Specific RNP capture with antisense LNA/DNA mixmers

Revealing the Arabidopsis AtGRP7 mRNA binding proteome by specific enhanced RNA interactome capture

BACKGROUND

The interaction of proteins with RNA in the cell is crucial to orchestrate all steps of RNA processing. RNA interactome capture (RIC) techniques have been implemented to catalogue RNA- binding proteins in the cell. In RIC, RNA-protein complexes are stabilized by UV crosslinking in vivo. Polyadenylated RNAs and associated proteins are pulled down from cell lysates using oligo(dT) beads and the RNA-binding proteome is identified by quantitative mass spectrometry. However, insights into the RNA-binding proteome of a single RNA that would yield mechanistic information on how RNA expression patterns are orchestrated, are scarce.

RESULTS

Here, we explored RIC in Arabidopsis to identify proteins interacting with a single mRNA, using the circadian clock-regulated Arabidopsis thaliana GLYCINE-RICH RNA-BINDING PROTEIN 7 (AtGRP7) transcript, one of the most abundant transcripts in Arabidopsis, as a showcase. Seedlings were treated with UV light to covalently crosslink RNA and proteins. The AtGRP7 transcript was captured from cell lysates with antisense oligonucleotides directed against the 5'untranslated region (UTR). The efficiency of RNA capture was greatly improved by using locked nucleic acid (LNA)/DNA oligonucleotides, as done in the enhanced RIC protocol. Furthermore, performing a tandem capture with two rounds of pulldown with the 5'UTR oligonucleotide increased the yield. In total, we identified 356 proteins enriched relative to a pulldown from atgrp7 mutant plants. These were benchmarked against proteins pulled down from nuclear lysates by AtGRP7 in vitro transcripts immobilized on beads. Among the proteins validated by in vitro interaction we found the family of Acetylation Lowers Binding Affinity (ALBA) proteins. Interaction of ALBA4 with the AtGRP7 RNA was independently validated via individual-nucleotide resolution crosslinking and immunoprecipitation (iCLIP). The expression of the AtGRP7 transcript in an alba loss-of-function mutant was slightly changed compared to wild-type, demonstrating the functional relevance of the interaction.

CONCLUSION

We adapted specific RNA interactome capture with LNA/DNA oligonucleotides for use in plants using AtGRP7 as a showcase. We anticipate that with further optimization and up scaling the protocol should be applicable for less abundant transcripts.

Sex-lethal regulates back-splicing and generation of the sex-differentially expressed circular RNAs

Conversely to canonical splicing, back-splicing connects the upstream 3' splice site (SS) with a downstream 5'SS and generates exonic circular RNAs (circRNAs) that are widely identified and have regulatory functions in eukaryotic gene expression. However, sex-specific back-splicing in Drosophila has not been investigated and its regulation remains unclear. Here, we performed multiple RNA analyses of a variety sex-specific Drosophila samples and identified over ten thousand circular RNAs, in which hundreds are sex-differentially and -specifically back-spliced. Intriguingly, we found that expression of SXL, an RNA-binding protein encoded by Sex-lethal (Sxl), the master Drosophila sex-determination gene that is only spliced into functional proteins in females, promoted back-splicing of many female-differential circRNAs in the male S2 cells, whereas expression of a SXL mutant (SXLRRM) did not promote those events. Using a monoclonal antibody, we further obtained the transcriptome-wide RNA-binding sites of SXL through PAR-CLIP. After splicing assay of mini-genes with mutations in the SXL-binding sites, we revealed that SXL-binding on flanking exons and introns of pre-mRNAs facilitates back-splicing, whereas SXL-binding on the circRNA exons inhibits back-splicing. This study provides strong evidence that SXL has a regulatory role in back-splicing to generate sex-specific and -differential circRNAs, as well as in the initiation of sex-determination cascade through canonical forward-splicing.

Toward a systems view on RNA-binding proteins and associated RNAs in plants: Guilt by association

RNA-binding proteins (RBPs) have a broad impact on most biochemical, physiological, and developmental processes in a plant's life. RBPs engage in an on-off relationship with their RNA partners, accompanying virtually every stage in RNA processing and function. While the function of a plethora of RBPs in plant development and stress responses has been described, we are lacking a systems-level understanding of components in RNA-based regulation. Novel techniques have substantially enlarged the compendium of proteins with experimental evidence for binding to RNAs in the cell, the RNA-binding proteome. Furthermore, ribonomics methods have been adapted for use in plants to profile the in vivo binding repertoire of RBPs genome-wide. Here, we discuss how recent technological achievements have provided novel insights into the mode of action of plant RBPs at a genome-wide scale. Furthermore, we touch upon two emerging topics, the connection of RBPs to phase separation in the cell and to extracellular RNAs. Finally, we define open questions to be addressed to move toward an integrated understanding of RBP function.

A widely applicable and cost-effective method for specific RNA-protein complex isolation

Although methodological advances have been made over the past years, a widely applicable, easily scalable and cost-effective procedure that can be routinely used to isolate specific ribonucleoprotein complexes (RNPs) remains elusive. We describe the "Silica-based Acidic Phase Separation (SAPS)-capture" workflow. This versatile method combines previously described techniques in a cost-effective, optimal and widely applicable protocol. The specific RNP isolation procedure is performed on a pre-purified RNP sample instead of cell lysate. This combination of protocols results in an increased RNP/bead ratio and by consequence a reduced experimental cost. To validate the method, the 18S rRNP of S. cerevisiae was captured and to illustrate its applicability we isolated the complete repertoire of RNPs in A. thaliana. The procedure we describe can provide the community with a powerful tool to advance the study of the ribonome of a specific RNA molecule in any organism or tissue type.

The mutual interaction of glycolytic enzymes and RNA in post-transcriptional regulation

About three decades ago, researchers suggested that metabolic enzymes participate in cellular processes that are unrelated to their catalytic activity, and the term "moonlighting functions" was proposed. Recently developed advanced technologies in the field of RNA interactome capture now unveil the unexpected RNA binding activity of many metabolic enzymes, as exemplified here for the enzymes of glycolysis. Although for most of these proteins a precise binding mechanism, binding conditions, and physiological relevance of the binding events still await in-depth clarification, several well explored examples demonstrate that metabolic enzymes hold crucial functions in post-transcriptional regulation of protein synthesis. This widely conserved RNA-binding function of glycolytic enzymes plays major roles in controlling cell activities. The best explored examples are glyceraldehyde 3-phosphate dehydrogenase, enolase, phosphoglycerate kinase, and pyruvate kinase. This review summarizes current knowledge about the RNA-binding activity of the ten core enzymes of glycolysis in plant, yeast, and animal cells, its regulation and physiological relevance. Apparently, a tight bidirectional regulation connects core metabolism and RNA biology, forcing us to rethink long established functional singularities.

Identification of Pri-miRNA Stem-Loop Interacting Proteins in Plants Using a Modified Version of the Csy4 CRISPR Endonuclease

Regulation at the RNA level by RNA-binding proteins (RBPs) and microRNAs (miRNAs) is key to coordinating eukaryotic gene expression. In plants, the importance of miRNAs is highlighted by severe developmental defects in mutants impaired in miRNA biogenesis. MiRNAs are processed from long primary-microRNAs (pri-miRNAs) with internal stem-loop structures by endonucleolytic cleavage. The highly structured stem-loops constitute the basis for the extensive regulation of miRNA biogenesis through interaction with RBPs. However, trans-acting regulators of the biogenesis of specific miRNAs are largely unknown in plants. Therefore, we exploit an RNA-centric approach based on modified versions of the conditional CRISPR nuclease Csy4* to pull down interactors of the Arabidopsis pri-miR398b stem-loop (pri-miR398b-SL) in vitro. We designed three epitope-tagged versions of the inactive Csy4* for the immobilization of the protein together with the pri-miR398b-SL bait on high affinity matrices. After incubation with nucleoplasmic extracts from Arabidopsis and extensive washing, pri-miR398b-SL, along with its specifically bound proteins, were released by re-activating the cleavage activity of the Csy4* upon the addition of imidazole. Co-purified proteins were identified via quantitative mass spectrometry and data sets were compared. In total, we identified more than 400 different proteins, of which 180 are co-purified in at least two out of three independent Csy4*-based RNA pulldowns. Among those, the glycine-rich RNA-binding protein AtRZ-1a was identified in all pulldowns. To analyze the role of AtRZ-1a in miRNA biogenesis, we determined the miR398 expression level in the atrz-1a mutant. Indeed, the absence of AtRZ-1a caused a decrease in the steady-state level of mature miR398 with a concomitant reduction in pri-miR398b levels. Overall, we show that our modified Csy4*-based RNA pulldown strategy is suitable to identify new trans-acting regulators of miRNA biogenesis and provides new insights into the post-transcriptional regulation of miRNA processing by plant RBPs.

Uncovering viral RNA-host cell interactions on a proteome-wide scale

RNA viruses interact with a wide range of cellular RNA-binding proteins (RBPs) during their life cycle. The prevalence of these host-virus interactions has been highlighted by new methods that elucidate the composition of viral ribonucleoproteins (vRNPs). Applied to 11 viruses so far, these approaches have revealed hundreds of cellular RBPs that interact with viral (v)RNA in infected cells. However, consistency across methods is limited, raising questions about methodological considerations when designing and interpreting these studies. Here, we discuss these caveats and, through comparing available vRNA interactomes, describe RBPs that are consistently identified as vRNP components and outline their potential roles in infection. In summary, these novel approaches have uncovered a new universe of host-virus interactions holding great therapeutic potential.

RNA Binding Proteins as Pioneer Determinants of Infection: Protective, Proviral, or Both?

As the first intracellular host factors that directly interact with the genomes of RNA viruses, RNA binding proteins (RBPs) have a profound impact on the outcome of an infection. Recent discoveries brought about by new methodologies have led to an unprecedented ability to peer into the earliest events between viral RNA and the RBPs that act upon them. These discoveries have sparked a re-evaluation of current paradigms surrounding RBPs and post-transcriptional gene regulation. Here, we highlight questions that have bloomed from the implementation of these novel approaches. Canonical RBPs can impact the fates of both cellular and viral RNA during infection, sometimes in conflicting ways. Noncanonical RBPs, some of which were first characterized via interactions with viral RNA, may encompass physiological roles beyond viral pathogenesis. We discuss how these RBPs might discriminate between an RNA of either cellular or viral origin and thus exert either pro- or antiviral effects—which is a particular challenge as viruses contain mechanisms to mimic molecular features of cellular RNA.

Short DNA/RNA heteroduplex oligonucleotide interacting proteins are key regulators of target gene silencing

Antisense oligonucleotide (ASO)-based therapy is one of the next-generation therapy, especially targeting neurological disorders. Many cases of ASO-dependent gene expression suppression have been reported. Recently, we developed a tocopherol conjugated DNA/RNA heteroduplex oligonucleotide (Toc-HDO) as a new type of drug. Toc-HDO is more potent, stable, and efficiently taken up by the target tissues compared to the parental ASO. However, the detailed mechanisms of Toc-HDO, including its binding proteins, are unknown. Here, we developed native gel shift assays with fluorescence-labeled nucleic acids samples extracted from mice livers. These assays revealed two Toc-HDO binding proteins, annexin A5 (ANXA5) and carbonic anhydrase 8 (CA8). Later, we identified two more proteins, apurinic/apyrimidinic endodeoxyribonuclease 1 (APEX1) and flap structure-specific endonuclease 1 (FEN1) by data mining. shRNA knockdown studies demonstrated that all four proteins regulated Toc-HDO activity in Hepa1-6, mouse hepatocellular cells. In vitro binding assays and fluorescence polarization assays with purified recombinant proteins characterized the identified proteins and pull-down assays with cell lysates demonstrated the protein binding to the Toc-HDO and ASO in a biological environment. Taken together, our findings provide a brand new molecular biological insight as well as future directions for HDO-based disease therapy.

Methods to study circRNA-protein interactions

Circular RNAs (circRNAs) have been studied extensively in the last few years, uncovering functional roles in a diverse range of cell types and organisms. As shown for a few cases, these functions may be mediated by trans-acting factors, in particular RNA-binding proteins (RBPs). However, the specific interaction partners for most circRNAs remain unknown. This is mainly due to technical difficulties in their identification and in differentiating between interactors of circRNAs and their linear counterparts. Here we review the currently used methodology to systematically study circRNA-protein complexes (circRNPs), focusing either on a specific RNA or protein, both on the gene-specific or global level, and discuss advantages and challenges of the available approaches.

RNA Proximity Labeling: A New Detection Tool for RNA-Protein Interactions

Multiple cellular functions are controlled by the interaction of RNAs and proteins. Together with the RNAs they control, RNA interacting proteins form RNA protein complexes, which are considered to serve as the true regulatory units for post-transcriptional gene expression. To understand how RNAs are modified, transported, and regulated therefore requires specific knowledge of their interaction partners. To this end, multiple techniques have been developed to characterize the interaction between RNAs and proteins. In this review, we briefly summarize the common methods to study RNA-protein interaction including crosslinking and immunoprecipitation (CLIP), and aptamer- or antisense oligonucleotide-based RNA affinity purification. Following this, we focus on in vivo proximity labeling to study RNA-protein interactions. In proximity labeling, a labeling enzyme like ascorbate peroxidase or biotin ligase is targeted to specific RNAs, RNA-binding proteins, or even cellular compartments and uses biotin to label the proteins and RNAs in its vicinity. The tagged molecules are then enriched and analyzed by mass spectrometry or RNA-Seq. We highlight the latest studies that exemplify the strength of this approach for the characterization of RNA protein complexes and distribution of RNAs in vivo.

RNA-Centric Approaches to Profile the RNA-Protein Interaction Landscape on Selected RNAs

RNA–protein interactions frame post-transcriptional regulatory networks and modulate transcription and epigenetics. While the technological advances in RNA sequencing have significantly expanded the repertoire of RNAs, recently developed biochemical approaches combined with sensitive mass-spectrometry have revealed hundreds of previously unrecognized and potentially novel RNA-binding proteins. Nevertheless, a major challenge remains to understand how the thousands of RNA molecules and their interacting proteins assemble and control the fate of each individual RNA in a cell. Here, I review recent methodological advances to approach this problem through systematic identification of proteins that interact with particular RNAs in living cells. Thereby, a specific focus is given to in vivo approaches that involve crosslinking of RNA–protein interactions through ultraviolet irradiation or treatment of cells with chemicals, followed by capture of the RNA under study with antisense-oligonucleotides and identification of bound proteins with mass-spectrometry. Several recent studies defining interactomes of long non-coding RNAs, viral RNAs, as well as mRNAs are highlighted, and short reference is given to recent in-cell protein labeling techniques. These recent experimental improvements could open the door for broader applications and to study the remodeling of RNA–protein complexes upon different environmental cues and in disease.

Sequencing of the Arabidopsis NOR2 reveals its distinct organization and tissue-specific rRNA ribosomal variants

Despite vast differences between organisms, some characteristics of their genomes are conserved, such as the nucleolus organizing region (NOR). The NOR is constituted of multiple, highly repetitive rDNA genes, encoding the catalytic ribosomal core RNAs which are transcribed from 45S rDNA units. Their precise sequence information and organization remain uncharacterized. Here, using a combination of long- and short-read sequencing technologies we assemble contigs of the Arabidopsis NOR2 rDNA domain. We identify several expressed rRNA gene variants which are integrated into translating ribosomes in a tissue-specific manner. These findings support the concept of tissue specific ribosome subpopulations that differ in their rRNA composition and provide insights into the higher order organization of NOR2.

The nucleolus organizing region (NOR) consists of multiple, highly repetitive rDNA genes. Here Sims et al. use both long- and short-read sequencing to determine the organization and sequence of Arabidopsis NOR2 rDNA and show that different rRNA gene variants are integrated into translating ribosomes in a tissue-specific manner.

Global analysis of RNA-binding protein dynamics by comparative and enhanced RNA interactome capture

Interactions between RNA-binding proteins (RBPs) and RNAs are critical to cell biology. However, methods to comprehensively and quantitatively assess these interactions within cells were lacking. RNA interactome capture (RIC) uses in vivo UV crosslinking, oligo(dT) capture, and proteomics to identify RNA-binding proteomes. Recent advances have empowered RIC to quantify RBP responses to biological cues such as metabolic imbalance or virus infection. Enhanced RIC exploits the stronger binding of locked nucleic acid (LNA)-containing oligo(dT) probes to poly(A) tails to maximize RNA capture selectivity and efficiency, profoundly improving signal-to-noise ratios. The subsequent analytical use of SILAC and TMT proteomic approaches, together with high-sensitivity sample preparation and tailored statistical data analysis, substantially improves RIC's quantitative accuracy and reproducibility. This optimized approach is an extension of the original RIC protocol. It takes 3 d plus 2 weeks for proteomics and data analysis and will enable the study of RBP dynamics under different physiological and pathological conditions.

RNA-protein interaction mapping via MS2- or Cas13-based APEX targeting

RNA-protein interactions underlie a wide range of cellular processes. Improved methods are needed to systematically map RNA-protein interactions in living cells in an unbiased manner. We used two approaches to target the engineered peroxidase APEX2 to specific cellular RNAs for RNA-centered proximity biotinylation of protein interaction partners. Both an MS2-MCP system and an engineered CRISPR-Cas13 system were used to deliver APEX2 to the human telomerase RNA hTR with high specificity. One-minute proximity biotinylation captured candidate binding partners for hTR, including more than a dozen proteins not previously linked to hTR. We validated the interaction between hTR and the N ⁶-methyladenosine (m⁶A) demethylase ALKBH5 and showed that ALKBH5 is able to erase the m⁶A modification on endogenous hTR. ALKBH5 also modulates telomerase complex assembly and activity. MS2- and Cas13-targeted APEX2 may facilitate the discovery of novel RNA-protein interactions in living cells.

Enzymatic RNA Biotinylation for Affinity Purification and Identification of RNA-Protein Interactions

Throughout their cellular lifetime, RNA transcripts are bound to proteins, playing crucial roles in RNA metabolism, trafficking, and function. Despite the importance of these interactions, identifying the proteins that interact with an RNA of interest in mammalian cells represents a major challenge in RNA biology. Leveraging the ability to site-specifically and covalently label an RNA of interest using E. coli tRNA guanine transglycosylase and an unnatural nucleobase substrate, we establish the identification of RNA-protein interactions and the selective enrichment of cellular RNA in mammalian systems. We demonstrate the utility of this approach through the identification of known binding partners of 7SK snRNA via mass spectrometry. Through a minimal 4-nucleotide mutation of the long noncoding RNA HOTAIR, enzymatic biotinylation enables identification of putative HOTAIR binding partners in MCF7 breast cancer cells that suggest new potential pathways for oncogenic function. Furthermore, using RNA sequencing and qPCR, we establish that an engineered enzyme variant achieves high levels of labeling selectivity against the human transcriptome allowing for 145-fold enrichment of cellular RNA directly from mammalian cell lysates. The flexibility and breadth of this approach suggests that this system could be routinely applied to the functional characterization of RNA, greatly expanding the toolbox available for studying mammalian RNA biology.

Single and Combined Methods to Specifically or Bulk-Purify RNA-Protein Complexes

The ribonome interconnects the proteome and the transcriptome. Specific biology is situated at this interface, which can be studied in bulk using omics approaches or specifically by targeting an individual protein or RNA species. In this review, we focus on both RNA- and ribonucleoprotein-(RNP) centric methods. These methods can be used to study the dynamics of the ribonome in response to a stimulus or to identify the proteins that interact with a specific RNA species. The purpose of this review is to provide and discuss an overview of strategies to cross-link RNA to proteins and the currently available RNA- and RNP-centric approaches to study RNPs. We elaborate on some major challenges common to most methods, involving RNP yield, purity and experimental cost. We identify the origin of these difficulties and propose to combine existing approaches to overcome these challenges. The solutions provided build on the recently developed organic phase separation protocols, such as Cross-Linked RNA eXtraction (XRNAX), orthogonal organic phase separation (OOPS) and Phenol-Toluol extraction (PTex).

Comparative Poly(A)+ RNA Interactome Capture of RNA Surveillance Mutants

RNA exosome complexes degrade many different RNA substrates. Substrate selection and targeting to the exosome complex rely on cofactors, which bind to the substrate RNA, recruit the exosome complex, and help to remodel the associated ribonucleoprotein particle to facilitate RNA degradation. These cofactors are RNA-binding proteins, but their interaction with RNA may be very transient because the RNAs they are bound to are rapidly turned over by the exosome complex. Hence, the cofactors involved in the degradation of many exosome substrates are unknown. Here, we describe comparative poly(A)+ RNA interactome capture as a method to screen for novel RNA-binding proteins involved in exosome-dependent RNA decay.For this, we compare the poly(A)+ RNA interactome of wild-type cells to that of RNA surveillance mutants, where the decay of exosome substrates is compromised and occupancy of exosome cofactors on RNA is strongly increased. More specifically, protein-RNA complexes in wild-type and mutant cells are UV-cross-linked in vivo after labeling with the photoactivatable nucleoside analogue 4-thiouracil. Following cell lysis, protein-RNA complexes are selected on oligo d(T) beads, subjected to stringent washes, and eluted in a low salt buffer. After RNase digestion of cross-linked RNA, RNA-binding proteins that are enriched in the mutant samples are identified by quantitative mass spectrometry. Here, we quantitatively compare the RNA-protein interactomes of wild-type and rrp6Δ cells to selectively determine cofactors of the nuclear RNA exosome complex in fission yeast. With minor modifications, the comparative interactome approach can easily be adapted to study a range of different RNA-dependent processes in various cellular systems.

From parts lists to functional significance-RNA-protein interactions in gene regulation

Hundreds of canonical RNA binding proteins facilitate diverse and essential RNA processing steps in cells forming a central regulatory point in gene expression. However, recent discoveries including the identification of a large number of noncanonical proteins bound to RNA have changed our view on RNA-protein interactions merely as necessary steps in RNA biogenesis. As the list of proteins interacting with RNA has expanded, so has the scope of regulation through RNA-protein interactions. In addition to facilitating RNA metabolism, RNA binding proteins help to form subcellular structures and membraneless organelles, and provide means to recruit components of macromolecular complexes to their sites of action. Moreover, RNA-protein interactions are not static in cells but the ribonucleoprotein (RNP) complexes are highly dynamic in response to cellular cues. The identification of novel proteins in complex with RNA and ways cells use these interactions to control cellular functions continues to broaden the scope of RNA regulation in cells and the current challenge is to move from cataloguing the components of RNPs into assigning them functions. This will not only facilitate our understanding of cellular homeostasis but may bring in key insights into human disease conditions where RNP components play a central role. This review brings together the classical view of regulation accomplished through RNA-protein interactions with the novel insights gained from the identification of RNA binding interactomes. We discuss the challenges in combining molecular mechanism with cellular functions on the journey towards a comprehensive understanding of the regulatory functions of RNA-protein interactions in cells. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications aRNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition.

Approaches for Understanding the Mechanisms of Long Noncoding RNA Regulation of Gene Expression

Mammalian genomes encode tens of thousands of long noncoding RNAs (lncRNAs) that have been implicated in a diverse array of biological processes and human diseases. In recent years, the development of new tools for studying lncRNAs has enabled important progress in defining the mechanisms by which Xist and other lncRNAs function. This collective work provides a framework for how to define the mechanisms by which lncRNAs act. This includes defining lncRNA function, identifying and characterizing lncRNA-protein interactions, and lncRNA localization in the cell. In this review, we discuss various experimental approaches for deciphering lncRNA mechanisms and discuss issues and limitations in interpreting these results. We explore what these data can reveal about lncRNA function and mechanism as well as emerging insights into lncRNA biology that have been derived from these studies.

Principles and Practices of Hybridization Capture Experiments to Study Long Noncoding RNAs That Act on Chromatin

The diverse roles of cellular RNAs can be studied by purifying RNAs of interest together with the biomolecules they bind. Biotinylated antisense oligonucleotides that hybridize specifically to the RNA of interest provide a general approach to develop affinity reagents for these experiments. Such oligonucleotides can be used to enrich endogenous RNAs from cross-linked chromatin extracts to study the genomic binding sites of RNAs. These hybridization capture protocols are evolving modular experiments that are compatible with a range of cross-linkers and conditions. This review discusses the principles of these hybridization capture experiments as well as considerations and controls necessary to interpret the resulting data without being misled by artifactual signals.

Drosophila Sister-of-Sex-lethal reinforces a male-specific gene expression pattern by controlling Sex-lethal alternative splicing

In Drosophila, female development is governed by a single RNA-binding protein, Sex-lethal (Sxl), that controls the expression of key factors involved in dosage compensation, germline homeostasis and the establishment of female morphology and behaviour. Sxl expression in female flies is maintained by an auto-regulatory, positive feedback loop with Sxl controlling splicing of its own mRNA. Until now, it remained unclear how males prevent accidental triggering of the Sxl expression cascade and protect themselves against runaway protein production. Here, we identify the protein Sister-of-Sex-lethal (Ssx) as an inhibitor of Sxl auto-regulatory splicing. Sxl and Ssx have a comparable RNA-binding specificity and compete for binding to RNA regulatory elements present in the Sxl transcript. In cultured Drosophila cells, Sxl-induced changes to alternative splicing can be reverted by the expression of Ssx. Moreover, in adult male flies ablation of the ssx gene results in a low level of productive Sxl mRNA splicing and Sxl protein production in isolated, clonal cell populations. In sum, this demonstrates that Ssx safeguards male animals against Sxl protein production to reinforce a stable, male-specific gene expression pattern.

Identification of proteins and miRNAs that specifically bind an mRNA in vivo

Understanding regulation of an mRNA requires knowledge of its regulators. However, methods for reliable de-novo identification of proteins binding to a particular RNA are scarce and were thus far only successfully applied to abundant noncoding RNAs in cell culture. Here, we present vIPR, an RNA-protein crosslink, RNA pulldown, and shotgun proteomics approach to identify proteins bound to selected mRNAs in C. elegans. Applying vIPR to the germline-specific transcript gld-1 led to enrichment of known and novel interactors. By comparing enrichment upon gld-1 and lin-41 pulldown, we demonstrate that vIPR recovers both common and specific RNA-binding proteins, and we validate DAZ-1 as a specific gld-1 regulator. Finally, combining vIPR with small RNA sequencing, we recover known and biologically important transcript-specific miRNA interactions, and we identify miR-84 as a specific interactor of the gld-1 transcript. We envision that vIPR will provide a platform for investigating RNA in vivo regulation in diverse biological systems.

Ribonucleoprotein Capture by in Vivo Expression of a Designer Pentatricopeptide Repeat Protein in Arabidopsis

Pentatricopeptide repeat (PPR) proteins bind RNA via a mechanism that facilitates the customization of sequence specificity. However, natural PPR proteins have irregular features that limit the degree to which their specificity can be predicted and customized. We demonstrate here that artificial PPR proteins built from consensus PPR motifs selectively bind the intended RNA in vivo, and we use this property to develop a new tool for ribonucleoprotein characterization. We show by RNA coimmunoprecipitation sequencing (RIP-seq) that artificial PPR proteins designed to bind the Arabidopsis (Arabidopsis thaliana) chloroplast psbA mRNA bind with high specificity to psbA mRNA in vivo. Analysis of coimmunoprecipitating proteins by mass spectrometry showed the psbA translational activator HCF173 and two RNA binding proteins of unknown function (CP33C and SRRP1) to be highly enriched. RIP-seq revealed that these proteins are bound primarily to psbA RNA in vivo, and precise mapping of the HCF173 and CP33C binding sites placed them in different locations on psbA mRNA. These results demonstrate that artificial PPR proteins can be tailored to bind specific endogenous RNAs in vivo, add to the toolkit for characterizing native ribonucleoproteins, and open the door to other applications that rely on the ability to target a protein to a specified RNA sequence.

Beyond CLIP: advances and opportunities to measure RBP-RNA and RNA-RNA interactions

RNA is an essential player in almost all biological processes, and has an ever-growing number of roles in regulating cellular growth and organization. RNA functions extend far beyond just coding for proteins and RNA has been shown to function in signaling events, chromatin organization and transcriptional regulation. Dissecting how the complex network of RNA-binding proteins (RBPs) and regulatory RNAs interact with their substrates within the cell is a real, but exciting, challenge for the RNA community. Investigating these biological questions has fueled the development of new quantitative technologies to measure how RNA and RBPs interact both locally and on a global scale. In this review, we provide an assessment of available approaches to enable researchers to select the protocol most applicable for their experimental question.

β-Actin mRNA interactome mapping by proximity biotinylation

The molecular function and fate of mRNAs are controlled by RNA-binding proteins (RBPs). Identification of the interacting proteome of a specific mRNA in vivo remains very challenging, however. Based on the widely used technique of RNA tagging with MS2 aptamers for RNA visualization, we developed a RNA proximity biotinylation (RNA-BioID) technique by tethering biotin ligase (BirA*) via MS2 coat protein at the 3' UTR of endogenous MS2-tagged β-actin mRNA in mouse embryonic fibroblasts. We demonstrate the dynamics of the β-actin mRNA interactome by characterizing its changes on serum-induced localization of the mRNA. Apart from the previously known interactors, we identified more than 60 additional β-actin-associated RBPs by RNA-BioID. Among these, the KH domain-containing protein FUBP3/MARTA2 has been shown to be required for β-actin mRNA localization. We found that FUBP3 binds to the 3' UTR of β-actin mRNA and is essential for β-actin mRNA localization, but does not interact with the characterized β-actin zipcode element. RNA-BioID provides a tool for identifying new mRNA interactors and studying the dynamic view of the interacting proteome of endogenous mRNAs in space and time.

Probing the RNA-Binding Proteome from Yeast to Man: Major Advances and Challenges

RNA-binding proteins are important for core cellular processes such as mRNA transcription, splicing, transport, translation, and degradation. Recently, hundreds of novel RNA-binders have been identified in vivo in various organisms and cell types. We discuss the RNA interactome capture technique which enabled this boost in identifying new RNA-binding proteins in eukaryotes. A focus of this chapter, however, is the presentation of different challenges and problems that need to be addressed to be able to understand the conserved mRNA-bound proteomes from yeast to man.

Beyond Transcription: Fine-Tuning of Circadian Timekeeping by Post-Transcriptional Regulation

The circadian clock is an important endogenous timekeeper, helping plants to prepare for the periodic changes of light and darkness in their environment. The clockwork of this molecular timer is made up of clock proteins that regulate transcription of their own genes with a 24 h rhythm. Furthermore, the rhythmically expressed clock proteins regulate time-of-day dependent transcription of downstream genes, causing messenger RNA (mRNA) oscillations of a large part of the transcriptome. On top of the transcriptional regulation by the clock, circadian rhythms in mRNAs rely in large parts on post-transcriptional regulation, including alternative pre-mRNA splicing, mRNA degradation, and translational control. Here, we present recent insights into the contribution of post-transcriptional regulation to core clock function and to regulation of circadian gene expression in Arabidopsis thaliana.

Identification of the RNA polymerase I-RNA interactome

Ribosome biogenesis is a complex process orchestrated by a host of ribosome assembly factors. Although it is known that many of the proteins involved in this process have RNA binding activity, the full repertoire of proteins that interact with the precursor ribosomal RNA is currently unknown. To gain a greater understanding of the extent to which RNA-protein interactions have the potential to control ribosome biogenesis, we used RNA affinity isolation coupled with proteomics to measure the changes in RNA-protein interactions that occur when rRNA transcription is blocked. Our analysis identified 211 out of 457 nuclear RNA binding proteins with a >3-fold decrease in RNA-protein interaction after inhibition of RNA polymerase I (RNAPI). We have designated these 211 RNA binding proteins as the RNAPI RNA interactome. As expected, the RNAPI RNA interactome is highly enriched for nucleolar proteins and proteins associated with ribosome biogenesis. Selected proteins from the interactome were shown to be nucleolar in location and to have RNA binding activity that was dependent on RNAPI activity. Furthermore, our data show that two proteins, which are required for rRNA maturation, AATF and NGDN, and which form part of the RNA interactome, both lack canonical RNA binding domains and yet are novel pre-rRNA binding proteins.

An Oligonucleotide-based Tandem RNA Isolation Procedure to Recover Eukaryotic mRNA-Protein Complexes

RNA-binding proteins (RBPs) play key roles in the post-transcriptional control of gene expression. Therefore, biochemical characterization of mRNA-protein complexes is essential to understanding mRNA regulation inferred by interacting proteins or non-coding RNAs. Herein, we describe a tandem RNA isolation procedure (TRIP) that enables the purification of endogenously formed mRNA-protein complexes from cellular extracts. The two-step protocol involves the isolation of polyadenylated mRNAs with antisense oligo(dT) beads and subsequent capture of an mRNA of interest with 3'-biotinylated 2'-O-methylated antisense RNA oligonucleotides, which can then be isolated with streptavidin beads. TRIP was used to recover in vivo crosslinked mRNA-ribonucleoprotein (mRNP) complexes from yeast, nematodes and human cells for further RNA and protein analysis. Thus, TRIP is a versatile approach that can be adapted to all types of polyadenylated RNAs across organisms to study the dynamic re-arrangement of mRNPs imposed by intracellular or environmental cues.

The Lsm1-7/Pat1 complex binds to stress-activated mRNAs and modulates the response to hyperosmotic shock

RNA-binding proteins (RBPs) establish the cellular fate of a transcript, but an understanding of these processes has been limited by a lack of identified specific interactions between RNA and protein molecules. Using MS2 RNA tagging, we have purified proteins associated with individual mRNA species induced by osmotic stress, STL1 and GPD1. We found members of the Lsm1-7/Pat1 RBP complex to preferentially bind these mRNAs, relative to the non-stress induced mRNAs, HYP2 and ASH1. To assess the functional importance, we mutated components of the Lsm1-7/Pat1 RBP complex and analyzed the impact on expression of osmostress gene products. We observed a defect in global translation inhibition under osmotic stress in pat1 and lsm1 mutants, which correlated with an abnormally high association of both non-stress and stress-induced mRNAs to translationally active polysomes. Additionally, for stress-induced proteins normally triggered only by moderate or high osmostress, in the mutants the protein levels rose high already at weak hyperosmosis. Analysis of ribosome passage on mRNAs through co-translational decay from the 5’ end (5P-Seq) showed increased ribosome accumulation in lsm1 and pat1 mutants upstream of the start codon. This effect was particularly strong for mRNAs induced under osmostress. Thus, our results indicate that, in addition to its role in degradation, the Lsm1-7/Pat1 complex acts as a selective translational repressor, having stronger effect over the translation initiation of heavily expressed mRNAs. Binding of the Lsm1-7/Pat1p complex to osmostress-induced mRNAs mitigates their translation, suppressing it in conditions of weak or no stress, and avoiding a hyperresponse when triggered.

When confronted with external physical or chemical stress, cells respond by increasing the mRNA output of a small number of genes required for stress survival, while shutting down the majority of other genes. Moreover, each mRNA is regulated under stress to either enhance or diminish its translation into proteins. The overall purpose is for the cell to optimize gene expression for survival and recovery during rapidly changing conditions. Much of this regulation is mediated by RNA-binding proteins. We have isolated proteins binding to specific mRNAs induced by stress, to investigate how they affect the stress response. We found members of one protein complex to be bound to stress-induced mRNAs. When mutants lacking these proteins were exposed to stress, ribosomes were more engaged with translating mRNAs than in the wild-type. In the mutants, it was also possible to trigger expression of stress proteins with only minimal stress levels. Tracing the passage of ribosomes over mRNAs, we saw that ribosomes accumulated around the start codon in the mutants. These findings indicate that the protein complex is required to moderate the stress response and prevent it from overreacting, which would be harmful for the cell.

RNA-protein interaction detection in living cells

RNA-protein interactions play numerous roles in cellular function and disease. Here we describe RNA-protein interaction detection (RaPID), which uses proximity-dependent protein labeling, based on the BirA* biotin ligase, to rapidly identify the proteins that bind RNA sequences of interest in living cells. RaPID displays utility in multiple applications, including in evaluating protein binding to mutant RNA motifs in human genetic disorders, in uncovering potential post-transcriptional networks in breast cancer, and in discovering essential host proteins that interact with Zika virus RNA. To improve the BirA*-labeling component of RaPID, moreover, a new mutant BirA* was engineered from Bacillus subtilis, termed BASU, that enables >1,000-fold faster kinetics and >30-fold increased signal-to-noise ratio over the prior standard Escherichia coli BirA*, thereby enabling direct study of RNA-protein interactions in living cells on a timescale as short as 1 min.

Drosophila Sister-of-Sex-lethal is a repressor of translation

The RNA-binding protein Sex-lethal (Sxl) is an important post-transcriptional regulator of sex determination and dosage compensation in female Drosophila To prevent the assembly of the MSL dosage compensation complex in female flies, Sxl acts as a repressor of male-specific lethal-2 (msl-2) mRNA translation. It uses two distinct and mutually reinforcing blocks to translation that operate on the 5' and 3' untranslated regions (UTRs) of msl-2 mRNA, respectively. While 5' UTR-mediated translational control involves an upstream open reading frame, 3' UTR-mediated regulation strictly requires the co-repressor protein Upstream of N-ras (Unr), which is recruited to the transcript by Sxl. We have identified the protein Sister-of-Sex-lethal (Ssx) as a novel repressor of translation with Sxl-like activity. Both proteins have a comparable RNA-binding specificity and can associate with uracil-rich RNA regulatory elements present in msl-2 mRNA. Moreover, both repress translation when bound to the 5' UTR of msl-2 However, Ssx is inactive in 3' UTR-mediated regulation, as it cannot engage the co-repressor protein Unr. The difference in activity maps to the first RNA-recognition motif (RRM) of Ssx. Conversion of three amino acids within this domain into their Sxl counterpart results in a gain of function and repression via the 3' UTR, allowing detailed insights into the evolutionary origin of the two proteins and into the molecular requirements of an important translation regulatory pathway.

Discovery of new RNA classes and global RNA-binding proteins

The identification of new RNA functions and the functional annotation of transcripts in genomes represent exciting yet challenging endeavours of modern biology. Crucial insights into the biological roles of RNA molecules can be gained from the identification of the proteins with which they form specific complexes. Modern interactome techniques permit to profile RNA-protein interactions in a genome-wide manner and identify new RNA classes associated with globally acting RNA-binding proteins. Applied to a variety of organisms, these methods are already revolutionising our understanding of RNA-mediated biological processes. Here, we focus on one such approach-Gradient sequencing or Grad-seq-which has recently guided the discovery of protein ProQ and its associated small RNAs as a new domain of post-transcriptional control in bacteria.

Identification of RNA-binding domains of RNA-binding proteins in cultured cells on a system-wide scale with RBDmap

This protocol is an extension to: Nat. Protoc. 8, 491-500 (2013); doi:10.1038/nprot.2013.020; published online 14 February 2013RBDmap is a method for identifying, in a proteome-wide manner, the regions of RNA-binding proteins (RBPs) engaged in native interactions with RNA. In brief, cells are irradiated with UV light to induce protein-RNA cross-links. Following stringent denaturing washes, the resulting covalently linked protein-RNA complexes are purified with oligo(dT) magnetic beads. After elution, RBPs are subjected to partial proteolysis, in which the protein regions still bound to the RNA and those released to the supernatant are separated by a second oligo(dT) selection. After sample preparation and mass-spectrometric analysis, peptide intensity ratios between the RNA-bound and released fractions are used to determine the RNA-binding regions. As a Protocol Extension, this article describes an adaptation of an existing Protocol and offers additional applications. The earlier protocol (for the RNA interactome capture method) describes how to identify the active RBPs in cultured cells, whereas this Protocol Extension also enables the identification of the RNA-binding domains of RBPs. The experimental workflow takes 1 week plus 2 additional weeks for proteomics and data analysis. Notably, RBDmap presents numerous advantages over classic methods for determining RNA-binding domains: it produces proteome-wide, high-resolution maps of the protein regions contacting the RNA in a physiological context and can be adapted to different biological systems and conditions. Because RBDmap relies on the isolation of polyadenylated RNA via oligo(dT), it will not provide RNA-binding information on proteins interacting exclusively with nonpolyadenylated transcripts. Applied to HeLa cells, RBDmap uncovered 1,174 RNA-binding sites in 529 proteins, many of which were previously unknown.