Abstract
Pre-mRNA splicing catalyzed by the spliceosome represents a critical step in the regulation of gene expression contributing to transcriptome and proteome diversity. The spliceosome consists of five small nuclear ribonucleoprotein particles (snRNPs), the biogenesis of which remains only partially understood. Here we define the evolutionarily conserved protein Ecdysoneless (Ecd) as a critical regulator of U5 snRNP assembly and Prp8 stability. Combining Drosophila genetics with proteomic approaches, we demonstrate the Ecd requirement for the maintenance of adult healthspan and lifespan and identify the Sm ring protein SmD3 as a novel interaction partner of Ecd. We show that the predominant task of Ecd is to deliver Prp8 to the emerging U5 snRNPs in the cytoplasm. Ecd deficiency, on the other hand, leads to reduced Prp8 protein levels and compromised U5 snRNP biogenesis, causing loss of splicing fidelity and transcriptome integrity. Based on our findings, we propose that Ecd chaperones Prp8 to the forming U5 snRNP allowing completion of the cytoplasmic part of the U5 snRNP biogenesis pathway necessary to meet the cellular demand for functional spliceosomes.

Abstract
Splicing of precursor mRNAs (pre-mRNA) is an important step during eukaryotic gene expression. The identification of the actual splice sites and the proper removal of introns are essential for the production of the desired mRNA isoforms and their encoded proteins. While the basic mechanisms of splicing regulation are well understood, recent work has uncovered a growing number of noncanonical splicing mechanisms that play key roles in the regulation of gene expression. In this review, we summarize the current principles of splicing regulation, including the impact of cis and trans regulatory elements, as well as the influence of chromatin structure, transcription, and RNA modifications. We further discuss the recent development of emerging splicing mechanisms, such as recursive and back splicing, and their impact on gene expression.

Abstract
Eukaryotic gene expression is constantly regulated and controlled by the translation-coupled nonsense-mediated mRNA decay (NMD) pathway. Aberrant translation termination leads to NMD activation and robust clearance of NMD targets via two seemingly independent and redundant mRNA degradation branches. Here, we uncover that the loss of the first SMG5-SMG7-dependent pathway also inactivates the second SMG6-dependent branch, indicating an unexpected functional hierarchy of the final NMD steps. Transcriptome-wide analyses of SMG5-SMG7-depleted cells confirm complete NMD inhibition resulting in massive transcriptomic alterations. The NMD activity conferred by SMG5-SMG7 is determined to varying degrees by their interaction with the central NMD factor UPF1, heterodimer formation and the initiation of deadenylation. Surprisingly, we find that SMG5 functionally substitutes SMG7 and vice versa. Our data support an improved model for NMD execution that requires two-factor authentication involving UPF1 phosphorylation and SMG5-SMG7 recruitment to access SMG6 activity.

Abstract
The exon junction complex (EJC) is an essential constituent and regulator of spliced messenger ribonucleoprotein particles (mRNPs) in metazoans. As a core component of the EJC, CASC3 was described to be pivotal for EJC-dependent nuclear and cytoplasmic processes. However, recent evidence suggests that CASC3 functions differently from other EJC core proteins. Here, we have established human CASC3 knockout cell lines to elucidate the cellular role of CASC3. In the knockout cells, overall EJC composition and EJC-dependent splicing are unchanged. A transcriptome-wide analysis reveals that hundreds of mRNA isoforms targeted by nonsense-mediated decay (NMD) are upregulated. Mechanistically, recruiting CASC3 to reporter mRNAs by direct tethering or via binding to the EJC stimulates mRNA decay and endonucleolytic cleavage at the termination codon. Building on existing EJC-NMD models, we propose that CASC3 equips the EJC with the persisting ability to communicate with the NMD machinery in the cytoplasm. Collectively, our results characterize CASC3 as a peripheral EJC protein that tailors the transcriptome by promoting the degradation of EJC-dependent NMD substrates.

Abstract
The exon junction complex (EJC) is an abundant messenger ribonucleoprotein (mRNP) component that is assembled during splicing and binds to mRNAs upstream of exon-exon junctions. EJCs accompany the mRNA during its entire life in the nucleus and the cytoplasm and communicate the information about the splicing process and the position of introns. Specifically, the EJC's core components and its associated proteins regulate different steps of gene expression, including pre-mRNA splicing, mRNA export, translation, and nonsense-mediated mRNA decay (NMD). This review summarizes the most important functions and main protagonists in the life of the EJC. It also provides an overview of the latest findings on the assembly, composition and molecular activities of the EJC and presents them in the chronological order, in which they play a role in the EJC's life cycle.

Abstract
The exon junction complex (EJC) is an essential constituent and regulator of spliced messenger ribonucleoprotein particles (mRNPs) in metazoans. As a core component of the EJC, CASC3 was described to be pivotal for EJC-dependent nuclear and cytoplasmic processes. However, recent evidence suggests that CASC3 functions differently from other EJC core proteins. Here, we have established human CASC3 knockout cell lines to elucidate the cellular role of CASC3. In the knockout cells, overall EJC composition and EJC-dependent splicing are unchanged, whereas mRNA isoforms targeted by nonsense-mediated decay (NMD) are upregulated on a transcriptome-wide scale. Mechanistically, recruiting CASC3 to reporter mRNAs by direct tethering or via binding to the EJC stimulates mRNA decay and endonucleolytic cleavage at the termination codon. Building on existing EJC-NMD models, we propose that CASC3 equips the EJC with the ability to communicate with the NMD machinery in the cytoplasm. Collectively, our results characterize CASC3 as a peripheral EJC protein that tailors the transcriptome by promoting the degradation of EJC-dependent NMD substrates.

Abstract
Modification with SUMO regulates many eukaryotic proteins. Down-regulation of sumoylated forms of proteins involves either their desumoylation, and hence recycling of the unmodified form, or their proteolytic targeting by ubiquitin ligases that recognize their SUMO modification (termed STUbL or ULS). STUbL enzymes such as Uls1 and Slx5-Slx8 in budding yeast or RNF4 and Arkadia/RNF111 in humans bear multiple SUMO interaction motifs to recognize substrates carrying poly-SUMO chains. Using yeast as experimental system and isothermal titration calorimetry, we here show that Arkadia specifically selects substrates carrying SUMO1-capped SUMO2/3 hybrid conjugates and targets them for proteasomal degradation. Our data suggest that a SUMO1-specific binding site in Arkadia with sequence similarity to a SUMO1-binding site in DPP9 is required for targeting endogenous hybrid SUMO conjugates and PML nuclear bodies in human cells. We thus characterize Arkadia as a STUbL with a preference for substrate proteins marked with distinct hybrid SUMO chains.

Abstract
UBR5 is an E3 ubiquitin ligase involved in distinct processes such as transcriptional regulation and development. UBR5 is highly upregulated in embryonic stem cells (ESCs), whereas its expression decreases with differentiation, suggesting a role for UBR5 in ESC function. However, little is known about how UBR5 regulates ESC identity. Here, we define the protein interactome of UBR5 in ESCs and find interactions with distinct components of the H/ACA ribonucleoprotein complex, which is required for proper maturation of ribosomal RNA (rRNA). Notably, loss of UBR5 induces an abnormal accumulation of rRNA processing intermediates, resulting in diminished ribosomal levels. Consequently, lack of UBR5 triggers an increase in p53 levels and a concomitant decrease in cellular proliferation rates. Thus, our results indicate a link between UBR5 and rRNA maturation.

Abstract
RNA degradation ensures appropriate levels of mRNA transcripts within cells and eliminates aberrant RNAs. Detailed studies of RNA degradation dynamics have been heretofore infeasible because of the inherent instability of degradation intermediates due to the high processivity of the enzymes involved. To visualize decay intermediates and to characterize the spatiotemporal dynamics of mRNA decay, we have developed a set of methods that apply XRN1-resistant RNA sequences (xrRNAs) to protect mRNA transcripts from 5′–3′ exonucleolytic digestion. To our knowledge, this approach is the only method that can detect the directionality of mRNA degradation and that allows tracking of degradation products in unperturbed cells. Here, we provide detailed procedures for xrRNA reporter design, transfection and cell line generation. We explain how to extract xrRNA reporter mRNAs from mammalian cells, as well as their detection and quantification using northern blotting and quantitative PCR. The procedure further focuses on how to detect and quantify intact reporter mRNAs and XRN1-resistant degradation intermediates using single-molecule fluorescence microscopy. It provides detailed instructions for sample preparation and image acquisition using fixed, as well as living, cells. The procedure puts special emphasis on detailed descriptions of high-throughput image analysis pipelines, which are provided along with the article and were designed to perform spot co-localization, detection efficiency normalization and the quality control steps necessary for interpretation of results. The aim of the analysis software published here is to enable nonexpert readers to detect and quantify RNA decay intermediates within 4–6 d after reporter mRNA expression.

Abstract
Productive splicing of human precursor messenger RNAs (pre-mRNAs) requires the correct selection of authentic splice sites (SS) from the large pool of potential SS. Although SS consensus sequence and splicing regulatory proteins are known to influence SS usage, the mechanisms ensuring the effective suppression of cryptic SS are insufficiently explored. Here, we find that many aberrant exonic SS are efficiently silenced by the exon junction complex (EJC), a multi-protein complex that is deposited on spliced mRNA near the exon-exon junction. Upon depletion of EJC proteins, cryptic SS are de-repressed, leading to the mis-splicing of a broad set of mRNAs. Mechanistically, the EJC-mediated recruitment of the splicing regulator RNPS1 inhibits cryptic 5'SS usage, while the deposition of the EJC core directly masks reconstituted 3'SS, thereby precluding transcript disintegration. Thus, the EJC protects the transcriptome of mammalian cells from inadvertent loss of exonic sequences and safeguards the expression of intact, full-length mRNAs.

Abstract
During splicing, the exon junction complex (EJC) is deposited upstream of exon-exon boundaries. The EJC and its peripheral bound proteins play an essential role in mediating mRNA export, translation and turnover. However, the exact sequence of EJC assembly and the involved factors during splicing remain elusive. Recently published structures of the human C* spliceosome clarified the position of the EJC at this phase of splicing and have given insight into previously unidentified interactions between the EJC and spliceosomal proteins. Here, these new observations are presented and the significance for EJC assembly is discussed. Furthermore, the vast landscape of EJC interacting proteins and their manifold functions are described. Finally, the factors involved in EJC disassembly and recycling are recapitulated. This review aims to integrate structural, biochemical and physiological data to obtain a comprehensive picture of EJC components during the lifetime of the EJC.

Abstract
Messenger RNA (mRNA) turnover is a crucial and highly regulated step of gene expression in mammalian cells. This includes mRNA surveillance pathways such as nonsense-mediated mRNA decay (NMD), which assesses the fidelity of transcripts and eliminates mRNAs containing a premature translation termination codon (PTC). When studying mRNA degradation pathways, reporter mRNAs are commonly expressed in cultivated cells. Traditionally, the molecular mechanism of NMD has been characterized using pairs of reporter constructs that express the same mRNA with ("PTC-containing mRNA") or without ("wild-type mRNA") a PTC. Cell lines stably expressing an NMD reporter have been reported to yield very robust and highly reproducible results, but establishing the cell lines can be very time-consuming. Therefore, transient transfection of such reporter constructs is frequently used and allows analysis of many samples within a short period of time. However, the behavior of transiently and stably transfected NMD constructs has not been systematically compared so far. Here, we report that not all commonly used human cell lines degrade NMD targets following transient transfection. Furthermore, the degradation efficiency of NMD substrates can depend on the manner of transfection within the same cell line. This has substantial implications for the interpretation of NMD assays based on transient transfections.

Abstract
Nonsense-mediated mRNA decay (NMD) controls gene expression by eliminating mRNAs with premature or aberrant translation termination. Degradation of NMD substrates is initiated by the central NMD factor UPF1, which recruits the endonuclease SMG6 and the deadenylation-promoting SMG5/7 complex. The extent to which SMG5/7 and SMG6 contribute to the degradation of individual substrates and their regulation by UPF1 remains elusive. Here we map transcriptome-wide sites of SMG6-mediated endocleavage via 3' fragment capture and degradome sequencing. This reveals that endogenous transcripts can have NMD-eliciting features at various positions, including upstream open reading frames (uORFs), premature termination codons (PTCs), and long 3' UTRs. We find that NMD substrates with PTCs undergo constitutive SMG6-dependent endocleavage, rather than SMG7-dependent exonucleolytic decay. In contrast, the turnover of NMD substrates containing uORFs and long 3' UTRs involves both SMG6- and SMG7-dependent endo- and exonucleolytic decay, respectively. This suggests that the extent to which SMG6 and SMG7 degrade NMD substrates is determined by the mRNA architecture.

Abstract
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.

Abstract
The turnover of messenger RNAs (mRNAs) is a key regulatory step of gene expression in eukaryotic cells. Due to the complexity of the mammalian degradation machinery, the contribution of decay factors to the directionality of mRNA decay is poorly understood. Here we characterize a molecular tool to interrogate mRNA turnover via the detection of XRN1-resistant decay fragments (xrFrag). Using nonsense-mediated mRNA decay (NMD) as a model pathway, we establish xrFrag analysis as a robust indicator of accelerated 5'-3' mRNA decay. In tethering assays, monitoring xrFrag accumulation allows to distinguish decapping and endocleavage activities from deadenylation. Moreover, xrFrag analysis of mRNA degradation induced by miRNAs, AU-rich elements (AREs) as well as the 3' UTRs of cytokine mRNAs reveals the contribution of 5'-3' decay and endonucleolytic cleavage. Our work uncovers formerly unrecognized modes of mRNA turnover and establishes xrFrag as a powerful tool for RNA decay analyses.

Abstract
Nonsense-mediated mRNA decay (NMD) is a cellular process that eliminates messenger RNA (mRNA) substrates with premature translation termination codons (PTCs). In addition, NMD regulates the expression of a number of physiological mRNAs, for example transcripts containing long 3' UTRs. Current models implicate the interaction between cytoplasmic poly(A)-binding protein (PABPC1) and translation termination in NMD. Accordingly, PABPC1 present within close proximity of a termination codon antagonizes NMD. Here, we use reporter mRNAs with different NMD-inducing 3' UTRs to establish a general NMD-inhibiting property of PABPC1. NMD-inhibition is not limited to PABPC1, but can also be achieved by peptides consisting of the PABP-interacting motif 2 (PAM2) of different proteins when recruited to an NMD-inhibiting position of NMD reporter transcripts. The short PAM2 peptides efficiently suppress NMD activated by a long 3' UTR, an exon-junction complex (EJC) and individual EJC components, and stabilize a PTC-containing β-globin mRNA. In conclusion, our results establish short PABPC1-recruiting peptides as potent but position-dependent inhibitors of mammalian NMD.

Abstract
The exon junction complex (EJC) is an RNA-binding protein complex that is assembled and deposited onto mRNAs during splicing. The EJC comprises four core components that bind to not only canonical sites upstream of exon-exon junctions, but also to noncanonical sites at other positions in exons. EJC-associated proteins are recruited by the EJC at different steps of gene expression to execute the multiple functions of the EJC. Recently, new insights have been obtained into how EJCs stimulate pre-mRNA splicing, and mRNA export, translation, and degradation. Furthermore, mutations in EJC core components were shown to result in severe disorders in humans, demonstrating the critical physiological role of the EJC. Hence, the EJC has been identified as an important player in post-transcriptional gene regulation in metazoans.

Abstract
Nonsense-mediated messenger RNA (mRNA) decay (NMD) is a quality control mechanism that degrades irregular or faulty mRNAs. NMD mainly degrades mRNAs, which contain a premature termination codon (PTC) and therefore encode a truncated protein. Furthermore, NMD alters the expression of different types of cellular mRNAs, the so-called endogenous NMD substrates. In this review, we focus on the impact of NMD on cellular and molecular physiology. We specify key classes of NMD substrates and provide a detailed overview of the physiological function of gene regulation by NMD. We also describe different mechanisms of NMD substrate degradation and how the regulation of the NMD machinery affects cellular physiology. Finally, we outline the physiological functions of central NMD factors.

Abstract
The export of messenger RNAs (mRNAs) is the final of several nuclear posttranscriptional steps of gene expression. The formation of export-competent mRNPs involves the recruitment of export factors that are assumed to facilitate transport of the mature mRNAs. Using in vitro splicing assays, we show that a core set of export factors, including ALYREF, UAP56 and DDX39, readily associate with the spliced RNAs in an EJC (exon junction complex)- and cap-dependent manner. In order to elucidate how ALYREF and other export adaptors mediate mRNA export, we conducted a computational analysis and discovered four short, conserved, linear motifs present in RNA-binding proteins. We show that mutation in one of the new motifs (WxHD) in an unstructured region of ALYREF reduced RNA binding and abolished the interaction with eIF4A3 and CBP80. Additionally, the mutation impaired proper localization to nuclear speckles and export of a spliced reporter mRNA. Our results reveal important details of the orchestrated recruitment of export factors during the formation of export competent mRNPs.