UV crosslinked mRNA-binding proteins captured from leaf mesophyll protoplasts

Defining context-dependent m6A RNA methylomes in Arabidopsis

N6-Methyladenosine (m6A) prevalently occurs on cellular RNA across almost all kingdoms of life. It governs RNA fate and is essential for development and stress responses. However, the dynamic, context-dependent m6A methylomes across tissues and in response to various stimuli remain largely unknown in multicellular organisms. Here, we generate a comprehensive census that identifies m6A methylomes in 100 samples during development or following exposure to various external conditions in Arabidopsis thaliana. We demonstrate that m6A is a suitable biomarker to reflect the developmental lineage, and that various stimuli rapidly affect m6A methylomes that constitute the regulatory network required for an effective response to the stimuli. Integrative analyses of the census and its correlation with m6A regulators identify multiple layers of regulation on highly context-dependent m6A modification in response to diverse developmental and environmental stimuli, providing insights into m6A modification dynamics in the myriad contexts of multicellular organisms.

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.

Convergent and/or parallel evolution of RNA-binding proteins in angiosperms after polyploidization

Increasing studies suggest that the biased retention of stress-related transcription factors (TFs) after whole-genome duplications (WGDs) could rewire gene transcriptional networks, facilitating plant adaptation to challenging environments. However, the role of posttranscriptional factors (e.g. RNA-binding proteins, RBPs) following WGDs has been largely ignored. Uncovering thousands of RBPs in 21 representative angiosperm species, we integrate genomic, transcriptomic, regulatomic, and paleotemperature datasets to unravel their evolutionary trajectories and roles in adapting to challenging environments. We reveal functional enrichments of RBP genes in stress responses and identify their convergent retention across diverse angiosperms from independent WGDs, coinciding with global cooling periods. Numerous RBP duplicates derived from WGDs are then identified as cold-induced. A significant overlap of 29 orthogroups between WGD-derived and cold-induced RBP genes across diverse angiosperms highlights a correlation between WGD and cold stress. Notably, we unveil an orthogroup (Glycine-rich RNA-binding Proteins 7/8, GRP7/8) and relevant TF duplicates (CCA1/LHY, RVE4/8, CBF2/4, etc.), co-retained in different angiosperms post-WGDs. Finally, we illustrate their roles in rewiring circadian and cold-regulatory networks at both transcriptional and posttranscriptional levels during global cooling. Altogether, we underline the adaptive evolution of RBPs in angiosperms after WGDs during global cooling, improving our understanding of plants surviving periods of environmental turmoil.

Mapping protein-RNA binding in plants with individual-nucleotide-resolution UV cross-linking and immunoprecipitation (plant iCLIP2)

Despite crucial roles of RNA-binding proteins (RBPs) in plant physiology and development, methods for determining their transcriptome-wide binding landscape are less developed than those used in other model organisms. Cross-linking and immunoprecipitation (CLIP) methods (based on UV-mediated generation of covalent bonds between RNAs and cognate RBPs in vivo, purification of the cross-linked complexes and identification of the co-purified RNAs by high-throughput sequencing) have been applied mainly in mammalian cells growing in monolayers or in translucent tissue. We have developed plant iCLIP2, an efficient protocol for performing individual-nucleotide-resolution CLIP (iCLIP) in plants, tailored to overcome the experimental hurdles posed by plant tissue. We optimized the UV dosage to efficiently cross-link RNA and proteins in plants and expressed epitope-tagged RBPs under the control of their native promoters in loss-of-function mutants. We select epitopes for which nanobodies are available, allowing stringent conditions for immunopurification of the RNA-protein complexes to be established. To overcome the inherently high RNase content of plant cells, RNase inhibitors are added and the limited RNA fragmentation step is modified. We combine the optimized isolation of RBP-bound RNAs with iCLIP2, a streamlined protocol that greatly enhances the efficiency of library preparation for high-throughput sequencing. Plant researchers with experience in molecular biology and handling of RNA can complete this iCLIP2 protocol in ~5 d. Finally, we describe a bioinformatics workflow to determine targets of Arabidopsis RBPs from iCLIP data, covering all steps from downloading sequencing reads to identifying cross-linking events ( https://github.com/malewins/Plant-iCLIPseq ), and present the R/Bioconductor package BindingSiteFinder to extract reproducible binding sites ( https://bioconductor.org/packages/release/bioc/html/BindingSiteFinder.html ).

Plant RNA-binding proteins: Phase separation dynamics and functional mechanisms underlying plant development and stress responses

RNA-binding proteins (RBPs) accompany RNA from synthesis to decay, mediating every aspect of RNA metabolism and impacting diverse cellular and developmental processes in eukaryotes. Many RBPs undergo phase separation along with their bound RNA to form and function in dynamic membraneless biomolecular condensates for spatiotemporal coordination or regulation of RNA metabolism. Increasing evidence suggests that phase-separating RBPs with RNA-binding domains and intrinsically disordered regions play important roles in plant development and stress adaptation. Here, we summarize the current knowledge about how dynamic partitioning of RBPs into condensates controls plant development and enables sensing of experimental changes to confer growth plasticity under stress conditions, with a focus on the dynamics and functional mechanisms of RBP-rich nuclear condensates and cytoplasmic granules in mediating RNA metabolism. We also discuss roles of multiple factors, such as environmental signals, protein modifications, and N6-methyladenosine RNA methylation, in modulating the phase separation behaviors of RBPs, and highlight the prospects and challenges for future research on phase-separating RBPs in crops.

Advances in Understanding the Mechanism of Cap-Independent Cucurbit Aphid-Borne Yellows Virus Protein Synthesis

Non-canonical translation mechanisms have been described for many viral RNAs. In the case of several plant viruses, their protein synthesis is controlled by RNA elements in their genomic 3'-ends that are able to enhance cap-independent translation (3'-CITE). The proposed general mechanism of 3'-CITEs includes their binding to eukaryotic translation initiation factors (eIFs) that reach the 5'-end and AUG start codon through 5'-3'-UTR-interactions. It was previously shown that cucurbit aphid-borne yellows virus (CABYV) has a 3'-CITE, which varies in sequence and structure depending on the phylogenetic group to which the isolate belongs, possibly as a result of adaptation to the different geographical regions. In this work, the cap-independent translation mechanisms of two CABYV 3'-CITEs belonging to the Mediterranean (CMTE) and Asian (CXTE) groups, respectively, were studied. In vivo cap-independent translation assays show that these 3'-CITEs require the presence of the CABYV short genomic 5'-UTR with at least 40% adenines in cis and an accessible 5'-end for its activity. Additionally, they suggest that the eIF4E-independent CABYV 3'-CITE activities may not require either eIF4A or the eIF4F complex, but may depend on eIF4G and PABP. By pulling down host proteins using RNA baits containing both 5'- and 3'-CABYV-UTRs, 80 RNA binding proteins were identified. These interacted preferentially with either CMTE, CXTE, or both. One of these proteins, specifically interacting with the RNA containing CMTE, was HSP70.2. Preliminary results suggested that HSP70.2 may be involved in CMTE- but not CXTE-mediated cap-independent translation activity.

RBPome identification in egg-cell like callus of Arabidopsis

RNA binding proteins (RBPs) have multiple and essential roles in transcriptional and posttranscriptional regulation of gene expression in all living organisms. Their biochemical identification in the proteome of a given cell or tissue requires significant protein amounts, which limits studies in rare and highly specialized cells. As a consequence, we know almost nothing about the role(s) of RBPs in reproductive processes such as egg cell development, fertilization and early embryogenesis in flowering plants. To systematically identify the RBPome of egg cells in the model plant Arabidopsis, we performed RNA interactome capture (RIC) experiments using the egg cell-like RKD2-callus and were able to identify 728 proteins associated with poly(A+)-RNA. Transcripts for 97 % of identified proteins could be verified in the egg cell transcriptome. 46 % of identified proteins can be associated with the RNA life cycle. Proteins involved in mRNA binding, RNA processing and metabolism are highly enriched. Compared with the few available RBPome datasets of vegetative plant tissues, we identified 475 egg cell-enriched RBPs, which will now serve as a resource to study RBP function(s) during egg cell development, fertilization and early embryogenesis. First candidates were already identified showing an egg cell-specific expression pattern in ovules.

Plant phase extraction: A method for enhanced discovery of the RNA-binding proteome and its dynamics in plants

RNA-binding proteins (RBPs) play critical roles in posttranscriptional gene regulation. Current methods of systematically profiling RBPs in plants have been predominantly limited to proteins interacting with polyadenylated (poly(A)) RNAs. We developed a method called plant phase extraction (PPE), which yielded a highly comprehensive RNA-binding proteome (RBPome), uncovering 2,517 RBPs from Arabidopsis (Arabidopsis thaliana) leaf and root samples with a highly diverse array of RNA-binding domains. We identified traditional RBPs that participate in various aspects of RNA metabolism and a plethora of nonclassical proteins moonlighting as RBPs. We uncovered constitutive and tissue-specific RBPs essential for normal development and, more importantly, revealed RBPs crucial for salinity stress responses from a RBP-RNA dynamics perspective. Remarkably, 40% of the RBPs are non-poly(A) RBPs that were not previously annotated as RBPs, signifying the advantage of PPE in unbiasedly retrieving RBPs. We propose that intrinsically disordered regions contribute to their nonclassical binding and provide evidence that enzymatic domains from metabolic enzymes have additional roles in RNA binding. Taken together, our findings demonstrate that PPE is an impactful approach for identifying RBPs from complex plant tissues and pave the way for investigating RBP functions under different physiological and stress conditions at the posttranscriptional level.

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 RNA-binding protein landscapes differ between mammalian organs and cultured cells

System-wide approaches have unveiled an unexpected breadth of the RNA-bound proteomes of cultured cells. Corresponding information regarding RNA-binding proteins (RBPs) of mammalian organs is still missing, largely due to technical challenges. Here, we describe ex vivo enhanced RNA interactome capture (eRIC) to characterize the RNA-bound proteomes of three different mouse organs. The resulting organ atlases encompass more than 1300 RBPs active in brain, kidney or liver. Nearly a quarter (291) of these had formerly not been identified in cultured cells, with more than 100 being metabolic enzymes. Remarkably, RBP activity differs between organs independent of RBP abundance, suggesting organ-specific levels of control. Similarly, we identify systematic differences in RNA binding between animal organs and cultured cells. The pervasive RNA binding of enzymes of intermediary metabolism in organs points to tightly knit connections between gene expression and metabolism, and displays a particular enrichment for enzymes that use nucleotide cofactors. We describe a generically applicable refinement of the eRIC technology and provide an instructive resource of RBPs active in intact mammalian organs, including the brain.

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.

Ribonomics Approaches to Identify RBPome in Plants and Other Eukaryotes: Current Progress and Future Prospects

RNA-binding proteins (RBPs) form complex interactions with RNA to regulate the cell’s activities including cell development and disease resistance. RNA-binding proteome (RBPome) aims to profile and characterize the RNAs and proteins that interact with each other to carry out biological functions. Generally, RNA-centric and protein-centric ribonomic approaches have been successfully developed to profile RBPome in different organisms including plants and animals. Further, more and more novel methods that were firstly devised and applied in mammalians have shown great potential to unravel RBPome in plants such as RNA-interactome capture (RIC) and orthogonal organic phase separation (OOPS). Despise the development of various robust and state-of-the-art ribonomics techniques, genome-wide RBP identifications and characterizations in plants are relatively fewer than those in other eukaryotes, indicating that ribonomics techniques have great opportunities in unraveling and characterizing the RNA–protein interactions in plant species. Here, we review all the available approaches for analyzing RBPs in living organisms. Additionally, we summarize the transcriptome-wide approaches to characterize both the coding and non-coding RBPs in plants and the promising use of RBPome for booming agriculture.

RNA-binding proteins and their role in translational regulation in plants

Translation is a fundamental process for life that needs to be finely adapted to the energetical, developmental and environmental conditions; however, the molecular mechanisms behind such adaptation are not yet fully understood. By directly recognizing and binding to cis-elements present in their target mRNAs, RBPs govern all post-transcriptional regulatory processes. They orchestrate the balance between mRNA stability, storage, decay, and translation of their client mRNAs, playing a crucial role in the modulation of gene expression. In the last years exciting discoveries have been made regarding the roles of RBPs in fine-tuning translation. In this review, we focus on how these RBPs recognize their targets and modulate their translation, highlighting the complex and diverse molecular mechanisms implicated. Since the repertoire of RBPs keeps growing, future research promises to uncover new fascinating means of translational modulation, and thus, of gene expression.

Characterization of RBPome in Oxidative Stress Conditions

Cellular redox signaling is triggered by accumulation of various reactive oxygen species (ROS) that integrate with other signaling cascades to enable plants to ultimately respond to (a)biotic stresses. The identification of key regulators underlying redox signaling networks is therefore of high priority. This chapter describes an improved mRNA interactome capture method that allows to systematically detect oxidative stress responsive regulators in the post-transcriptional gene regulation (PTGR) pathway. The protocol includes PSB-D suspension cell culture preparation, setup of oxidative stress conditions, short-term exposure to UV irradiation, cell lysis, pull-down and purification of crosslinked messenger ribonucleoproteins, their mass spectrometric analyses, and identification of proteome by statistical analyses. As result, a comprehensive inventory of the functional oxidative stress responsive RBPome (OxRBPome) is generated, which paves the way toward new insights into PTGR processes in redox signaling.

The mRNA-binding proteome of a critical phase transition during Arabidopsis seed germination

Arabidopsis thaliana seed germination is marked by extensive translational control at two critical phase transitions. The first transition refers to the start of hydration, the hydration translational shift. The second shift, the germination translational shift (GTS) is the phase between testa rupture and radicle protrusion at which the seed makes the all or nothing decision to germinate. The mechanism behind the translational regulation at these phase transitions is unknown. RNA binding proteins (RBPs) are versatile players in the post-transcriptional control of messenger RNAs (mRNAs) and as such candidates for regulating translation during seed germination. Here, we report the mRNA binding protein repertoire of seeds during the GTS. Thirty seed specific RBPs and 22 dynamic RBPs were identified during the GTS, like the putative RBP Vacuolar ATPase subunit A and RBP HSP101. Several stress granule markers were identified in this study, which suggests that seeds are prepared to quickly adapt the translation of specific mRNAs in response to changes in environmental conditions during the GTS. Taken together this study provides a detailed insight into the world of RBPs during seed germination and their possible regulatory role during this developmentally regulated process.

The expanding world of metabolic enzymes moonlighting as RNA binding proteins

RNA binding proteins play key roles in many aspects of RNA metabolism and function, including splicing, transport, translation, localization, stability and degradation. Within the past few years, proteomics studies have identified dozens of enzymes in intermediary metabolism that bind to RNA. The wide occurrence and conservation of RNA binding ability across distant branches of the evolutionary tree suggest that these moonlighting enzymes are involved in connections between intermediary metabolism and gene expression that comprise far more extensive regulatory networks than previously thought. There are many outstanding questions about the molecular structures and mechanisms involved, the effects of these interactions on enzyme and RNA functions, and the factors that regulate the interactions. The effects on RNA function are likely to be wider than regulation of translation, and some enzyme-RNA interactions have been found to regulate the enzyme's catalytic activity. Several enzyme-RNA interactions have been shown to be affected by cellular factors that change under different intracellular and environmental conditions, including concentrations of substrates and cofactors. Understanding the molecular mechanisms involved in the interactions between the enzymes and RNA, the factors involved in regulation, and the effects of the enzyme-RNA interactions on both the enzyme and RNA functions will lead to a better understanding of the role of the many newly identified enzyme-RNA interactions in connecting intermediary metabolism and gene expression.

The Landscape of RNA-Protein Interactions in Plants: Approaches and Current Status

RNAs transmit information from DNA to encode proteins that perform all cellular processes and regulate gene expression in multiple ways. From the time of synthesis to degradation, RNA molecules are associated with proteins called RNA-binding proteins (RBPs). The RBPs play diverse roles in many aspects of gene expression including pre-mRNA processing and post-transcriptional and translational regulation. In the last decade, the application of modern techniques to identify RNA-protein interactions with individual proteins, RNAs, and the whole transcriptome has led to the discovery of a hidden landscape of these interactions in plants. Global approaches such as RNA interactome capture (RIC) to identify proteins that bind protein-coding transcripts have led to the identification of close to 2000 putative RBPs in plants. Interestingly, many of these were found to be metabolic enzymes with no known canonical RNA-binding domains. Here, we review the methods used to analyze RNA-protein interactions in plants thus far and highlight the understanding of plant RNA-protein interactions these techniques have provided us. We also review some recent protein-centric, RNA-centric, and global approaches developed with non-plant systems and discuss their potential application to plants. We also provide an overview of results from classical studies of RNA-protein interaction in plants and discuss the significance of the increasingly evident ubiquity of RNA-protein interactions for the study of gene regulation and RNA biology in plants.

Highly Efficient Leaf Base Protoplast Isolation and Transient Expression Systems for Orchids and Other Important Monocot Crops

Versatile protoplast platforms greatly facilitate the development of modern botany. However, efficient protoplast-based systems are still challenging for numerous horticultural plants and crops. Orchids are globally cultivated ornamental and medicinal monocot plants, but few efficient protoplast isolation and transient expression systems have been developed. In this study, we established a highly efficient orchid protoplast isolation protocol by selecting suitable source materials and optimizing the enzymatic conditions, which required optimal D-mannitol concentrations (0.4-0.6 M) combined with optimal 1.2% cellulose and 0.6% macerozyme, 5 μM of 2-mercaptoethanol and 6 h digestion. Tissue- and organ-specific protoplasts were successfully isolated from young leaves [∼3.22 × 106/g fresh weight (FW)], flower pedicels (∼5.26 × 106/g FW), and young root tips (∼7.66 × 105/g FW) of Cymbidium orchids. This protocol recommends the leaf base tissues (the tender part of young leaves attached to the stem) as better source materials. High yielding viable protoplasts were isolated from the leaf base of Cymbidium (∼2.50 × 107/g FW), Phalaenopsis (1.83 × 107/g FW), Paphiopedilum (1.10 × 107/g FW), Dendrobium (8.21 × 106/g FW), Arundina (3.78 × 106/g FW) orchids, and other economically important monocot crops including maize (Zea mays) (3.25 × 107/g FW) and rice (Oryza sativa) (4.31 × 107/g FW), which showed marked advantages over previous mesophyll protoplast isolation protocols. Leaf base protoplasts of Cymbidium orchids were used for polyethylene glycol (PEG)-mediated transfection, and a transfection efficiency of more than 80% was achieved. This leaf base protoplast system was applied successfully to analyze the CsDELLA-mediated gibberellin signaling in Cymbidium orchids. We investigated the subcellular localization of the CsDELLA-green fluorescent protein fusion and analyzed the role of CsDELLA in the regulation of gibberellin to flowering-related genes via efficient transient overexpression and gene silencing of CsDELLA in Cymbidium protoplasts. This protoplast isolation and transient expression system is the most efficient based on the documented results to date. It can be widely used for cellular and molecular studies in orchids and other economically important monocot crops, especially for those lacking an efficient genetic transformation system in vivo.

AIRBP: Accurate identification of RNA-binding proteins using machine learning techniques

Identification of RNA-binding proteins (RBPs) that bind to ribonucleic acid molecules is an important problem in Computational Biology and Bioinformatics. It becomes indispensable to identify RBPs as they play crucial roles in post-transcriptional control of RNAs and RNA metabolism as well as have diverse roles in various biological processes such as splicing, mRNA stabilization, mRNA localization, and translation, RNA synthesis, folding-unfolding, modification, processing, and degradation. The existing experimental techniques for identifying RBPs are time-consuming and expensive. Therefore, identifying RBPs directly from the sequence using computational methods can be useful to annotate RBPs and assist the experimental design efficiently. In this work, we present a method called AIRBP, which is designed using an advanced machine learning technique, called stacking, to effectively predict RBPs by utilizing features extracted from evolutionary information, physiochemical properties, and disordered properties. Moreover, our method, AIRBP, use the majority vote from RBPPred, DeepRBPPred, and the stacking model for the prediction for RBPs. The results show that AIRBP attains Accuracy (ACC), Balanced Accuracy (BACC), F1-score, and Mathews Correlation Coefficient (MCC) of 95.84 %, 94.71 %, 0.928, and 0.899, respectively, based on the training dataset, using 10-fold cross-validation (CV). Further evaluation of AIRBP on independent test set reveals that it achieves ACC, BACC, F1-score, and MCC of 94.36 %, 94.28 %, 0.897, and 0.860, for Human test set; 91.25 %, 93.00 %, 0.896, and 0.835 for S. cerevisiae test set; and 90.60 %, 90.41 %, 0.934, and 0.775 for A. thaliana test set, respectively. These results indicate that the AIRBP outperforms the existing Deep- and TriPepSVM methods. Therefore, the proposed better-performing AIRBP can be useful for accurate identification and annotation of RBPs directly from the sequence and help gain valuable insight to treat critical diseases. Availability: Code-data is available here: http://cs.uno.edu/∼tamjid/Software/AIRBP/code_data.zip.

Functional Roles of RNA-Binding Proteins in Plant Signaling

RNA-binding proteins (RBPs) are typical proteins that bind RNA through single or multiple RNA-binding domains (RBDs). These proteins have a functional role in determining the fate or function of the bound RNAs. A few hundred RBPs were known through in silico prediction based on computational assignment informed by structural similarity and the presence of classical RBDs. However, RBPs lacking such conventional RBDs were omitted. Owing to the recent mRNA interactome capture technology based on UV-crosslinking and fixing proteins to their mRNA targets followed by affinity capture purification and identification of RBPs by tandem mass spectrometry, several hundreds of RBPs have recently been discovered. These proteome-wide studies have colossally increased the number of proteins implicated in RNA binding and unearthed hundreds of novel RBPs lacking classical RBDs, such as proteins involved in intermediary metabolism. These discoveries provide wide insights into the post-transcriptional gene regulation players and their role in plant signaling, such as environmental stress conditions. In this review, novel discoveries of RBPs are explored, particularly on the evolving knowledge of their role in stress responses. The molecular functions of these RBPs, particularly focusing on those that do not have classical RBDs, are also elucidated at the systems level.

The increasing diversity and complexity of the RNA-binding protein repertoire in plants

Post-transcriptional regulation has far-reaching implications on the fate of RNAs. It is gaining increasing momentum as a critical component in adjusting global cellular transcript levels during development and in response to environmental stresses. In this process, RNA-binding proteins (RBPs) are indispensable chaperones that naturally bind RNA via one or multiple globular RNA-binding domains (RBDs) changing the function or fate of the bound RNAs. Despite the technical challenges faced in plants in large-scale studies, several hundreds of these RBPs have been discovered and elucidated globally over the past few years. Recent discoveries have more than doubled the number of proteins implicated in RNA interaction, including identification of RBPs lacking classical RBDs. This review will discuss these new emerging classes of RBPs, focusing on the current state of the RBP repertoire in Arabidopsis thaliana, including the diverse functional roles derived from quantitative studies implicating RBPs in abiotic stress responses. Notably, this review highlights that 836 RBPs are enriched as Arabidopsis RBPs while 1865 can be classified as candidate RBPs. The review will also outline outstanding areas within this field that require addressing to advance our understanding and potential biotechnological applications of RBPs.

Emerging Roles of RNA-Binding Proteins in Seed Development and Performance

Seed development, dormancy, and germination are key physiological events that are not only important for seed generation, survival, and dispersal, but also contribute to agricultural production. RNA-binding proteins (RBPs) directly interact with target mRNAs and fine-tune mRNA metabolism by governing post-transcriptional regulation, including RNA processing, intron splicing, nuclear export, trafficking, stability/decay, and translational control. Recent studies have functionally characterized increasing numbers of diverse RBPs and shown that they participate in seed development and performance, providing significant insight into the role of RBP-mRNA interactions in seed processes. In this review, we discuss recent research progress on newly defined RBPs that have crucial roles in RNA metabolism and affect seed development, dormancy, and germination.

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).

Plant PUF RNA-binding proteins: A wealth of diversity for post-transcriptional gene regulation

PUF proteins are a conserved group of sequence-specific RNA-binding proteins that typically function to negatively regulate mRNA stability and translation. PUFs are well characterized at the molecular, structural and functional levels in Drosophila, Caenorhabditis elegans, budding yeast and human systems. Although usually encoded by small gene families, PUFs are over-represented in the plant genome, with up to 36 genes identified in a single species. PUF gene expansion in plants has resulted in extensive variability in gene expression patterns, diversity in predicted RNA-binding domain structure, and novel combinations of key amino acids involved in modular nucleotide binding. Reports on the characterization of plant PUF structure and function continue to expand, and include RNA target identification, subcellular distribution, crystal structure, and molecular mechanisms. Arabidopsis PUF mutant analysis has provided insight into biological function, and has identified roles related to development and environmental stress tolerance. The diversity of plant PUFs implies an extensive role for this family of proteins in post-transcriptional gene regulation. This diversity also holds the potential for providing novel RNA-binding domains that could be engineered to produce designer PUFs to alter the metabolism of target RNAs in the cell.

Plant Individual Nucleotide Resolution Cross-Linking and Immunoprecipitation to Characterize RNA-Protein Complexes

In recent years, it has become increasingly recognized that regulation at the RNA level pervasively shapes the transcriptome in eukaryotic cells. This has fostered an interest in the mode of action of RNA-binding proteins that, via interaction with specific RNA sequence motifs, modulate gene expression. Understanding such posttranscriptional networks controlled by an RNA-binding protein requires a comprehensive identification of its in vivo targets. In metazoans and yeast, methods have been devised to stabilize RNA-protein interactions by UV cross-linking before isolating RNA-protein complexes using antibodies, followed by identification of associated RNAs by next-generation sequencing. These methods are collectively referred to as CLIP-Seq (cross-linking immunoprecipitation-high-throughput sequencing). Here, we present a version of the individual nucleotide resolution cross-linking and immunoprecipitation procedure that is suitable for use in the model plant Arabidopsis thaliana.

Marking RNA: m6A writers, readers, and functions in Arabidopsis

N6-methyladenosine (m6A) emerges as an important modification in eukaryotic mRNAs. m6A has first been reported in 1974, and its functional significance in mammalian gene regulation and importance for proper development have been well established. An arsenal of writer, eraser, and reader proteins accomplish deposition, removal, and interpretation of the m6A mark, resulting in dynamic function. This led to the concept of an epitranscriptome, the compendium of RNA species with chemical modification of the nucleobases in the cell, in analogy to the epigenome. While m6A has long been known to also exist in plant mRNAs, proteins involved in m6A metabolism have only recently been detected by mutant analysis, homology search, and mRNA interactome capture in the reference plant Arabidopsis thaliana. Dysregulation of the m6A modification causes severe developmental abnormalities of leaves and roots and altered timing of reproductive development. Furthermore, m6A modification affects viral infection. Here, we discuss recent progress in identifying m6A sites transcriptome-wide, in identifying the molecular players involved in writing, removing, and reading the mark, and in assigning functions to this RNA modification in A. thaliana. We highlight similarities and differences to m6A modification in mammals and provide an outlook on important questions that remain to be addressed.

Comparative analysis of RNA-binding proteomes under Arabidopsis thaliana-Pst DC3000-PAMP interaction by orthogonal organic phase separation

RNA-binding proteins (RBPs) are pivotal participants in post-transcriptional gene regulation. They interact with RNA directly to perform several post-transcriptional RNA regulatory functions or direct metabolic processes. Despite the essential importance, the understanding of plant RBPs is elementary, which derives mainly from other kingdoms via bioinformatic extrapolation or mRNA-binding proteins captured through UV crosslinked method. Recently, orthogonal organic phase separation (OOPS) method for RBP identification has been used in mammals and Escherichia coli. And plentiful RBPs were enriched without molecular tagging or capture of polyadenylated RNA in an unbiased way. In our study, OOPS was conducted on Arabidopsis and 468 RBPs were discovered including 244 putative RBPs. There were 17 peroxidases in 232 RBPs with enzymatic activities. In addition, Arabidopsis thaliana-Pst DC3000-chitinpentaose interaction system was chosen to explore whether OOPS can be used to dig specific RBPs under special physiological conditions. Eighty-four differential RBPs in this system were found and some of them involved in reactive oxygen species (ROS) metabolic pathway. These results showed OOPS can be applied to plants successfully and would be a useful method to identify RBPomes and specific RBPs.

Discovering the RNA-Binding Proteome of Plant Leaves with an Improved RNA Interactome Capture Method

RNA-binding proteins (RBPs) play a crucial role in regulating RNA function and fate. However, the full complement of RBPs has only recently begun to be uncovered through proteome-wide approaches such as RNA interactome capture (RIC). RIC has been applied to various cell lines and organisms, including plants, greatly expanding the repertoire of RBPs. However, several technical challenges have limited the efficacy of RIC when applied to plant tissues. Here, we report an improved version of RIC that overcomes the difficulties imposed by leaf tissue. Using this improved RIC method in Arabidopsis leaves, we identified 717 RBPs, generating a deep RNA-binding proteome for leaf tissues. While 75% of these RBPs can be linked to RNA biology, the remaining 25% were previously not known to interact with RNA. Interestingly, we observed that a large number of proteins related to photosynthesis associate with RNA in vivo, including proteins from the four major photosynthetic supercomplexes. As has previously been reported for mammals, a large proportion of leaf RBPs lack known RNA-binding domains, suggesting unconventional modes of RNA binding. We anticipate that this improved RIC method will provide critical insights into RNA metabolism in plants, including how cellular RBPs respond to environmental, physiological and pathological cues.

Emerging roles of RNA-binding proteins in plant development

RNA-binding proteins (RBPs) influence the fate of target RNAs via direct interactions. During transcription, RBPs and interacting partners are recruited to and modify transcripts, after which they may also participate in critical steps to generate functional RNA. RBP-RNA interactions govern post-transcriptional processing of RNA, consequently regulating gene expression in a spatio-temporal manner. In plants, an increasing number of proteins have been classified as RBPs, many of which have been shown to function as key players in diverse developmental processes. However, a comprehensive understanding of how RBPs function, which RNAs are targeted, and where RBP-RNA interactions occur within plant cells is lacking. Here, we discuss recent findings in the field and newly defined roles for RBPs in plant growth and development. We also describe the mechanistic effects of RBPs on target RNA metabolism and translation.

CLIP and RNA interactome studies to unravel genome-wide RNA-protein interactions in vivo in Arabidopsis thaliana

Post-transcriptional regulation makes an important contribution to adjusting the transcriptome to environmental changes in plants. RNA-binding proteins are key players that interact specifically with mRNAs to co-ordinate their fate. While the regulatory interactions between proteins and RNA are well understood in animals, until recently little information was available on the global binding landscape of RNA-binding proteins in higher plants. This is not least due to technical challenges in plants. In turn, while numerous RNA-binding proteins have been identified through mutant analysis and homology-based searches in plants, only recently a full compendium of proteins with RNA-binding activity has been experimentally determined for the reference plant Arabidopsis thaliana. State-of-the-art techniques to determine RNA-protein interactions genome-wide in animals are based on the covalent fixation of RNA and protein in vivo by UV light. This has only recently been successfully applied to plants. Here, we present practical considerations on the application of UV irradiation based methods to comprehensively determine in vivo RNA-protein interactions in Arabidopsis thaliana, focussing on individual nucleotide resolution crosslinking immunoprecipitation (iCLIP) and mRNA interactome capture.

Seeds: A Unique System to Study Translational Regulation

Seeds accumulate mRNA during their development and have the ability to store these mRNAs over extended periods of time. On imbibition, seeds transform from a quiescent dry state (no translation) to a fully active metabolic state, and selectively translate subsets of these stored mRNA. Thus, seeds provide a unique developmentally regulated 'on/off' switch for translation. Additionally, there is extensive translational control during seed germination. Here we discuss new findings and hypotheses linked to mRNA fate and the role of translational regulation in seeds. Translation is an understated yet important mode of gene regulation. We propose seeds as a novel system to study developmentally and physiologically regulated translation.

Changes in the Arabidopsis RNA-binding proteome reveal novel stress response mechanisms

BACKGROUND

RNA-binding proteins (RBPs) are increasingly recognized as regulatory component of post-transcriptional gene expression. RBPs interact with mRNAs via RNA-binding domains and these interactions affect RNA availability for translation, RNA stability and turn-over thus affecting both RNA and protein expression essential for developmental and stimulus specific responses. Here we investigate the effect of severe drought stress on the RNA-binding proteome to gain insights into the mechanisms that govern drought stress responses at the systems level.

RESULTS

Label-free mass spectrometry enabled the identification 567 proteins of which 150 significantly responded to the drought-induced treatment. A gene ontology analysis revealed enrichment in the "RNA binding" and "RNA processing" categories as well as biological processes such as "response to abscisic acid" and "response to water deprivation". Importantly, a large number of the stress responsive proteins have not previously been identified as RBPs and include proteins in carbohydrate metabolism and in the glycolytic and citric acid pathways in particular. This suggests that RBPs have hitherto unknown roles in processes that govern metabolic changes during stress responses. Furthermore, a comparative analysis of RBP domain architectures shows both, plant specific and common domain architectures between plants and animals. The latter could be an indication that RBPs are part of an ancient stress response.

CONCLUSION

This study establishes mRNA interactome capture technique as an approach to study stress signal responses implicated in environmental changes. Our findings denote RBP changes in the proteome as critical components in plant adaptation to changing environments and in particular drought stress protein-dependent changes in RNA metabolism.

Histone 2B monoubiquitination complex integrates transcript elongation with RNA processing at circadian clock and flowering regulators

HISTONE MONOUBIQUITINATION1 (HUB1) and its paralog HUB2 act in a conserved heterotetrameric complex in the chromatin-mediated transcriptional modulation of developmental programs, such as flowering time, dormancy, and the circadian clock. The KHD1 and SPEN3 proteins were identified as interactors of the HUB1 and HUB2 proteins with in vitro RNA-binding activity. Mutants in SPEN3 and KHD1 had reduced rosette and leaf areas. Strikingly, in spen3 mutants, the flowering time was slightly, but significantly, delayed, as opposed to the early flowering time in the hub1-4 mutant. The mutant phenotypes in biomass and flowering time suggested a deregulation of their respective regulatory genes CIRCADIAN CLOCK-ASSOCIATED1 (CCA1) and FLOWERING LOCUS C (FLC) that are known targets of the HUB1-mediated histone H2B monoubiquitination (H2Bub). Indeed, in the spen3-1 and hub1-4 mutants, the circadian clock period was shortened as observed by luciferase reporter assays, the levels of the CCA1α and CCA1β splice forms were altered, and the CCA1 expression and H2Bub levels were reduced. In the spen3-1 mutant, the delay in flowering time was correlated with an enhanced FLC expression, possibly due to an increased distal versus proximal ratio of its antisense COOLAIR transcript. Together with transcriptomic and double-mutant analyses, our data revealed that the HUB1 interaction with SPEN3 links H2Bub during transcript elongation with pre-mRNA processing at CCA1 Furthermore, the presence of an intact HUB1 at the FLC is required for SPEN3 function in the formation of the FLC-derived antisense COOLAIR transcripts.

Capture and Identification of RNA-binding Proteins by Using Click Chemistry-assisted RNA-interactome Capture (CARIC) Strategy

A comprehensive identification of RNA-binding proteins (RBPs) is key to understanding the posttranscriptional regulatory network in cells. A widely used strategy for RBP capture exploits the polyadenylation [poly(A)] of target RNAs, which mostly occurs on eukaryotic mature mRNAs, leaving most binding proteins of non-poly(A) RNAs unidentified. Here we describe the detailed procedures of a recently reported method termed click chemistry-assisted RNA-interactome capture (CARIC), which enables the transcriptome-wide capture of both poly(A) and non-poly(A) RBPs by combining the metabolic labeling of RNAs, in vivo UV cross-linking, and bioorthogonal tagging.

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.

Post-transcriptional regulation of the oxidative stress response in plants

Due to their sessile lifestyle, plants can be exposed to several kinds of stresses that will increase the production of reactive oxygen species (ROS), such as hydrogen peroxide, singlet oxygen, and hydroxyl radicals, in the plant cells and activate several signaling pathways that cause alterations in the cellular metabolism. Nevertheless, when ROS production outreaches a certain level, oxidative damage to nucleic acids, lipids, metabolites, and proteins will occur, finally leading to cell death. Until now, the most comprehensive and detailed readout of oxidative stress responses is undoubtedly obtained at the transcriptome level. However, transcript levels often do not correlate with the corresponding protein levels. Indeed, together with transcriptional regulations, post-transcriptional, translational, and/or post-translational regulations will shape the active proteome. Here, we review the current knowledge on the post-transcriptional gene regulation during the oxidative stress responses in planta.

Transcriptome-wide discovery of coding and noncoding RNA-binding proteins

Transcriptome-wide identification of RNA-binding proteins (RBPs) is a prerequisite for understanding the posttranscriptional gene regulation networks. However, proteomic profiling of RBPs has been mostly limited to polyadenylated mRNA-binding proteins, leaving RBPs on nonpoly(A) RNAs, including most noncoding RNAs (ncRNAs) and pre-mRNAs, largely undiscovered. Here we present a click chemistry-assisted RNA interactome capture (CARIC) strategy, which enables unbiased identification of RBPs, independent of the polyadenylation state of RNAs. CARIC combines metabolic labeling of RNAs with an alkynyl uridine analog and in vivo RNA-protein photocross-linking, followed by click reaction with azide-biotin, affinity enrichment, and proteomic analysis. Applying CARIC, we identified 597 RBPs in HeLa cells, including 130 previously unknown RBPs. These newly discovered RBPs can likely bind ncRNAs, thus uncovering potential involvement of ncRNAs in processes previously unknown to be ncRNA-related, such as proteasome function and intermediary metabolism. The CARIC strategy should be broadly applicable across various organisms to complete the census of RBPs.

Plant Ribonomics: Proteins in Search of RNA Partners

Research into the regulation of gene expression underwent a shift from focusing on DNA-binding proteins as key transcriptional regulators to RNA-binding proteins (RBPs) that come into play once transcription has been initiated. RBPs orchestrate all RNA-processing steps in the cell. To obtain a global view of in vivo targets, the RNA complement associated with particular RBPs is determined via immunoprecipitation of the RBP and subsequent identification of bound RNAs via RNA-seq. Here, we describe technical advances in identifying RBP in vivo targets and their binding motifs. We provide an up-to-date view of targets of nucleocytoplasmic RBPs collected in arabidopsis. We also discuss current experimental limitations and provide an outlook on how the approaches may advance our understanding of post-transcriptional networks.

A brave new world of RNA-binding proteins

RNA-binding proteins (RBPs) are typically thought of as proteins that bind RNA through one or multiple globular RNA-binding domains (RBDs) and change the fate or function of the bound RNAs. Several hundred such RBPs have been discovered and investigated over the years. Recent proteome-wide studies have more than doubled the number of proteins implicated in RNA binding and uncovered hundreds of additional RBPs lacking conventional RBDs. In this Review, we discuss these new RBPs and the emerging understanding of their unexpected modes of RNA binding, which can be mediated by intrinsically disordered regions, protein-protein interaction interfaces and enzymatic cores, among others. We also discuss the RNA targets and molecular and cellular functions of the new RBPs, as well as the possibility that some RBPs may be regulated by RNA rather than regulate RNA.

Systems Approaches to Map In Vivo RNA–Protein Interactions in Arabidopsis thaliana

Proteins that specifically interact with mRNAs orchestrate mRNA processing steps all the way from transcription to decay. Thus, these RNA-binding proteins represent an important control mechanism to double check which proportion of nascent pre-mRNAs is ultimately available for translation into distinct proteins. Here, we discuss recent progress to obtain a systems-level understanding of in vivo RNA–protein interactions in the reference plant Arabidopsis thaliana using protein-centric and RNA-centric methods as well as combined protein binding site and structure probing.

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.

Adaptation of iCLIP to plants determines the binding landscape of the clock-regulated RNA-binding protein AtGRP7

Background

Functions for RNA-binding proteins in orchestrating plant development and environmental responses are well established. However, the lack of a genome-wide view of their in vivo binding targets and binding landscapes represents a gap in understanding the mode of action of plant RNA-binding proteins. Here, we adapt individual nucleotide resolution crosslinking and immunoprecipitation (iCLIP) genome-wide to determine the binding repertoire of the circadian clock-regulated Arabidopsis thaliana glycine-rich RNA-binding protein AtGRP7.

Results

iCLIP identifies 858 transcripts with significantly enriched crosslink sites in plants expressing AtGRP7-GFP that are absent in plants expressing an RNA-binding-dead AtGRP7 variant or GFP alone. To independently validate the targets, we performed RNA immunoprecipitation (RIP)-sequencing of AtGRP7-GFP plants subjected to formaldehyde fixation. Of the iCLIP targets, 452 were also identified by RIP-seq and represent a set of high-confidence binders. AtGRP7 can bind to all transcript regions, with a preference for 3′ untranslated regions. In the vicinity of crosslink sites, U/C-rich motifs are overrepresented. Cross-referencing the targets against transcriptome changes in AtGRP7 loss-of-function mutants or AtGRP7-overexpressing plants reveals a predominantly negative effect of AtGRP7 on its targets. In particular, elevated AtGRP7 levels lead to damping of circadian oscillations of transcripts, including DORMANCY/AUXIN ASSOCIATED FAMILY PROTEIN2 and CCR-LIKE. Furthermore, several targets show changes in alternative splicing or polyadenylation in response to altered AtGRP7 levels.

Conclusions

We have established iCLIP for plants to identify target transcripts of the RNA-binding protein AtGRP7. This paves the way to investigate the dynamics of posttranscriptional networks in response to exogenous and endogenous cues.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1332-x) contains supplementary material, which is available to authorized users.

mRNA Interactome Capture from Plant Protoplasts

RNA-binding proteins (RBPs) determine the fates of RNAs. They participate in all RNA biogenesis pathways and especially contribute to post-transcriptional gene regulation (PTGR) of messenger RNAs (mRNAs). In the past few years, a number of mRNA-bound proteomes from yeast and mammalian cell lines have been successfully isolated through the use of a novel method called "mRNA interactome capture," which allows for the identification of mRNA-binding proteins (mRBPs) directly from a physiological environment. The method is composed of in vivo ultraviolet (UV) crosslinking, pull-down and purification of messenger ribonucleoprotein complexes (mRNPs) by oligo(dT) beads, and the subsequent identification of the crosslinked proteins by mass spectrometry (MS). Very recently, by applying the same method, several plant mRNA-bound proteomes have been reported simultaneously from different Arabidopsis tissue sources: etiolated seedlings, leaf tissue, leaf mesophyll protoplasts, and cultured root cells. Here, we present the optimized mRNA interactome capture method for Arabidopsis thaliana leaf mesophyll protoplasts, a cell type that serves as a versatile tool for experiments that include various cellular assays. The conditions for optimal protein yield include the amount of starting tissue and the duration of UV irradiation. In the mRNA-bound proteome obtained from a medium-scale experiment (10⁷ cells), RBPs noted to have RNA-binding capacity were found to be overrepresented, and many novel RBPs were identified. The experiment can be scaled up (10⁹ cells), and the optimized method can be applied to other plant cell types and species to broadly isolate, catalog, and compare mRNA-bound proteomes in plants.

Plant RNA Interactome Capture: Revealing the Plant RBPome

The application of RNA interactome capture to plants has enabled comprehensive determination of the plant RNA-binding proteome and the identification of novel families of RNA-binding proteins (RBPs). The technique is providing insight into the evolution of the eukaryotic repertoire of RBPs and will enhance prospects for engineering RBPs to improve crop traits.

RNA-Binding Proteins Revisited - The Emerging Arabidopsis mRNA Interactome

RNA-protein interaction is an important checkpoint to tune gene expression at the RNA level. Global identification of proteins binding in vivo to mRNA has been possible through interactome capture - where proteins are fixed to target RNAs by UV crosslinking and purified through affinity capture of polyadenylated RNA. In Arabidopsis over 500 RNA-binding proteins (RBPs) enriched in UV-crosslinked samples have been identified. As in mammals and yeast, the mRNA interactomes came with a few surprises. For example, a plethora of the proteins caught on RNA had not previously been linked to RNA-mediated processes, for example proteins of intermediary metabolism. Thus, the studies provide unprecedented insights into the composition of the mRNA interactome, highlighting the complexity of RNA-mediated processes.