A novel mode of RBD-protein recognition in the Y14-Mago complex

Messenger RNA Surveillance: Current Understanding, Regulatory Mechanisms, and Future Implications

Nonsense-mediated mRNA decay (NMD) is an evolutionarily conserved surveillance mechanism in eukaryotes primarily deployed to ensure RNA quality control by eliminating aberrant transcripts and also involved in modulating the expression of several physiological transcripts. NMD, the mRNA surveillance pathway, is a major form of gene regulation in eukaryotes. NMD serves as one of the most significant quality control mechanisms as it primarily scans the newly synthesized transcripts and differentiates the aberrant and non-aberrant transcripts. The synthesis of truncated proteins is restricted, which would otherwise lead to cellular dysfunctions. The up-frameshift factors (UPFs) play a central role in executing the NMD event, largely by recognizing and recruiting multiple protein factors that result in the decay of non-physiological mRNAs. NMD exhibits astounding variability in its ability across eukaryotes in an array of pathological and physiological contexts. The detailed understanding of NMD and the underlying molecular mechanisms remains blurred. This review outlines our current understanding of NMD, in regulating multifaceted cellular events during development and disease. It also attempts to identify unanswered questions that deserve further investigation.

Surmounting Cancer Drug Resistance: New Perspective on RNA-Binding Proteins

RNA-binding proteins (RBPs), being pivotal elements in both physiological and pathological processes, possess the ability to directly impact RNA, thereby exerting a profound influence on cellular life. Furthermore, the dysregulation of RBPs not only induces alterations in the expression levels of genes associated with cancer but also impairs the occurrence of post-transcriptional regulatory mechanisms. Consequently, these circumstances can give rise to aberrations in cellular processes, ultimately resulting in alterations within the proteome. An aberrant proteome can disrupt the equilibrium between oncogenes and tumor suppressor genes, promoting cancer progression. Given their significant role in modulating gene expression and post-transcriptional regulation, directing therapeutic interventions towards RBPs represents a viable strategy for combating drug resistance in cancer treatment. RBPs possess significant potential as diagnostic and prognostic markers for diverse cancer types. Gaining comprehensive insights into the structure and functionality of RBPs, along with delving deeper into the molecular mechanisms underlying RBPs in tumor drug resistance, can enhance cancer treatment strategies and augment the prognostic outcomes for individuals afflicted with cancer.

Co-phase separation of Y14 and RNA in vitro and its implication for DNA repair

The multifunctional RNA recognition motif-containing protein Y14/RBM8A participates in mRNA metabolism and is essential for the efficient repair of DNA double-strand breaks (DSBs). Y14 contains highly charged, low-complexity sequences in both the amino- and carboxy-terminal domains. The feature of charge segregation suggests that Y14 may undergo liquid-liquid phase separation (LLPS). Recombinant Y14 formed phase-separated droplets, which were sensitive to pH and salt concentration. Domain mapping suggested that LLPS of Y14 involves multivalent electrostatic interactions and is partly determined by the net charge of its low-complexity regions. Phospho-mimicry of the carboxy-terminal arginine-serine dipeptides of Y14 suppressed phase separation. Moreover, RNA could phase separate into Y14 droplets and modulate Y14 LLPS in a concentration-dependent manner. Finally, the capacity of Y14 in LLPS and coacervation with RNA in vitro correlated with its activity in DSB repair. These results reveal a molecular rule for LLPS of Y14 in vitro and an implication for its co-condensation with RNA in genome stability.

Multifaceted roles of MAGOH Proteins

MAGOH and MAGOHB are paralog proteins that can substitute each other in the exon junction complex (EJC). The EJC is formed of core components EIF4A3, RBM8A, and MAGOH/MAGOHB. As a part of the EJC, MAGOH proteins are required for mRNA splicing, export, translation and nonsense-mediated mRNA decay (NMD). MAGOH is also essential for embryonic development and normal cellular functioning. The haploinsufficiency of MAGOH results in disorders such as microcephaly and cancer. The present review discusses the discovery of MAGOH, its paralog MAGOHB, their roles in cellular function as part of the EJC, and other cellular roles that are not directly associated with mRNA processing. We also discuss how MAGOH haploinsufficiency in cancer cells can be exploited to develop a novel targeted cancer treatment.

Systematic exploration of dynamic splicing networks reveals conserved multistage regulators of neurogenesis

Alternative splicing (AS) is a critical regulatory layer; yet, factors controlling functionally coordinated splicing programs during developmental transitions are poorly understood. Here, we employ a screening strategy to identify factors controlling dynamic splicing events important for mammalian neurogenesis. Among previously unknown regulators, Rbm38 acts widely to negatively control neural AS, in part through interactions mediated by the established repressor of splicing, Ptbp1. Puf60, a ubiquitous factor, is surprisingly found to promote neural splicing patterns. This activity requires a conserved, neural-differential exon that remodels Puf60 co-factor interactions. Ablation of this exon rewires distinct AS networks in embryonic stem cells and at different stages of mouse neurogenesis. Single-cell transcriptome analyses further reveal distinct roles for Rbm38 and Puf60 isoforms in establishing neuronal identity. Our results describe important roles for previously unknown regulators of neurogenesis and establish how an alternative exon in a widely expressed splicing factor orchestrates temporal control over cell differentiation.

No-nonsense: insights into the functional interplay of nonsense-mediated mRNA decay factors

Nonsense-mediated messenger RNA decay (NMD) represents one of the main surveillance pathways used by eukaryotic cells to control the quality and abundance of mRNAs and to degrade viral RNA. NMD recognises mRNAs with a premature termination codon (PTC) and targets them to decay. Markers for a mRNA with a PTC, and thus NMD, are a long a 3′-untranslated region and the presence of an exon-junction complex (EJC) downstream of the stop codon. Here, we review our structural understanding of mammalian NMD factors and their functional interplay leading to a branched network of different interconnected but specialised mRNA decay pathways. We discuss recent insights into the potential impact of EJC composition on NMD pathway choice. We highlight the coexistence and function of different isoforms of up-frameshift protein 1 (UPF1) with an emphasis of their role at the endoplasmic reticulum and during stress, and the role of the paralogs UPF3B and UPF3A, underscoring that gene regulation by mammalian NMD is tightly controlled and context-dependent being conditional on developmental stage, tissue and cell types.

Nonsense-mediated mRNA decay and metal ion homeostasis and detoxification in Saccharomyces cerevisiae

The highly conserved Nonsense-mediated mRNA decay (NMD) pathway is a translation dependent mRNA degradation pathway. Although NMD is best known for its role in degrading mRNAs with premature termination codons (PTCs) generated during transcription, splicing, or damage to the mRNAs, NMD is now also recognized as a pathway with additional important functions. Notably, NMD precisely regulates protein coding natural mRNAs, hence controlling gene expression within several physiologically significant pathways. Such pathways affected by NMD include nutritional bio-metal homeostasis and metal ion detoxification, as well as crosstalk between these pathways. Here, we focus on the relationships between NMD and various metal homeostasis and detoxification pathways. We review the described role that the NMD pathway plays in magnesium, zinc, iron, and copper homeostasis, as well as cadmium detoxification.

Exon junction complex dependent mRNA localization is linked to centrosome organization during ciliogenesis

Exon junction complexes (EJCs) mark untranslated spliced mRNAs and are crucial for the mRNA lifecycle. An imbalance in EJC dosage alters mouse neural stem cell (mNSC) division and is linked to human neurodevelopmental disorders. In quiescent mNSC and immortalized human retinal pigment epithelial (RPE1) cells, centrioles form a basal body for ciliogenesis. Here, we report that EJCs accumulate at basal bodies of mNSC or RPE1 cells and decline when these cells differentiate or resume growth. A high-throughput smFISH screen identifies two transcripts accumulating at centrosomes in quiescent cells, NIN and BICD2. In contrast to BICD2, the localization of NIN transcripts is EJC-dependent. NIN mRNA encodes a core component of centrosomes required for microtubule nucleation and anchoring. We find that EJC down-regulation impairs both pericentriolar material organization and ciliogenesis. An EJC-dependent mRNA trafficking towards centrosome and basal bodies might contribute to proper mNSC division and brain development.

Exon junction complexes (EJCs) that mark untranslated mRNA are involved in transport, translation and nonsense-mediated mRNA decay. Here the authors show centrosomal localization of EJCs which appears to be required for both the localization of NIN mRNA around centrosomes and ciliogenesis.

eIF4A3 Phosphorylation by CDKs Affects NMD during the Cell Cycle

Exon junction complexes (EJCs) loaded onto spliced mRNAs during splicing serve as molecular markers for various post-transcriptional gene-regulatory processes, including nonsense-mediated mRNA decay (NMD). Although the composition and structure of EJCs are well characterized, the mechanism regulating EJC deposition remains unknown. Here we find that threonine 163 (T163) within the RNA-binding motif of eIF4A3 (a core EJC component) is phosphorylated by cyclin-dependent protein kinases 1 and 2 in a cell cycle-dependent manner. T163 phosphorylation hinders binding of eIF4A3 to spliced mRNAs and other EJC components. Instead, it promotes association of eIF4A3 with CWC22, which guides eIF4A3 to an active spliceosome. These molecular events ensure the fidelity of specific deposition of the EJC ∼20-24 nt upstream of an exon-exon junction. Accordingly, NMD is affected by T163 phosphorylation. Collectively, our data provide evidence that T163 phosphorylation affects EJC formation and, consequently, NMD efficiency in a cell cycle-dependent manner.

Prognostic value of increased expression of RBM8A in gastric cancer

This study was designed to investigate the expression of RBM8A protein in patients with gastric cancer (GC) and to explore its correlation with clinical pathological features as well as prognosis. One hundred pairs of gastric carcinoma tissues and adjacent tissues from patients undergoing gastrectomy for GC were included in this study. The protein expression level of RBM8A was determined by immunohistochemistry using tissue microarrays. We also detected the mRNA expression level of RBM8A in 16 pairs of gastric carcinoma tissues and adjacent tissues. Meanwhile, we predicted the potential correlation between RBM8A and tumor stages as well as survival condition in patents with GC based on The Cancer Genome Atlas (TCGA) database. The correlation of RBM8A with the clinical pathological features and prognosis of the 100 patients with GC was also elucidated. The expression level of RBM8A was significantly higher in gastric carcinoma tissues compared to the adjacent tissues. The protein level of RBM8A was correlated with tumor size (P=0.031), depth of invasion (P<0.001), lymph node metastasis (P<0.001), TNM stage (<0.001), and distant metastasis (P=0.001). Patients with increased RBM8A expression (P<0.0018, 95%CI=0.322−0.871), higher TNM stage (P<0.001, 95%CI=4.990−11.283), and lymph node metastasis (P<0.001, 95%CI=2.873−4.002) had a lower overall survival. Taken together, our study demonstrated that RBM8A may act as a proto-oncogene, which could be a promising biomarker and therapeutic target in the diagnosis and treatment of GC.

RRMdb-an evolutionary-oriented database of RNA recognition motif sequences

RNA-recognition motif (RRM) is an RNA-interacting protein domain that plays an important role in the processes of RNA metabolism such as the splicing, editing, export, degradation, and regulation of translation. Here, we present the RNA-recognition motif database (RRMdb), which affords rapid identification and annotation of RRM domains in a given protein sequence. The RRMdb database is compiled from ~57 000 collected representative RRM domain sequences, classified into 415 families. Whenever possible, the families are associated with the available literature and structural data. Moreover, the RRM families are organized into a network of sequence similarities that allows for the assessment of the evolutionary relationships between them.

Exon junction complex (EJC) core genes play multiple developmental roles in Physalis floridana

KEY MESSAGE

Molecular and functional characterization of four gene families of the Physalis exon junction complex (EJC) core improved our understanding of the evolution and function of EJC core genes in plants. The exon junction complex (EJC) plays significant roles in posttranscriptional regulation of genes in eukaryotes. However, its developmental roles in plants are poorly known. We characterized four EJC core genes from Physalis floridana that were named PFMAGO, PFY14, PFeIF4AIII and PFBTZ. They shared a similar phylogenetic topology and were expressed in all examined organs. PFMAGO, PFY14 and PFeIF4AIII were localized in both the nucleus and cytoplasm while PFBTZ was mainly localized in the cytoplasm. No protein homodimerization was observed, but they could form heterodimers excluding the PFY14-PFBTZ heterodimerization. Virus-induced gene silencing (VIGS) of PFMAGO or PFY14 aborted pollen development and resulted in low plant survival due to a leaf-blight-like phenotype in the shoot apex. Carpel functionality was also impaired in the PFY14 knockdowns, whereas pollen maturation was uniquely affected in PFBTZ-VIGS plants. Once PFeIF4AIII was strongly downregulated, plant survival was reduced via a decomposing root collar after flowering and Chinese lantern morphology was distorted. The expression of Physalis orthologous genes in the DYT1-TDF1-AMS-bHLH91 regulatory cascade that is associated with pollen maturation was significantly downregulated in PFMAGO-, PFY14- and PFBTZ-VIGS flowers. Intron-retention in the transcripts of P. floridana dysfunctional tapetum1 (PFDYT1) occurred in these mutated flowers. Additionally, the expression level of WRKY genes in defense-related pathways in the shoot apex of PFMAGO- or PFY14-VIGS plants and in the root collar of PFeIF4AIII-VIGS plants was significantly downregulated. Taken together, the Physalis EJC core genes play multiple roles including a conserved role in male fertility and newly discovered roles in Chinese lantern development, carpel functionality and defense-related processes. These data increase our understanding of the evolution and functions of EJC core genes in plants.

Ribonucleoprotein particles: advances and challenges in computational methods

RNA-binding proteins (RBPs) interact with RNA to form Ribonucleoprotein Particles (RNPs). The interaction between RBPs and their RNA partners are traditionally thought to be mediated by highly conserved RNA-binding domains (RBDs). Recently, high-throughput studies led to the discovery of hundreds of novel proteins and domains, of which many do not follow the classical definition of RNA-binding. Despite technological innovations, experimental screenings are currently limited to the detection of specific types of RNPs, underscoring the importance of computational methods for predicting novel RBPs and RNA interacting residues and interfaces. Here, we discuss major challenges in computational prediction of RBPs and RBDs and outline new strategies to circumvent current limitations of experimental techniques.

The translational regulation of maternal mRNAs in time and space

Since their discovery, the study of maternal mRNAs has led to the identification of mechanisms underlying their spatiotemporal regulation within the context of oogenesis and early embryogenesis. Following synthesis in the oocyte, maternal mRNAs are translationally silenced and sequestered into storage in cytoplasmic granules. At the same time, their unique distribution patterns throughout the oocyte and embryo are tightly controlled and connected to their functions in downstream embryonic processes. At certain points in oogenesis and early embryogenesis, maternal mRNAs are translationally activated to perform their functions in a timely manner. The cytoplasmic polyadenylation machinery is responsible for the translational activation of maternal mRNAs, and its role in initiating the maternal to zygotic transition events has recently come to light. Here, we summarize the current knowledge on maternal mRNA regulation, with particular focus on cytoplasmic polyadenylation as a mechanism for translational regulation.

The exon junction complex factor Y14 is dynamic in the nucleus of the beetle Tribolium castaneum during late oogenesis

Background

The oocyte chromosomes of the red flour beetle, Tribolium castaneum, are gathered into a knot, forming a karyosphere at the diplotene stage of meiotic prophase. Chromatin rearrangement, which is a characteristic feature of oocyte maturation, is well documented. The T. castaneum karyosphere is surrounded by a complex extrachromosomal structure termed the karyosphere capsule. The capsule contains the vast majority of oocyte RNA. We have previously shown using a BrUTP assay that oocyte chromosomes in T. castaneum maintain residual transcription up to the very end of oocyte maturation. Karyosphere transcription requires evidently not only transcription factors but also mRNA processing factors, including the components of the exon junction complex with its core component, the splicing factor Y14. We employed a gene engineering approach with injection of mRNA derived from the Myc-tagged Y14 plasmid-based construct in order to monitor the newly synthesized fusion protein in the oocyte nuclei.

Results

Our preliminary data have been presented as a brief correspondence elsewhere. Here, we provide a full-length article including immunoelectron-microscopy localization data on Y14–Myc distribution in the nucleus of previtellogenic and vitellogenic oocytes. The injections of the fusion protein Y14–Myc mRNA into the oocytes showed a dynamic pattern of the protein distribution. At the previtellogenic stage, there are two main locations for the protein: SC35 domains (the analogues of interchromatin granule clusters or nuclear speckles) and the karyosphere capsule. At the vitellogenic stage, SC35 domains were devoid of labels, and Y14–Myc was found in the perichromatin region of the karyosphere, presumably at the places of residual transcription. We show that karyosphere formation is accompanied by the movement of a nuclear protein while the residual transcription occurs during genome inactivation.

Conclusions

Our data indicate that the karyosphere capsule, being a destination site for a protein involved in mRNA splicing and export, is not only a specializes part of nuclear matrix separating the karyosphere from the products of chromosome activity, as believed previously, but represents a special nuclear compartment involved in the processes of gene expression in the case the karyosphere retains residual transcription activity.

Electronic supplementary material

The online version of this article (10.1186/s13039-017-0342-4) contains supplementary material, which is available to authorized users.

Examining the intersection between splicing, nuclear export and small RNA pathways

BACKGROUND

Nuclear Argonaute/small RNA pathways in a variety of eukaryotic species are generally known to regulate gene expression via chromatin modulation and transcription attenuation in a process known as transcriptional gene silencing (TGS). However, recent data, including genetic screens, phylogenetic profiling, and molecular mechanistic studies, also point to a novel and emerging intersection between the splicing and nuclear export machinery with nuclear Argonaute/small RNA pathways in many organisms.

SCOPE OF REVIEW

In this review, we summarize the field's current understanding regarding the relationship between splicing, export and small RNA pathways, and consider the biological implications for coordinated regulation of transcripts by these pathways. We also address the importance and available approaches for understanding the RNA regulatory logic generated by the intersection of these particular pathways in the context of synthetic biology.

MAJOR CONCLUSIONS

The interactions between various eukaryotic RNA regulatory pathways, particularly splicing, nuclear export and small RNA pathways provide a type of combinatorial code that informs the identity ("self" versus "non-self") and dictates the fate of each transcript in a cell. Although the molecular mechanisms for how splicing and nuclear export impact small RNA pathways are not entirely clear at this early stage, the links between these pathways are widespread across eukaryotic phyla.

GENERAL SIGNIFICANCE

The link between splicing, nuclear export, and small RNA pathways is emerging and establishes a new frontier for understanding the combinatorial logic of gene regulation across species that could someday be harnessed for therapeutic, biotechnology and agricultural applications. This article is part of a Special Issue entitled "Biochemistry of Synthetic Biology - Recent Developments" Guest Editor: Dr. Ilka Heinemann and Dr. Patrick O'Donoghue.

Deciphering the mRNP Code: RNA-Bound Determinants of Post-Transcriptional Gene Regulation

Eukaryotic cells determine the final protein output of their genetic program not only by controlling transcription but also by regulating the localization, translation and turnover rates of their mRNAs. Ultimately, the fate of any given mRNA is determined by the ensemble of all associated RNA-binding proteins (RBPs), non-coding RNAs and metabolites collectively known as the messenger ribonucleoprotein particle (mRNP). Although many mRNA-associated factors have been identified over the past years, little is known about the composition of individual mRNPs and the cooperation of their constituents. In this review we discuss recent progress that has been made on how this 'mRNP code' is established on individual transcripts and how it is interpreted during gene expression in eukaryotic cells.

Unmasking the U2AF homology motif family: a bona fide protein-protein interaction motif in disguise

U2AF homology motifs (UHM) that recognize U2AF ligand motifs (ULM) are an emerging family of protein-protein interaction modules. UHM-ULM interactions recur in pre-mRNA splicing factors including U2AF1 and SF3b1, which are frequently mutated in myelodysplastic syndromes. The core topology of the UHM resembles an RNA recognition motif and is often mistakenly classified within this large family. Here, we unmask the charade and review recent discoveries of UHM-ULM modules for protein-protein interactions. Diverse polypeptide extensions and selective phosphorylation of UHM and ULM family members offer new molecular mechanisms for the assembly of specific partners in the early-stage spliceosome.

SLIRP stabilizes LRPPRC via an RRM-PPR protein interface

LRPPRC is a protein that has attracted interest both for its role in post-transcriptional regulation of mitochondrial gene expression and more recently because numerous mutated variants have been characterized as causing severe infantile mitochondrial neurodegeneration. LRPPRC belongs to the pentatricopeptide repeat (PPR) protein family, originally defined by their RNA binding capacity, and forms a complex with SLIRP that harbours an RNA recognition motif (RRM) domain. We show here that LRPPRC displays a broad and strong RNA binding capacity in vitro in contrast to SLIRP that associates only weakly with RNA. The LRPPRC–SLIRP complex comprises a hetero-dimer via interactions by polar amino acids in the single RRM domain of SLIRP and three neighbouring PPR motifs in the second quarter of LRPPRC, which critically contribute to the LRPPRC–SLIRP binding interface to enhance its stability. Unexpectedly, specific amino acids at this interface are located within the PPRs of LRPPRC at positions predicted to interact with RNA and within the RNP1 motif of SLIRP's RRM domain. Our findings thus unexpectedly establish that despite the prediction that these residues in LRPPRC and SLIRP should bind RNA, they are instead used to facilitate protein–protein interactions, enabling the formation of a stable complex between these two proteins.

The molecular basis for ANE syndrome revealed by the large ribosomal subunit processome interactome

ANE syndrome is a ribosomopathy caused by a mutation in an RNA recognition motif of RBM28, a nucleolar protein conserved to yeast (Nop4). While patients with ANE syndrome have fewer mature ribosomes, it is unclear how this mutation disrupts ribosome assembly. Here we use yeast as a model system and show that the mutation confers growth and pre-rRNA processing defects. Recently, we found that Nop4 is a hub protein in the nucleolar large subunit (LSU) processome interactome. Here we demonstrate that the ANE syndrome mutation disrupts Nop4’s hub function by abrogating several of Nop4’s protein-protein interactions. Circular dichroism and NMR demonstrate that the ANE syndrome mutation in RRM3 of human RBM28 disrupts domain folding. We conclude that the ANE syndrome mutation generates defective protein folding which abrogates protein-protein interactions and causes faulty pre-LSU rRNA processing, thus revealing one aspect of the molecular basis of this human disease.

DOI:http://dx.doi.org/10.7554/eLife.16381.001

ANE syndrome is a rare genetic disease that causes many problems including hair loss, mental retardation and a failure to develop normally during puberty. A study of 5 boys in the same family that were all born with the condition revealed that the disease is caused by a small change (or mutation) in a protein called RBM28. While little is known about the role of human RBM28, it is known that the equivalent protein in yeast – known as Nop4 – plays a critical role in forming a network of proteins needed to assemble ribosomes, the machines that make proteins.

McCann et al. investigated how such a small mutation in human RBM28 could cause disease and whether this involves interrupting the assembly of ribosomes. The experiments show that introducing the same mutation into yeast Nop4 impaired the ability of Nop4 to form the network of proteins needed for ribosomes to assemble. This ultimately restricted the growth of the yeast.

Further experiments revealed that the mutation also alters the shape of the human RBM28 protein. The main challenges for the future are to find out whether human RBM28 plays a similar role in ribosome assembly as the yeast protein, and to work out how disrupting ribosome assembly could lead to the symptoms of ANE syndrome.

DOI:http://dx.doi.org/10.7554/eLife.16381.002

Identification and characterization of MAGO and Y14 genes in Hevea brasiliensis

Mago nashi (MAGO) and Y14 proteins are highly conserved among eukaryotes. In this study, we identified two MAGO (designated as HbMAGO1 andHbMAGO2) and two Y14 (designated as HbY14aand HbY14b) genes in the rubber tree (Hevea brasiliensis) genome annotation. Multiple amino acid sequence alignments predicted that HbMAGO and HbY14 proteins are structurally similar to homologous proteins from other species. Tissue-specific expression profiles showed that HbMAGO and HbY14 genes were expressed in at least one of the tissues (bark, flower, latex, leaf and root) examined. HbMAGOs and HbY14s were predominately located in the nucleus and were found to interact in yeast two-hybrid analysis (YTH) and bimolecular fluorescence complementation (BiFC) assays. HbMAGOs and HbY14s showed the highest transcription in latex and were regulated by ethylene and jasmonate. Interaction between HbMAGO2 and gp91^phox (a large subunit of nicotinamide adenine dinucleotide phosphate) was identified using YTH and BiFC assays. These findings suggested that HbMAGO may be involved in the aggregation of rubber particles in H. brasiliensis.

A Point Mutation in the Exon Junction Complex Factor Y14 Disrupts Its Function in mRNA Cap Binding and Translation Enhancement

Eukaryotic mRNA biogenesis involves a series of interconnected steps mediated by RNA-binding proteins. The exon junction complex core protein Y14 is required for nonsense-mediated mRNA decay (NMD) and promotes translation. Moreover, Y14 binds the cap structure of mRNAs and inhibits the activity of the decapping enzyme Dcp2. In this report, we show that an evolutionarily conserved tryptophan residue (Trp-73) of Y14 is critical for its binding to the mRNA cap structure. A Trp-73 mutant (W73V) bound weakly to mRNAs and failed to protect them from degradation. However, this mutant could still interact with the NMD and mRNA degradation factors and retained partial NMD activity. In addition, we found that the W73V mutant could not interact with translation initiation factors. Overexpression of W73V suppressed reporter mRNA translation in vitro and in vivo and reduced the level of a set of nascent proteins. These results reveal a residue of Y14 that confers cap-binding activity and is essential for Y14-mediated enhancement of translation. Finally, we demonstrated that Y14 may selectively and differentially modulate protein biosynthesis.

Interactions between RNA-binding proteins and P32 homologues in trypanosomes and human cells

RNA-binding proteins (RBPs) are involved in many aspects of mRNA metabolism such as splicing, nuclear export, translation, silencing, and decay. To cope with these tasks, these proteins use specialized domains such as the RNA recognition motif (RRM), the most abundant and widely spread RNA-binding domain. Although this domain was first described as a dedicated RNA-binding moiety, current evidence indicates these motifs can also engage in direct protein-protein interactions. Here, we discuss recent evidence describing the interaction between the RRM of the trypanosomatid RBP UBP1 and P22, the homolog of the human multifunctional protein P32/C1QBP. Human P32 was also identified while performing a similar interaction screening using both RRMs of TDP-43, an RBP involved in splicing regulation and Amyotrophic Lateral Sclerosis. Furthermore, we show that this interaction is mediated by RRM1. The relevance of this interaction is discussed in the context of recent TDP-43 interactomic approaches that identified P32, and the numerous evidences supporting interactions between P32 and RBPs. Finally, we discuss the vast universe of interactions involving P32, supporting its role as a molecular chaperone regulating the function of its ligands.

Mutagenesis of ARS2 Domains To Assess Possible Roles in Cell Cycle Progression and MicroRNA and Replication-Dependent Histone mRNA Biogenesis

ARS2 is a regulator of RNA polymerase II transcript processing through its role in the maturation of distinct nuclear cap-binding complex (CBC)-controlled RNA families. In this study, we examined ARS2 domain function in transcript processing. Structural modeling based on the plant ARS2 orthologue, SERRATE, revealed 2 previously uncharacterized domains in mammalian ARS2: an N-terminal domain of unknown function (DUF3546), which is also present in SERRATE, and an RNA recognition motif (RRM) that is present in metazoan ARS2 but not in plants. Both the DUF3546 and zinc finger domain (ZnF) were required for association with microRNA and replication-dependent histone mRNA. Mutations in the ZnF disrupted interaction with FLASH, a key component in histone pre-mRNA processing. Mutations targeting the Mid domain implicated it in DROSHA interaction and microRNA biogenesis. The unstructured C terminus was required for interaction with the CBC protein CBP20, while the RRM was required for cell cycle progression and for binding to FLASH. Together, our results support a bridging model in which ARS2 plays a central role in RNA recognition and processing through multiple protein and RNA interactions.

Function and pathological implications of exon junction complex factor Y14

Eukaryotic mRNA biogenesis involves a series of interconnected steps, including nuclear pre-mRNA processing, mRNA export, and surveillance. The exon-junction complex (EJC) is deposited on newly spliced mRNAs and coordinates several downstream steps of mRNA biogenesis. The EJC core protein, Y14, functions with its partners in nonsense-mediated mRNA decay and translational enhancement. Y14 plays additional roles in mRNA metabolism, some of which are independent of the EJC, and it is also involved in other cellular processes. Genetic mutations or aberrant expression of Y14 results in physiological abnormality and may cause disease. Therefore, it is important to understand the various functions of Y14 and its physiological and pathological roles.

Slow co-evolution of the MAGO and Y14 protein families is required for the maintenance of their obligate heterodimerization mode

The exon junction complex (EJC) plays important roles in RNA metabolisms and the development of eukaryotic organisms. MAGO (short form of MAGO NASHI) and Y14 (also Tsunagi or RBM8) are the EJC core components. Their biological roles have been well investigated in various species, but the evolutionary patterns of the two gene families and their protein-protein interactions are poorly known. Genome-wide survey suggested that the MAGO and Y14 two gene families originated in eukaryotic organisms with the maintenance of a low copy. We found that the two protein families evolved slowly; however, the MAGO family under stringent purifying selection evolved more slowly than the Y14 family that was under relative relaxed purifying selection. MAGO and Y14 were obliged to form heterodimer in a eukaryotic organism, and this obligate mode was plesiomorphic. Lack of binding of MAGO to Y14 as functional barrier was observed only among distantly species, suggesting that a slow co-evolution of the two protein families. Inter-protein co-evolutionary signal was further quantified in analyses of the Tol-MirroTree and co-evolution analysis using protein sequences. About 20% of the 41 significantly correlated mutation groups (involving 97 residues) predicted between the two families was clade-specific. Moreover, around half of the predicted co-evolved groups and nearly all clade-specific residues fell into the minimal interaction domains of the two protein families. The mutagenesis effects of the clade-specific residues strengthened that the co-evolution is required for obligate MAGO-Y14 heterodimerization mode. In turn, the obliged heterodimerization in an organism serves as a strong functional constraint for the co-evolution of the MAGO and Y14 families. Such a co-evolution allows maintaining the interaction between the proteins through large evolutionary time scales. Our work shed a light on functional evolution of the EJC genes in eukaryotes, and facilitates to understand the co-evolutionary processes among protein families.

Nonsense-mediated mRNA decay: inter-individual variability and human disease

Nonsense-mediated mRNA decay (NMD) is a regulatory pathway that functions to degrade transcripts containing premature termination codons (PTCs) and to maintain normal transcriptome homeostasis. Nonsense and frameshift mutations that generate PTCs cause approximately one-third of all known human genetic diseases and thus NMD has a potentially important role in human disease. In genetic disorders in which the affected genes carry PTC-generating mutations, NMD acts as a double-edge sword. While it can benefit the patient by degrading PTC-containing mRNAs that encode detrimental, dominant-negative truncated proteins, it can also make the disease worse when a PTC-containing mRNA is degraded that encodes a mutant but still functional protein. There is evidence that the magnitude of NMD varies between individuals, which, in turn, has been shown to correlate with both clinical presentations and the patients' responses to drugs that promote read-through of PTCs. In this review, we examine the evidence supporting the existence of inter-individual variability in NMD efficiency and discuss the genetic factors that underlie this variability. We propose that inter-individual variability in NMD efficiency is a common phenomenon in human populations and that an individual's NMD efficiency should be taken into consideration when testing, developing, and making therapeutic decisions for diseases caused by genes harboring PTCs.

Structure and RNA-binding properties of the type III-A CRISPR-associated protein Csm3

The prokaryotic adaptive immune system is based on the incorporation of genome fragments of invading viral genetic elements into clusters of regulatory interspaced short palindromic repeats (CRISPRs). The CRISPR loci are transcribed and processed into crRNAs, which are then used to target the invading nucleic acid for degradation. The large family of CRISPR-associated (Cas) proteins mediates this interference response. We have characterized Methanopyrus kandleri Csm3, a protein of the type III-A CRISPR-Cas complex. The 2.4 Å resolution crystal structure shows an elaborate four-domain fold organized around a core RRM-like domain. The overall architecture highlights the structural homology to Cas7, the Cas protein that forms the backbone of type I interference complexes. Csm3 binds unstructured RNAs in a sequence non-specific manner, suggesting that it interacts with the variable spacer sequence of the crRNA. The structural and biochemical data provide insights into the similarities and differences in this group of Cas proteins.

MARVELD1 Inhibits Nonsense-Mediated RNA Decay by Repressing Serine Phosphorylation of UPF1

We have observed low expression levels of MARVELD1, a novel tumor repressor, in multiple tumors; however, its function in normal cells has not been explored. We recently reported that MARVELD1 interacts with importin β1, which plays an important role in nonsense-mediated RNA decay(NMD). Here, we demonstrate that MARVELD1 substantially inhibits nonsense-mediated RNA decay by decreasing the pioneer round of translation but not steady-state translation, and we identify MARVELD1 as an important component of the molecular machinery containing UPF1 and Y14. Furthermore, we determined the specific regions of MARVELD1 and UPF1 responsible for their interaction. We also showed that MARVELD1 promotes the dissociation of SMG1 from UPF1, resulting in the repression of serine phosphorylation of UPF1, and subsequently blocks the recruitment of SMG5, which is required for ensuing SMG5-mediated exonucleolytic decay. Our observations provide molecular insight into the potential function of MARVELD1 in nonsense-mediated RNA decay.

Nuclear magnetic resonance solution structure of the peptidoglycan-binding SPOR domain from Escherichia coli DamX: insights into septal localization

SPOR domains are present in thousands of bacterial proteins and probably bind septal peptidoglycan (PG), but the details of the SPOR-PG interaction have yet to be elucidated. Here we characterize the structure and function of the SPOR domain for an Escherichia coli division protein named DamX. Nuclear magnetic resonance revealed the domain comprises a four-stranded antiparallel β-sheet buttressed on one side by two α-helices. A third helix, designated α3, associates with the other face of the β-sheet, but this helix is relatively mobile. Site-directed mutagenesis revealed the face of the β-sheet that interacts with α3 is important for septal localization and binding to PG sacculi. The position and mobility of α3 suggest it might regulate PG binding, but although α3 deletion mutants still localized to the septal ring, they were too unstable to use in a PG binding assay. Finally, to assess the importance of the SPOR domain in DamX function, we constructed and characterized E. coli mutants that produced DamX proteins with SPOR domain point mutations or SPOR domain deletions. These studies revealed the SPOR domain is important for multiple activities associated with DamX: targeting the protein to the division site, conferring full resistance to the bile salt deoxycholate, improving the efficiency of cell division when DamX is produced at normal levels, and inhibiting cell division when DamX is overproduced.

The RNA-binding protein Y14 inhibits mRNA decapping and modulates processing body formation

The exon-junction complex (EJC) deposited on a newly spliced mRNA plays an important role in subsequent mRNA metabolic events. Here we show that an EJC core heterodimer, Y14/Magoh, specifically associates with mRNA-degradation factors, including the mRNA-decapping complex and exoribonucleases, whereas another core factor, eIF4AIII/MLN51, does not. We also demonstrate that Y14 interacts directly with the decapping factor Dcp2 and the 5' cap structure of mRNAs via different but overlapping domains and that Y14 inhibits the mRNA-decapping activity of Dcp2 in vitro. Accordingly, overexpression of Y14 prolongs the half-life of a reporter mRNA. Therefore Y14 may function independently of the EJC in preventing mRNA decapping and decay. Furthermore, we observe that depletion of Y14 disrupts the formation of processing bodies, whereas overexpression of a phosphomimetic Y14 considerably increases the number of processing bodies, perhaps by sequestering the mRNA-degradation factors. In conclusion, this report provides unprecedented evidence for a role of Y14 in regulating mRNA degradation and processing body formation and reinforces the influence of phosphorylation of Y14 on its activity in postsplicing mRNA metabolism.

Analysis of nonsense-mediated mRNA decay in mammalian cells

The nonsense-mediated mRNA decay (NMD) pathway acts to selectively identify and degrade mRNAs that contain a premature translation termination codon (PTC), and hence reduce the accumulation of potentially toxic truncated proteins. NMD is one of the best studied mRNA quality-control mechanisms in eukaryotes, and it has become clear during recent years that many physiological mRNAs are also NMD substrates, signifying a role for NMD beyond mRNA quality control as a translation-dependent post-transcriptional regulator of gene expression. Despite a great deal of scientific research for over twenty years, the process of NMD is far from being fully understood with regard to its physiological relevance to the cell, the molecular mechanisms that underpin this pathway, all of the factors that are involved, and the exact cellular locations of NMD. This unit details some of the fundamental RNA based approaches taken to examine aspects of NMD, such as creating PTC+ reporter genes, knocking down key NMD factors via RNAi, elucidating the important functions of NMD factors by complementation assays or Tethered Function Assays, and measuring RNA levels by reverse-transcription quantitative PCR.

CAVIN-3 regulates circadian period length and PER:CRY protein abundance and interactions

In mammals, transcriptional autorepression by Period (PER) and Cryptochrome (CRY) protein complexes is essential for the generation of circadian rhythms. We have identified CAVIN-3 as a new, cytoplasmic PER2-interacting protein influencing circadian clock properties. Thus, CAVIN-3 loss- and gain-of-function shortened and lengthened, respectively, the circadian period in fibroblasts and affected PER:CRY protein abundance and interaction. While depletion of protein kinase Cδ (PKCδ), a known partner of CAVIN-3, had little effect on circadian gene expression, CAVIN-3 required the PKCδ-binding site to exert its effect on period length. This suggests the involvement of yet uncharacterized protein kinases. Finally, CAVIN-3 activity in circadian gene expression was independent of caveolae.

Regulation of nonsense-mediated mRNA decay

Nonsense-mediated mRNA decay (NMD) is a highly conserved pathway that was originally identified as a RNA surveillance mechanism that degrades aberrant mRNAs harboring premature termination (nonsense) codons. Recently, it was discovered that NMD also regulates normal gene expression. Genome-wide studies showed that ablation of NMD alters the expression of ∼10% of transcripts in a wide variety of eukaryotes. In general, NMD specifically targets normal transcripts that harbor a stop codon in a premature context. The finding that NMD regulates normal gene expression raises the possibility that NMD itself is subject to regulation. Indeed, recent studies have shown that NMD efficiency varies in different cell types and tissues. NMD is also subject to developmental control in both higher and lower eukaryotic species. Molecular mechanisms have been defined-including those involving microRNAs and other RNA decay pathways-that regulate the magnitude of NMD in some developmental settings. This developmental regulation of NMD appears to have physiological roles, at least in some model systems. In addition to mechanisms that modulate the efficiency of NMD, mechanisms have recently been identified that serve the opposite purpose: to maintain the efficiency of NMD in the face of insults. This 'buffering' is achieved by feedback networks that serve to regulate the stability of NMD factors. The discovery of NMD homeostasis and NMD regulatory mechanisms has important implications for how NMD acts in biological processes and how its magnitude could potentially be manipulated for clinical benefit.

Multifunctional G-rich and RRM-containing domains of TbRGG2 perform separate yet essential functions in trypanosome RNA editing

Efficient editing of Trypanosoma brucei mitochondrial RNAs involves the actions of multiple accessory factors. T. brucei RGG2 (TbRGG2) is an essential protein crucial for initiation and 3'-to-5' progression of editing. TbRGG2 comprises an N-terminal G-rich region containing GWG and RG repeats and a C-terminal RNA recognition motif (RRM)-containing domain. Here, we perform in vitro and in vivo separation-of-function studies to interrogate the mechanism of TbRGG2 action in RNA editing. TbRGG2 preferentially binds preedited mRNA in vitro with high affinity attributable to its G-rich region. RNA-annealing and -melting activities are separable, carried out primarily by the G-rich and RRM domains, respectively. In vivo, the G-rich domain partially complements TbRGG2 knockdown, but the RRM domain is also required. Notably, TbRGG2's RNA-melting activity is dispensable for RNA editing in vivo. Interactions between TbRGG2 and MRB1 complex proteins are mediated by both G-rich and RRM-containing domains, depending on the binding partner. Overall, our results are consistent with a model in which the high-affinity RNA binding and RNA-annealing activities of the G-rich domain are essential for RNA editing in vivo. The RRM domain may have key functions involving interactions with the MRB1 complex and/or regulation of the activities of the G-rich domain.

Transient CPEB dimerization and translational control

During oocyte development, the cytoplasmic polyadenylation element-binding protein (CPEB) nucleates a set of factors on mRNA that controls cytoplasmic polyadenylation and translation. The regulation of polyadenylation is mediated in part through serial phosphorylations of CPEB, which control both the dynamic integrity of the cytoplasmic polyadenylation apparatus and CPEB stability, events necessary for meiotic progression. Because the precise stoichiometry between CPEB and CPE-containing RNA is responsible for the temporal order of mRNA polyadenylation during meiosis, we hypothesized that, if CPEB production exceeded the amount required to bind mRNA, the excess would be sequestered in an inactive form. One attractive possibility for the sequestration is protein dimerization. We demonstrate that not only does CPEB form a dimer, but dimerization requires its RNA-binding domains. Dimer formation prevents CPEB from being UV cross-linked to RNA, which establishes a second pool of CPEB that is inert for polyadenylation and translational control. During oocyte maturation, the dimers are degraded much more rapidly than the CPEB monomers, due to their greater affinity for polo-like kinase 1 (plx1) and the ubiquitin E3 ligase β-TrCP. Because dimeric CPEB also binds cytoplasmic polyadenylation factors with greater affinity than monomeric CPEB, it may act as a hub or reservoir for the polyadenylation machinery. We propose that the balance between CPEB and its target mRNAs is maintained by CPEB dimerization, which inactivates spare proteins and prevents them from inducing polyadenylation of RNAs with low affinity binding sites. In addition, the dimers might serve as molecular hubs that release polyadenylation factors for translational activation upon CPEB dimer destruction.

Perispeckles are major assembly sites for the exon junction core complex

The exon junction complex (EJC) allows the spliceosome to communicate with other cellular machinery. This study shows that assembled EJC cores are enriched in nuclear regions around speckles, called perispeckles. Speckles and perispeckles may represent specialized nuclear regions for messenger ribonucleoprotein maturation.

The exon junction complex (EJC) is loaded onto mRNAs as a consequence of splicing and regulates multiple posttranscriptional events. MLN51, Magoh, Y14, and eIF4A3 form a highly stable EJC core, but where this tetrameric complex is assembled in the cell remains unclear. Here we show that EJC factors are enriched in domains that we term perispeckles and are visible as doughnuts around nuclear speckles. Fluorescence resonance energy transfer analyses and EJC assembly mutants show that perispeckles do not store free subunits, but instead are enriched for assembled cores. At the ultrastructural level, perispeckles are distinct from interchromatin granule clusters that may function as storage sites for splicing factors and intermingle with perichromatin fibrils, where nascent RNAs and active RNA Pol II are present. These results support a model in which perispeckles are major assembly sites for the tetrameric EJC core. This subnuclear territory thus represents an intermediate region important for mRNA maturation, between transcription sites and splicing factor reservoirs and assembly sites.

The structure of the ASAP core complex reveals the existence of a Pinin-containing PSAP complex

The ASAP complex interacts with the exon-junction complex (EJC), a messenger ribonucleoprotein complex involved in post-transcriptional regulation. The three ASAP subunits (Acinus, RNPS1 and SAP18) have been individually implicated in transcriptional regulation, pre-mRNA splicing and mRNA quality control. To shed light on the basis for and consequences of ASAP's interaction with the EJC, we have determined the 1.9-Å resolution structure of a eukaryotic ASAP core complex. The RNA-recognition motif of RNPS1 binds to a conserved motif of Acinus with a recognition mode similar to that observed in splicing U2AF proteins. The Acinus-RNPS1 platform recruits the ubiquitin-like domain of SAP18, forming a ternary complex that has both RNA- and protein-binding properties. Unexpectedly, our structural analysis identified an Acinus-like motif in Pinin, another EJC-associated splicing factor. We show that Pinin physically interacts with RNPS1 and SAP18, forming an alternative ternary complex, PSAP.

Structural insight into RNA recognition motifs: versatile molecular Lego building blocks for biological systems

'RNA recognition motifs (RRMs)' are common domain-folds composed of 80-90 amino-acid residues in eukaryotes, and have been identified in many cellular proteins. At first they were known as RNA binding domains. Through discoveries over the past 20 years, however, the RRMs have been shown to exhibit versatile molecular recognition activities and to behave as molecular Lego building blocks to construct biological systems. Novel RNA/protein recognition modes by RRMs are being identified, and more information about the molecular recognition by RRMs is becoming available. These RNA/protein recognition modes are strongly correlated with their biological significance. In this review, we would like to survey the recent progress on these versatile molecular recognition modules.

Three RNA recognition motifs participate in RNA recognition and structural organization by the pro-apoptotic factor TIA-1

T-cell intracellular antigen-1 (TIA-1) regulates developmental and stress-responsive pathways through distinct activities at the levels of alternative pre-mRNA splicing and mRNA translation. The TIA-1 polypeptide contains three RNA recognition motifs (RRMs). The central RRM2 and C-terminal RRM3 associate with cellular mRNAs. The N-terminal RRM1 enhances interactions of a C-terminal Q-rich domain of TIA-1 with the U1-C splicing factor, despite linear separation of the domains in the TIA-1 sequence. Given the expanded functional repertoire of the RRM family, it was unknown whether TIA-1 RRM1 contributes to RNA binding as well as documented protein interactions. To address this question, we used isothermal titration calorimetry and small-angle X-ray scattering to dissect the roles of the TIA-1 RRMs in RNA recognition. Notably, the fas RNA exhibited two binding sites with indistinguishable affinities for TIA-1. Analyses of TIA-1 variants established that RRM1 was dispensable for binding AU-rich fas sites, yet all three RRMs were required to bind a polyU RNA with high affinity. Small-angle X-ray scattering analyses demonstrated a "V" shape for a TIA-1 construct comprising the three RRMs and revealed that its dimensions became more compact in the RNA-bound state. The sequence-selective involvement of TIA-1 RRM1 in RNA recognition suggests a possible role for RNA sequences in regulating the distinct functions of TIA-1. Further implications for U1-C recruitment by the adjacent TIA-1 binding sites of the fas pre-mRNA and the bent TIA-1 shape, which organizes the N- and C-termini on the same side of the protein, are discussed.

Protein-RNA and protein-protein recognition by dual KH1/2 domains of the neuronal splicing factor Nova-1

Nova onconeural antigens are neuron-specific RNA-binding proteins implicated in paraneoplastic opsoclonus-myoclonus-ataxia (POMA) syndrome. Nova harbors three K-homology (KH) motifs implicated in alternate splicing regulation of genes involved in inhibitory synaptic transmission. We report the crystal structure of the first two KH domains (KH1/2) of Nova-1 bound to an in vitro selected RNA hairpin, containing a UCAG-UCAC high-affinity binding site. Sequence-specific intermolecular contacts in the complex involve KH1 and the second UCAC repeat, with the RNA scaffold buttressed by interactions between repeats. Whereas the canonical RNA-binding surface of KH2 in the above complex engages in protein-protein interactions in the crystalline state, the individual KH2 domain can sequence-specifically target the UCAC RNA element in solution. The observed antiparallel alignment of KH1 and KH2 domains in the crystal structure of the complex generates a scaffold that could facilitate target pre-mRNA looping on Nova binding, thereby potentially explaining Nova's functional role in splicing regulation.

Functional interconnections of Arabidopsis exon junction complex proteins and genes at multiple steps of gene expression

The exon junction complex (EJC) is deposited on mRNA after splicing and participates in several aspects of RNA metabolism, from intracellular transport to translation. In this work, the functional and molecular interactions of Arabidopsis homologues of Mago, Y14, and PYM, three EJC components that participate in intron-mediated enhancement of gene expression in animals, have been analysed. AtMago, AtY14, and AtPYM are encoded by single genes that show similar expression patterns and contain common regulatory elements, known as site II, that are required for expression. AtPYM and AtY14 are phosphorylated by plant extracts and this modification regulates complex formation between both proteins. In addition, overexpression of AtMago and AtY14 in plants produces an increase in AtPYM protein levels, while overexpression of AtPYM results in increased formation of a complex that contains the three proteins. The effect of AtMago and AtY14 on AtPYM expression is most likely to be due to intron-mediated enhacement of AtPYM expression, since the AtPYM gene contains a leader intron that is required for expression. Indeed, transient transformation asssays indicated that the three proteins are able to increase expression from reporter constructs that contain leader introns required for the expression of different genes. The results indicate that the plant homologues of Mago, Y14, and PYM are closely interconnected, not only through their function as EJC components but also at different steps of their own gene expression mechanisms, probably reflecting the importance of their interaction for the correct expression of plant genes.

Specific Y14 domains mediate its nucleo-cytoplasmic shuttling and association with spliced mRNA

Pre-mRNA splicing deposits multi-protein complexes, termed exon junction complexes (EJCs), on mRNAs near exon-exon junctions. The core of EJC consists of four proteins, eIF4AIII, MLN51, Y14 and Magoh. Y14 is a nuclear protein that can shuttle between the nucleus and the cytoplasm, and binds specifically to Magoh. Here we delineate a Y14 nuclear localization signal that also confers its nuclear export, which we name YNS. We further identified a 12-amino-acid peptide near Y14's carboxyl terminus that is required for its association with spliced mRNAs, as well as for Magoh binding. Furthermore, the Y14 mutants, which are deficient in binding to Magoh, could still be localized to the nucleus, suggesting the existence of both the nuclear import pathway and function for Y14 unaccompanied by Magoh.

A novel occluded RNA recognition motif in Prp24 unwinds the U6 RNA internal stem loop

The essential splicing factor Prp24 contains four RNA Recognition Motif (RRM) domains, and functions to anneal U6 and U4 RNAs during spliceosome assembly. Here, we report the structure and characterization of the C-terminal RRM4. This domain adopts a novel non-canonical RRM fold with two additional flanking α-helices that occlude its β-sheet face, forming an occluded RRM (oRRM) domain. The flanking helices form a large electropositive surface. oRRM4 binds to and unwinds the U6 internal stem loop (U6 ISL), a stable helix that must be unwound during U4/U6 assembly. NMR data indicate that the process starts with the terminal base pairs of the helix and proceeds toward the loop. We propose a mechanistic and structural model of Prp24′s annealing activity in which oRRM4 functions to destabilize the U6 ISL during U4/U6 assembly.