Ribotrap : targeted purification of RNA-specific RNPs from cell lysates through immunoaffinity precipitation to identify regulatory proteins and RNAs

Purification of In Vivo or In Vitro-Assembled RNA-Protein Complexes by RNA Centric Methods

Throughout their entire life cycle, RNAs are associated with RNA-binding proteins (RBPs), forming ribonucleoprotein (RNP) complexes with highly dynamic compositions and very diverse functions in RNA metabolism, including splicing, translational regulation, ribosome assembly. Many RNPs remain poorly characterized due to the challenges inherent in their purification and subsequent biochemical characterization. Therefore, developing methods to isolate specific RNA-protein complexes is an important initial step toward understanding their function. Many elegant methodologies have been developed to isolate RNPs. This chapter describes different approaches and methods devised for RNA-specific purification of a target RNP. We focused on general methods for selecting RNPs that target a given RNA under conditions favourable for the copurification of associated factors including RNAs and protein components of the RNP.

Capturing Endogenous Long Noncoding RNAs and Their Binding Proteins Using Chromatin Isolation by RNA Purification

Long noncoding RNAs (lncRNAs) exert their functions through binding to other RNA, genomic DNA, or proteins. The identification of proteins that associate with the lncRNA of interest sheds light on the molecular basis of its biological functions. This can be achieved by tagging the lncRNA with chemically modified ribonucleotides, or by using in vitro transcribed lncRNA to retrieve proteins from cell lysates. Alternatively, endogenous lncRNAs can be pulled down from cells or tissues with biotinylated antisense DNA oligonucleotides, which may overcome the potential pitfalls of using tagged lncRNAs, such as artifacts caused by tagging or non-physiological interactions. Here we describe the detailed protocol for chromatin isolation by RNA purification (ChIRP) from mammalian cell lines and mouse tissues, which captures endogenous lncRNAs and enables subsequent identification of their physiologically relevant binding partners.

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).

RNA: interactions drive functionalities

RNA is produced from the majority of human genomic sequences, although only a relatively small portion of these transcripts has known functions. Diverse RNA species interact with RNA, DNA, proteins, lipids, and metabolites to form intricate molecular networks. In this review, we attempt to delineate diverse RNA functions by interaction types between RNA and other macromolecules. Through such interactions RNAs participate in essentially every major molecular function and process, including information flow and storage, environment sensing, signal transduction, and gene regulation at transcriptional and posttranscriptional levels. Through such interactions, RNAs promote or inhibit diverse biological processes, and act as catalyzer or quencher to modulate the pace of these progresses. Alterations and personal variations of these interactions are mechanistically coupled with disease etiology and phenotypical variations for clinical use.

Transcript specific mRNP capture from Drosophila egg-chambers for proteomic analysis

mRNA binding proteins (RBPs) play a major role in post-transcriptional control of gene expression. To understand the complex regulatory processes regulating a specific mRNA during its life-time, a comprehensive view of the bound RBPs is essential. Here, we describe a method for transcript-specific isolation of endogenous ribonucleoprotein complexes (RNPs) from Drosophila egg-chambers. The method, which is based on in-solution hybridization of short biotinylated antisense DNA oligonucleotide probes to multiple segments of a transcript of interest allows unbiased identification of associated proteins by quantitative proteomics.

Current approaches for RNA-labelling to identify RNA-binding proteins

RNA is involved in all domains of life, playing critical roles in a host of gene expression processes, host-defense mechanisms, cell proliferation, and diseases. A critical component in many of these events is the ability for RNA to interact with proteins. Over the past few decades, our understanding of such RNA-protein interactions and their importance has driven the search and development of new techniques for the identification of RNA-binding proteins. In determining which proteins bind to the RNA of interest, it is often useful to use the approach where the RNA molecule is the "bait" and allow it to capture proteins from a lysate or other relevant solution. Here, we review a collection of methods for modifying RNA to capture RNA-binding proteins. These include small-molecule modification, the addition of aptamers, DNA-anchoring, and nucleotide substitution. With each, we provide examples of their application, as well as highlight their advantages and potential challenges.

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.

A versatile tandem RNA isolation procedure to capture in vivo formed mRNA-protein complexes

We describe a tandem RNA isolation procedure (TRIP) that enables purification of in vivo formed messenger ribonucleoprotein (mRNP) complexes. The procedure relies on the purification of polyadenylated mRNAs with oligo(dT) beads from cellular extracts, followed by the capture of specific mRNAs with 3'-biotinylated 2'-O-methylated antisense RNA oligonucleotides, which are recovered with streptavidin beads. TRIP was applied to isolate in vivo crosslinked mRNP complexes from yeast, nematodes and human cells for subsequent analysis of RNAs and bound proteins. The method provides a basis for adaptation to other types of polyadenylated RNAs, enabling the comprehensive identification of bound proteins/RNAs, and the investigation of dynamic rearrangement of mRNPs imposed by cellular or environmental cues.

Identification of protein-interacting nucleotides in a RNA sequence using composition profile of tri-nucleotides

The RNA-protein interactions play a diverse role in the cells, thus identification of RNA-protein interface is essential for the biologist to understand their function. In the past, several methods have been developed for predicting RNA interacting residues in proteins, but limited efforts have been made for the identification of protein-interacting nucleotides in RNAs. In order to discriminate protein-interacting and non-interacting nucleotides, we used various classifiers (NaiveBayes, NaiveBayesMultinomial, BayesNet, ComplementNaiveBayes, MultilayerPerceptron, J48, SMO, RandomForest, SMO and SVM(light)) for prediction model development using various features and achieved highest 83.92% sensitivity, 84.82 specificity, 84.62% accuracy and 0.62 Matthew's correlation coefficient by SVM(light) based models. We observed that certain tri-nucleotides like ACA, ACC, AGA, CAC, CCA, GAG, UGA, and UUU preferred in protein-interaction. All the models have been developed using a non-redundant dataset and are evaluated using five-fold cross validation technique. A web-server called RNApin has been developed for the scientific community (http://crdd.osdd.net/raghava/rnapin/).

Ribotrap analysis of proteins associated with FHL3 3'untranslated region in glioma cells

OBJECTIVE

To screen the proteins associated with four-and-a-half LIM domains 3 (FHL3) 3' untranslated region (3'UTR) in glioma cells.

METHODS

Western blot was adopted to detect the regulatory effect of poly(C)-binding protein 2 (PCBP2) on FHL3. Biotin pull-down and sliver staining were employed to screen and verify the candidate binding proteins of FHL3 3'UTR. Then liquid chromatography-tandem mass spectrometry (LC-MS/MS) and molecule annotation system were used to identify and analyze the candidate binding proteins. Immuno- precipitation was conducted to study the interaction between PCBP2 and polypyrimidine tract-binding protein 1 (PTBP1), a binding protein identified by LC-MS/MS.

RESULTS

PCBP2 could bind to FHL3 mRNA 3'UTR-A and inhibited the expression of FHL3 in T98G glioms cells. 22 candidate binding proteins were identified. Among them, there were 11 RNA binding proteins, including PCBP2. PTBP1 associated with FHL3 mRNA 3'UTR and interacted with PCBP2 protein.

CONCLUSIONS

PCBP2 and PTBP1 can both associate with FHL3 mRNA 3'UTR through forming a protein complex.

Ribonomic approaches to study the RNA-binding proteome

Gene expression is controlled through a complex interplay among mRNAs, non-coding RNAs and RNA-binding proteins (RBPs), which all assemble along with other RNA-associated factors in dynamic and functional ribonucleoprotein complexes (RNPs). To date, our understanding of RBPs is largely limited to proteins with known or predicted RNA-binding domains. However, various methods have been recently developed to capture an RNA of interest and comprehensively identify its associated RBPs. In this review, we discuss the RNA-affinity purification methods followed by mass spectrometry analysis (AP-MS); RBP screening within protein libraries and computational methods that can be used to study the RNA-binding proteome (RBPome).

Transactivation of programmed ribosomal frameshifting by a viral protein

Programmed -1 ribosomal frameshifting (-1 PRF) is a widely used translational mechanism facilitating the expression of two polypeptides from a single mRNA. Commonly, the ribosome interacts with an mRNA secondary structure that promotes -1 frameshifting on a homopolymeric slippery sequence. Recently, we described an unusual -2 frameshifting (-2 PRF) signal directing efficient expression of a transframe protein [nonstructural protein 2TF (nsp2TF)] of porcine reproductive and respiratory syndrome virus (PRRSV) from an alternative reading frame overlapping the viral replicase gene. Unusually, this arterivirus PRF signal lacks an obvious stimulatory RNA secondary structure, but as confirmed here, can also direct the occurrence of -1 PRF, yielding a third, truncated nsp2 variant named "nsp2N." Remarkably, we now show that both -2 and -1 PRF are transactivated by a protein factor, specifically a PRRSV replicase subunit (nsp1β). Embedded in nsp1β's papain-like autoproteinase domain, we identified a highly conserved, putative RNA-binding motif that is critical for PRF transactivation. The minimal RNA sequence required for PRF was mapped within a 34-nt region that includes the slippery sequence and a downstream conserved CCCANCUCC motif. Interaction of nsp1β with the PRF signal was demonstrated in pull-down assays. These studies demonstrate for the first time, to our knowledge, that a protein can function as a transactivator of ribosomal frameshifting. The newly identified frameshifting determinants provide potential antiviral targets for arterivirus disease control and prevention. Moreover, protein-induced transactivation of frameshifting may be a widely used mechanism, potentially including previously undiscovered viral strategies to regulate viral gene expression and/or modulate host cell translation upon infection.

RNA-RNA interactions and pre-mRNA mislocalization as drivers of group II intron loss from nuclear genomes

Group II introns are commonly believed to be the progenitors of spliceosomal introns, but they are notably absent from nuclear genomes. Barriers to group II intron function in nuclear genomes therefore beg examination. A previous study showed that nuclear expression of a group II intron in yeast results in nonsense-mediated decay and translational repression of mRNA, and that these roadblocks to expression are group II intron-specific. To determine the molecular basis for repression of gene expression, we investigated cellular dynamics of processed group II intron RNAs, from transcription to cellular localization. Our data show pre-mRNA mislocalization to the cytoplasm, where the RNAs are targeted to foci. Furthermore, tenacious mRNA-pre-mRNA interactions, based on intron-exon binding sequences, result in reduced abundance of spliced mRNAs. Nuclear retention of pre-mRNA prevents this interaction and relieves these expression blocks. In addition to providing a mechanistic rationale for group II intron-specific repression, our data support the hypothesis that RNA silencing of the host gene contributed to expulsion of group II introns from nuclear genomes and drove the evolution of spliceosomal introns.

From specific to global analysis of posttranscriptional regulation in eukaryotes: posttranscriptional regulatory networks

Regulation of gene expression occurs at several levels in eukaryotic organisms and is a highly controlled process. Although RNAs have been traditionally viewed as passive molecules in the pathway from transcription to translation, there is mounting evidence that their metabolism is controlled by a class of proteins called RNA-binding proteins (RBPs), as well as a number of small RNAs. In this review, I provide an overview of the recent developments in our understanding of the repertoire of RBPs across diverse model systems, and discuss the computational and experimental approaches currently available for the construction of posttranscriptional networks governed by them. I also present an overview of the different roles played by RBPs in the cellular context, based on their cis-regulatory modules identified in the literature and discuss how their interplay can result in the dynamic, spatial and tissue-specific expression maps of RNAs. I finally present the concept of posttranscriptional network of RBPs and their cognate RNA targets and discuss their cross-talk with other important posttranscriptional regulatory molecules such as microRNAs s, resulting in diverse functional network motifs. I argue that with rapid developments in the genome-wide elucidation of posttranscriptional networks it would not only be possible to gain a deeper understanding of regulation at a level that has been under-appreciated in the past, but would also allow us to use the newly developed high-throughput approaches to interrogate the prevalence of these phenomena in different states, and thereby study their relevance to physiology and disease across organisms.

Two steps forward--one step back: advances in affinity purification mass spectrometry of macromolecular complexes

Cellular functions are defined by the dynamic interactions of proteins within macromolecular networks. Deciphering these complex interplays is the key to getting a comprehensive picture of cellular behavior and to understanding biological systems, from a simple bacterial cell to highly regulated neuronal cells or cancerous tissue. In the last decade, affinity purification (AP) coupled to mass spectrometry has emerged as a powerful tool to comprehensively study interaction networks and their macromolecular assemblies. This review discusses recent advances in AP approaches, from cell lysis to the importance of sample preparation and the choice of AP matrix as well as the development of different epitope tags and strategies to study dynamic interactions, with an emphasis on RNA-protein interaction networks.

MS2-TRAP (MS2-tagged RNA affinity purification): tagging RNA to identify associated miRNAs

Cellular transcripts of all types, including coding messenger (m)RNAs and noncoding (nc)RNAs, are subject to extensive post-transcriptional regulation. Among the factors that elicit post-transcriptional control, microRNAs (miRNAs) have emerged as a major class of small regulatory RNAs. Since RNA-RNA interactions can be modeled computationally, several excellent programs have been developed to predict the interaction of miRNAs with target transcripts. However, many such predictions are not realized for different reasons, including absent or low-abundance expression of the miRNA in the cell, the existence of competing factors or conformational changes masking the microRNA site, and the possibility that target transcripts are not present in the prediction databases, as is the case for long ncRNAs. Here, we provide a systematic approach termed MS2-TRAP (tagged RNA affinity purification) for identifying miRNAs associated with a target transcript in the cellular context. We illustrate the use of this methodology by identifying microRNAs that associate with a long intergenic (li)ncRNA, based on the expression of the lincRNA tagged with MS2 RNA hairpins (lincRNA-p21-MS2) and the concomitant expression of a fusion protein recognizing the MS2 RNA hairpins, MS2-GST. After affinity pulldown of the ribonucleoprotein (RNP) complex comprising [MS2-GST/lincRNA-p21-MS2], the RNA in the pulldown material was isolated and reverse transcribed (RT). Subsequent assessment of the microRNAs present in the pulldown complex by using real-time quantitative (q)PCR analysis led to the identification of bona fide miRNAs that interact with and control the abundance of lincRNA-p21. We describe alternative designs and applications of this approach, and discuss its implications in deciphering post-transcriptional gene regulatory schemes.

From cis-regulatory elements to complex RNPs and back

Messenger RNAs (mRNAs), the templates for translation, have evolved to harbor abundant cis-acting sequences that affect their posttranscriptional fates. These elements are frequently located in the untranslated regions and serve as binding sites for trans-acting factors, RNA-binding proteins, and/or small non-coding RNAs. This article provides a systematic synopsis of cis-acting elements, trans-acting factors, and the mechanisms by which they affect translation. It also highlights recent technical advances that have ushered in the era of transcriptome-wide studies of the ribonucleoprotein complexes formed by mRNAs and their trans-acting factors.

Synthetic antibodies as tools to probe RNA-binding protein function

RNA-binding proteins (RBPs) have essential roles in post-transcriptional regulation of gene expression. They bind sequence elements in specific mRNAs and control their splicing, transport, localization, translation, and stability. A complete understanding of RBP function requires identification of the target RNAs that an RBP regulates, the mechanisms by which the RBP regulates these targets, and the biological consequences for the cell in which these transactions occur. Antibodies are key tools in such studies: first, mRNA targets of RBPs can be identified by co-immunoprecipitation of RBPs with their associated RNAs followed by microarray analysis or sequencing; second, partner proteins can be identified by immunoprecipitation of the RBP followed by mass spectrometry; third, the mechanisms and functions of RBPs can be inferred from loss-of-function studies employing antibodies that block RBP-RNA interactions. One potentially powerful approach to making antibodies for such studies is the generation of synthetic antibodies using phage display, which involves in vitro selection using a human-designed antibody library to generate antibodies that recognize a target protein. Using two well-characterized Drosophila RNA-binding proteins, Staufen and Smaug, for proof-of-principle, we demonstrate that synthetic antibodies can be generated and used either to perform RNA-coimmunoprecipitations (RIPs) to identify RBP-bound mRNAs, or to block RBP-RNA interactions. Given that synthetic antibody selection protocols are amenable to high-throughput antibody production, these results demonstrate that synthetic antibodies can be powerful tools for genome-wide studies of RBP function.

MicroRNA regulation by RNA-binding proteins and its implications for cancer

Non-protein-coding transcripts have been conserved throughout evolution, indicating that crucial functions exist for these RNAs. For example, microRNAs (miRNAs) have been found to modulate most cellular processes. The protein classes of RNA-binding proteins include essential regulators of miRNA biogenesis, turnover and activity. RNA-RNA and protein-RNA interactions are essential for post-transcriptional regulation in normal development and may be deregulated in disease. In reviewing emerging concepts of the interplay between miRNAs and RNA-binding proteins, we highlight the implications of these complex layers of regulation in cancer initiation and progression.

RaPID: an aptamer-based mRNA affinity purification technique for the identification of RNA and protein factors present in ribonucleoprotein complexes

RNA metabolism involves regulatory processes, such as transcription, splicing, nuclear export, transport and localization, association with sites of RNA modification, silencing and decay, and necessitates a wide variety of diverse RNA-interacting proteins. These interactions can be direct via RNA-binding proteins (RBPs) or indirect via other proteins and RNAs that form ribonucleoprotein complexes that together control RNA fate. While pull-down methods for the isolation of known RBPs are commonly used, strategies have also been described for the direct isolation of messenger RNAs (mRNAs) and their associated factors. The latter techniques allow for the identification of interacting proteins and RNAs, but most suffer from problems of low sensitivity and high background. Here we describe a simple and highly effective method for RNA purification and identification (RaPID) that allows for the isolation of specific mRNAs of interest from yeast and mammalian cells, and subsequent analysis of the associated proteins and RNAs using mass spectrometry and reverse transcription-PCR, respectively. This method employs the MS2 coat RBP fused to both GFP and streptavidin-binding protein to precipitate MS2 aptamer-tagged RNAs using immobilized streptavidin.

Polyadenylation state microarray (PASTA) analysis

Nearly all eukaryotic mRNAs terminate in a poly(A) tail that serves important roles in mRNA utilization. In the cytoplasm, the poly(A) tail promotes both mRNA stability and translation, and these functions are frequently regulated through changes in tail length. To identify the scope of poly(A) tail length control in a transcriptome, we developed the polyadenylation state microarray (PASTA) method. It involves the purification of mRNA based on poly(A) tail length using thermal elution from poly(U) sepharose, followed by microarray analysis of the resulting fractions. In this chapter we detail our PASTA approach and describe some methods for bulk and mRNA-specific poly(A) tail length measurements of use to monitor the procedure and independently verify the microarray data.

A novel mRNA affinity purification technique for the identification of interacting proteins and transcripts in ribonucleoprotein complexes

Intracellular mRNA targeting and localized translation are potential determinants for protein localization. To facilitate targeting, mRNAs possess specific cis-acting sequence motifs that are recognized by trans-acting RNA-binding proteins (RBPs). While many mRNAs are trafficked, our knowledge of the RBPs involved and presence of additional transcripts within these ribonucleoprotein (RNP) complexes is limited. To facilitate the identification of RBPs and transcripts that bind to specific mRNAs, we developed RNA-binding protein purification and identification (RaPID), a novel technique that allows for the affinity purification of MS2 aptamer-tagged mRNAs and subsequent detection of bound RBPs and transcripts using mass-spectometry and RT-PCR, respectively. RaPID effectively isolated specific mRNAs from both yeast and mammalian cells, and identified known mRNA-RBP interactions (e.g., ASH1-She2; β-Actin-IMP1). By isolating tagged OXA1 mRNA using RaPID, we could identify a yeast COPI subunit (i.e., Sec27) as a candidate interacting protein. This finding was strengthened by the observation that a portion of OXA1 mRNA was delocalized in a sec27-1 temperature-sensitive mutant at restrictive temperatures. Finally, RaPID could also be used to show biochemically the coexistence of different RNA species within the same RNP complex (e.g., coprecipitation of the yeast SRO7, WSC2, SEC3, and IST2 mRNAs with ASH1 mRNA) for the first time.

Functionally related transcripts have common RNA motifs for specific RNA-binding proteins in trypanosomes

Background

Trypanosomes mostly control gene expression by post-transcriptional events such as modulation of mRNA stability and translational efficiency. These mechanisms involve RNA-binding proteins (RBPs), which associate with transcripts to form messenger ribonucleoprotein (mRNP) complexes.

Results

In this study, we report the identification of mRNA targets for Trypanosoma cruzi U-rich RBP 1 (TcUBP1) and T. cruzi RBP 3 (TcRBP3), two phylogenetically conserved proteins among Kinetoplastids. Co-immunoprecipitated RBP-associated RNAs were extracted from mRNP complexes and binding of RBPs to several targets was confirmed by independent experimental assays. Analysis of target transcript sequences allowed the identification of different signature RNA motifs for each protein. Cis-elements for RBP binding have a stem-loop structure of 30–35 bases and are more frequently represented in the 3'-untranslated region (UTR) of mRNAs. Insertion of the correctly folded RNA elements to a non-specific mRNA rendered it into a target transcript, whereas substitution of the RNA elements abolished RBP interaction. In addition, RBPs competed for RNA-binding sites in accordance with the distribution of different and overlapping motifs in the 3'-UTRs of common mRNAs.

Conclusion

Functionally related transcripts were preferentially associated with a given RBP; TcUBP1 targets were enriched in genes encoding proteins involved in metabolism, whereas ribosomal protein-encoding transcripts were the largest group within TcRBP3 targets. Together, these results suggest coordinated control of different mRNA subsets at the post-transcriptional level by specific RBPs.

RNA-binding proteins in human genetic disease

RNA-binding proteins (RBPs) are key components in RNA metabolism, regulating the temporal, spatial and functional dynamics of RNAs. Altering the expression of RBPs has profound implications for cellular physiology, affecting RNA processes from pre-mRNA splicing to protein translation. Recent genetic and proteomic data and evidence from animal models reveal that RBPs are involved in many human diseases ranging from neurologic disorders to cancer. Here we review the emerging evidence showing the involvement of RBPs in many disease networks and conclude that defects in RNA metabolism caused by aberrations in RBPs might underlie a broader spectrum of complex human disorders.

RNA recognition by 3'-to-5' exonucleases: the substrate perspective

The 3'-to-5' exonucleolytic decay and processing of a variety of RNAs is an essential feature of RNA metabolism in all cells. The 3'-5' exonucleases, and in particular the exosome, are involved in a large number of pathways from 3' processing of rRNA, snRNA and snoRNA, to decay of mRNAs and mRNA surveillance. The potent enzymes performing these reactions are regulated to prevent processing of inappropriate substrates whilst mature RNA molecules exhibit several attributes that enable them to evade 3'-5' attack. How does an enzyme perform such selective activities on different substrates? The goal of this review is to provide an overview and perspective of available data on the underlying principles for the recognition of RNA substrates by 3'-to-5' exonucleases.