAs (ncRNAs), especially in long non-coding RNAs (lncRNAs) that are of emerging interest and significance in biology. Herein, we report that MALAT1 lncRNA contains conserved rG4 motifs, forming thermostable rG4 structures with parallel topology. We also show that rG4s in MALAT1 lncRNA can interact with NONO protein with high specificity and affinity in vitro and in nuclear cell lysate, and we provide cellular data to support that NONO protein recognizes MALAT1 lncRNA via rG4 motifs. Notably, we demonstrate that rG4s in MALAT1 lncRNA can be targeted by the rG4-specific small molecule, peptide, and L-aptamer, leading to the dissociation of MALAT1 rG4-NONO protein interaction. Altogether, this study uncovers new and important rG4s in MALAT1 lncRNAs, reveals their specific interactions with NONO protein, offers multiple strategies for targeting MALAT1 and its RNA-protein complex via its rG4 structure and illustrates the prevalence and significance of rG4s in ncRNAs.

The Cellular Functions and Molecular Mechanisms of G-Quadruplex Unwinding Helicases in Humans

G-quadruplexes (G4s) are stable non-canonical secondary structures formed by G-rich DNA or RNA sequences. They play various regulatory roles in many biological processes. It is commonly agreed that G4 unwinding helicases play key roles in G4 metabolism and function, and these processes are closely related to physiological and pathological processes. In recent years, more and more functional and mechanistic details of G4 helicases have been discovered; therefore, it is necessary to carefully sort out the current research efforts. Here, we provide a systematic summary of G4 unwinding helicases from the perspective of functions and molecular mechanisms. First, we provide a general introduction about helicases and G4s. Next, we comprehensively summarize G4 unfolding helicases in humans and their proposed cellular functions. Then, we review their study methods and molecular mechanisms. Finally, we share our perspective on further prospects. We believe this review will provide opportunities for researchers to reach the frontiers in the functions and molecular mechanisms of human G4 unwinding helicases.

The different activities of RNA G-quadruplex structures are controlled by flanking sequences

The role of G-quadruplex (G4) RNA structures is multifaceted and controversial. Here, we have used as a model the EBV-encoded EBNA1 and the Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded LANA1 mRNAs. We have compared the G4s in these two messages in terms of nucleolin binding, nuclear mRNA retention, and mRNA translation inhibition and their effects on immune evasion. The G4s in the EBNA1 message are clustered in one repeat sequence and the G4 ligand PhenDH2 prevents all G4-associated activities. The RNA G4s in the LANA1 message take part in similar multiple mRNA functions but are spread throughout the message. The different G4 activities depend on flanking coding and non-coding sequences and, interestingly, can be separated individually. Together, the results illustrate the multifunctional, dynamic and context-dependent nature of G4 RNAs and highlight the possibility to develop ligands targeting specific RNA G4 functions. The data also suggest a common multifunctional repertoire of viral G4 RNA activities for immune evasion.

Single-cell Visualization of Monogenic RNA G-quadruplex and Occupied G-quadruplex Ratio through Module Assembled Multifunctional Probes Assay (MAMPA)

G-quadruplexes (G4s), non-canonical nucleic acid secondary structure, regulate many biological functions and are considered as potential molecular targets for therapeutics of cancers. However, due to the lack of analytical methods, the regulating mechanism of monogenic G4s is still unclear. Here, we developed a Module Assembled Multifunctional Probes Assay (MAMPA) for visualizing endogenous G4s in individual genes in single cells. Two modular probes separately recognize G4 structures and the adjacent RNA sequences, and the module assembly enables imaging of G4s in an individual RNA with high specificity. Through imaging G4s in several individual genes, we found that G4s were steadily occupied by G4 Binding Proteins (G4BPs) in various mRNAs in every cell line and defined "Occupied G4 Ratio". In all, we demonstrated MAMPA was suitable for most experiment situations and found that Occupied G4 Ratios had the potential to become a new parameter for the study of G4s in living cells.

Analysis of mRNA Translation by Polysome Profiling

mRNA translation is a key step in gene expression that allows the cell to qualitatively and quantitatively modulate the cell's proteome according to intra- or extracellular signals. Polysome profiling is the most comprehensive technique to study both the translation state of mRNAs and the protein machinery associated with the mRNAs being translated. Here we describe the procedure commonly used in our laboratory to gain insights into the molecular mechanisms underlying translation regulation under pathophysiological conditions.

ERα is an RNA-binding protein sustaining tumor cell survival and drug resistance

Estrogen receptor α (ERα) is a hormone receptor and key driver for over 70% of breast cancers that has been studied for decades as a transcription factor. Unexpectedly, we discover that ERα is a potent non-canonical RNA-binding protein. We show that ERα RNA binding function is uncoupled from its activity to bind DNA and critical for breast cancer progression. Employing genome-wide cross-linking immunoprecipitation (CLIP) sequencing and a functional CRISPRi screen, we find that ERα-associated mRNAs sustain cancer cell fitness and elicit cellular responses to stress. Mechanistically, ERα controls different steps of RNA metabolism. In particular, we demonstrate that ERα RNA binding mediates alternative splicing of XBP1 and translation of the eIF4G2 and MCL1 mRNAs, which facilitates survival upon stress conditions and sustains tamoxifen resistance of cancer cells. ERα is therefore a multifaceted RNA-binding protein, and this activity transforms our knowledge of post-transcriptional regulation underlying cancer development and drug response.

Action and function of helicases on RNA G-quadruplexes

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Methodological progresses and piling evidence prove the rG4 biology in vivo.

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rG4s step in virtually every aspect of RNA biology.

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Helicases unwinding of rG4s is a fine regulatory layer to the downstream processes and general cell homeostasis.

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The current knowledge is however limited to a few cell lines.

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The regulation of helicases themselves is delineating as a important question.

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Non-helicase rG4-processing proteins likely play a role.

The nucleic acid structure called G-quadruplex (G4) is currently discussed to function in nucleic acid-based mechanisms that influence several cellular processes. They can modulate the cellular machinery either positively or negatively, both at the DNA and RNA level. The majority of what we know about G4 biology comes from DNA G4 (dG4) research. RNA G4s (rG4), on the other hand, are gaining interest as researchers become more aware of their role in several aspects of cellular homeostasis. In either case, the correct regulation of G4 structures within cells is essential and demands specialized proteins able to resolve them. Small changes in the formation and unfolding of G4 structures can have severe consequences for the cells that could even stimulate genome instability, apoptosis or proliferation. Helicases are the most relevant negative G4 regulators, which prevent and unfold G4 formation within cells during different pathways. Yet, and despite their importance only a handful of rG4 unwinding helicases have been identified and characterized thus far. This review addresses the current knowledge on rG4s-processing helicases with a focus on methodological approaches. An example of a non-helicase rG4s-unwinding protein is also briefly described.

RNA G-quadruplexes (rG4s): genomics and biological functions

G-quadruplexes (G4s) are non-classical DNA or RNA secondary structures that have been first observed decades ago. Over the years, these four-stranded structural motifs have been demonstrated to have significant regulatory roles in diverse biological processes, but challenges remain in detecting them globally and reliably. Compared to DNA G4s (dG4s), the study of RNA G4s (rG4s) has received less attention until recently. In this review, we will summarize the innovative high-throughput methods recently developed to detect rG4s on a transcriptome-wide scale, highlight the many novel and important functions of rG4 being discovered in vivo across the tree of life, and discuss the key biological questions to be addressed in the near future.

Constrained G4 structures unveil topology specificity of known and new G4 binding proteins

G-quadruplexes (G4) are non-canonical secondary structures consisting in stacked tetrads of hydrogen-bonded guanines bases. An essential feature of G4 is their intrinsic polymorphic nature, which is characterized by the equilibrium between several conformations (also called topologies) and the presence of different types of loops with variable lengths. In cells, G4 functions rely on protein or enzymatic factors that recognize and promote or resolve these structures. In order to characterize new G4-dependent mechanisms, extensive researches aimed at identifying new G4 binding proteins. Using G-rich single-stranded oligonucleotides that adopt non-controlled G4 conformations, a large number of G4-binding proteins have been identified in vitro, but their specificity towards G4 topology remained unknown. Constrained G4 structures are biomolecular objects based on the use of a rigid cyclic peptide scaffold as a template for directing the intramolecular assembly of the anchored oligonucleotides into a single and stabilized G4 topology. Here, using various constrained RNA or DNA G4 as baits in human cell extracts, we establish the topology preference of several well-known G4-interacting factors. Moreover, we identify new G4-interacting proteins such as the NELF complex involved in the RNA-Pol II pausing mechanism, and we show that it impacts the clastogenic effect of the G4-ligand pyridostatin.

The m6A-epitranscriptome in brain plasticity, learning and memory

Activity-dependent gene expression and protein translation underlie the ability of neurons to dynamically adjust their synaptic strength in response to sensory experience and during learning. The emerging field of epitranscriptomics (RNA modifications) has rapidly shifted our views on the mechanisms that regulate gene expression. Among hundreds of biochemical modifications on RNA, N⁶-methyladenosine (m⁶A) is the most abundant reversible mRNA modification in the brain. Its dynamic nature and ability to regulate all aspects of mRNA processing have positioned m⁶A as an important and versatile regulator of nervous system functions, including neuronal plasticity, learning and memory. In this review, we summarise recent experimental evidence that supports the role of m⁶A signalling in learning and memory, as well as providing an overview of the underlying molecular mechanisms in neurons. We also discuss the consequences of perturbed m⁶A signalling and/or its regulatory networks which are increasingly being linked to various cognitive disorders in humans.

Long, Noncoding RNA Dysregulation in Glioblastoma

Simple Summary

Developing effective therapies for glioblastoma (GBM), the most common primary brain cancer, remains challenging due to the heterogeneity within tumors and therapeutic resistance that drives recurrence. Noncoding RNAs are transcribed from a large proportion of the genome and remain largely unexplored in their contribution to the evolution of GBM tumors. Here, we will review the general mechanisms of long, noncoding RNAs and the current knowledge of how these impact heterogeneity and therapeutic resistance in GBM. A better understanding of the molecular drivers required for these aggressive tumors is necessary to improve the management and outcomes of this challenging disease.

Abstract

Transcription occurs across more than 70% of the human genome and more than half of currently annotated genes produce functional noncoding RNAs. Of these transcripts, the majority—long, noncoding RNAs (lncRNAs)—are greater than 200 nucleotides in length and are necessary for various roles in the cell. It is increasingly appreciated that these lncRNAs are relevant in both health and disease states, with the brain expressing the largest number of lncRNAs compared to other organs. Glioblastoma (GBM) is an aggressive, fatal brain tumor that demonstrates remarkable intratumoral heterogeneity, which has made the development of effective therapies challenging. The cooperation between genetic and epigenetic alterations drives rapid adaptation that allows therapeutic evasion and recurrence. Given the large repertoire of lncRNAs in normal brain tissue and the well-described roles of lncRNAs in molecular and cellular processes, these transcripts are important to consider in the context of GBM heterogeneity and treatment resistance. Herein, we review the general mechanisms and biological roles of lncRNAs, with a focus on GBM, as well as RNA-based therapeutics currently in development.

Recent advances in the development of small molecules targeting RNA G-quadruplexes for drug discovery

Extensive evidence indicates that RNA G-quadruplexes have associated with some important cellular events. Investigation of RNA G-quadruplexes is thus vital to revealing their biofunctions. Several small molecules have been developed to target RNA G-quadruplexes to date. Some of the small molecules showed significantly light-up fluorescence signals upon binding to RNA G-quadruplexes, while some of them regulated the biofunctions of RNA G-quadruplexes. In this mini-review, the small molecules divided into four kinds are expounded which focused mainly on their structural features and biological activities. Moreover, we raised the current challenges and promising prospects. This mini-review might contribute to exploiting more sophisticated small molecules targeting RNA G-quadruplexes with high specificity based on the reported chemical structural features.

Transcriptional Riboswitches Integrate Timescales for Bacterial Gene Expression Control

Transcriptional riboswitches involve RNA aptamers that are typically found in the 5' untranslated regions (UTRs) of bacterial mRNAs and form alternative secondary structures upon binding to cognate ligands. Alteration of the riboswitch's secondary structure results in perturbations of an adjacent expression platform that controls transcription elongation and termination, thus turning downstream gene expression "on" or "off." Riboswitch ligands are typically small metabolites, divalent cations, anions, signaling molecules, or other RNAs, and can be part of larger signaling cascades. The interconnectedness of ligand binding, RNA folding, RNA transcription, and gene expression empowers riboswitches to integrate cellular processes and environmental conditions across multiple timescales. For a successful response to an environmental cue that may determine a bacterium's chance of survival, a coordinated coupling of timescales from microseconds to minutes must be achieved. This review focuses on recent advances in our understanding of how riboswitches affect such critical gene expression control across time.

Properties and biological impact of RNA G-quadruplexes: from order to turmoil and back

Guanine-quadruplexes (G4s) are non-canonical four-stranded structures that can be formed in guanine (G) rich nucleic acid sequences. A great number of G-rich sequences capable of forming G4 structures have been described based on in vitro analysis, and evidence supporting their formation in live cells continues to accumulate. While formation of DNA G4s (dG4s) within chromatin in vivo has been supported by different chemical, imaging and genomic approaches, formation of RNA G4s (rG4s) in vivo remains a matter of discussion. Recent data support the dynamic nature of G4 formation in the transcriptome. Such dynamic fluctuation of rG4 folding-unfolding underpins the biological significance of these structures in the regulation of RNA metabolism. Moreover, rG4-mediated functions may ultimately be connected to mechanisms underlying disease pathologies and, potentially, provide novel options for therapeutics. In this framework, we will review the landscape of rG4s within the transcriptome, focus on their potential impact on biological processes, and consider an emerging connection of these functions in human health and disease.

G-Quadruplexes in RNA Biology: Recent Advances and Future Directions

RNA G-quadruplexes (RG4s) are four-stranded structures known to control gene expression mechanisms, from transcription to protein synthesis, and DNA-related processes. Their potential impact on RNA biology allows these structures to shape cellular processes relevant to disease development, making their targeting for therapeutic purposes an attractive option. We review here the current knowledge on RG4s, focusing on the latest breakthroughs supporting the notion of transient structures that fluctuate dynamically in cellulo, their interplay with RNA modifications, their role in cell compartmentalization, and their deregulation impacting the host immune response. We emphasize RG4-binding proteins as determinants of their transient conformation and effectors of their biological functions.

The Small Nuclear Ribonucleoprotein Polypeptide A (SNRPA) binds to the G-quadruplex of the BAG-1 5'UTR

The protein "BCL-2-associated athanogene-1" (BAG-1), which exists in multiple isoforms, promotes cancer cell survival and is overexpressed in many different cancers. As a result, BAG-1-targeted therapy appears to be a promising strategy with which to treat cancer. It has previously been shown that the 5'UTR of the BAG-1 mRNA contains a guanine rich region that folds into a G-quadruplex structure which can modulate both its cap-dependent and its cap-independent translation. Accumulating data regarding G-quadruplex binding proteins suggest that these proteins can play a central role in gene expression. Consequently, the identification of the proteins that could potentially bind to the G-quadruplex of the BAG-1 mRNA was undertaken. Label-free RNA pulldown assays were performed using protein extracts from colorectal cancer cells and this leads to the detection of RNA G4 binding proteins by LC-MS/MS. The use of G-quadruplex containing RNA, as well as of a mutated version, ensured that the proteins identified were specific for the RNA G-quadruplex structure and not just general RNA binding proteins. Following confirmation of the interaction, the Small Nuclear Ribonucleoprotein Polypeptide A (SNRPA) was shown to bind directly to the BAG-1 mRNA through the G-quadruplex, and knock down experiments in colorectal cancer cells suggested that it can modulate the expression level of BAG-1.