Mechanical devices of the spliceosome: motors, clocks, springs, and things

Rearrangements within the U6 snRNA Core during the Transition between the Two Catalytic Steps of Splicing

The RNA catalytic core of spliceosomes as visualized by cryoelectron microscopy (cryo-EM) remains unchanged at different stages of splicing. However, we demonstrate that mutations within the core of yeast U6 snRNA modulate conformational changes between the two catalytic steps. We propose that the intramolecular stem-loop (ISL) of U6 exists in two competing states, changing between a default, non-catalytic conformation and a transient, catalytic conformation. Whereas stable interactions in the catalytic triplex promote catalysis and their disruptions favor exit from the catalytic conformation, destabilization of the lower ISL stem promotes catalysis and its stabilization supports exit from the catalytic conformation. Thus, in addition to the catalytic triplex, U6-ISL acts as an important dynamic component of the catalytic center. The relative flexibility of the lower U6-ISL stem is conserved across eukaryotes. Similar features are found in U6atac and domain V of group II introns, arguing for the generality of the proposed mechanism.

Early splicing functions of fission yeast Prp16 and its unexpected requirement for gene Silencing is governed by intronic features

Prp16 is a DEAH box pre-mRNA splicing factor that triggers a key spliceosome conformational switch to facilitate second step splicing in Saccharomyces cerevisiae. However, Prp16 functions are largely unexplored in Schizosaccharomyces pombe, an attractive model with exon-intron architecture more relevant to several other eukaryotes. Here, we generated mis-sense alleles in SpPrp16 whose consequences on genome-wide splicing uncover its nearly global splicing role with only a small subset of unaffected introns. Prp16 dependent and independent intron categories displayed a striking difference in the strength of intronic 5' splice site (5'SS)-U6 snRNA and branch site (BS)-U2 snRNA interactions. Selective weakening of these interactions could convert a Prp16 dependent intron into an independent one. These results point to the role of SpPrp16 in destabilizing 5'SS-U6snRNA and BS-U2snRNA interactions which plausibly trigger structural alterations in the spliceosome to facilitate first step catalysis. Our data suggest that SpPrp16 interactions with early acting factors, its enzymatic activities and association with intronic elements collectively account for efficient and accurate first step catalysis. In addition to splicing derangements in the spprp16F528S mutant, we show that SpPrp16 influences cell cycle progression and centromeric heterochromatinization. We propose that strong 5'SS-U6 snRNA and BS-U2 snRNA complementarity of intron-like elements in non-coding RNAs which lead to complete splicing arrest and impaired Seb1 functions at the pericentromeric loci may cumulatively account for the heterochromatin defects in spprp16F528S cells. These findings suggest that the diverse Prp16 functions within a genome are likely governed by its intronic features that influence splice site-snRNA interaction strength.

Intrinsically Disordered Protein Ntr2 Modulates the Spliceosomal RNA Helicase Brr2

Precursor messenger RNA splicing is mediated by the spliceosome, a large and dynamic molecular machine composed of five small nuclear RNAs and numerous proteins. Many spliceosomal proteins are predicted to be intrinsically disordered or contain large disordered regions, but experimental validation of these predictions is scarce, and the precise functions of these proteins are often unclear. Here, we show via circular dichroism spectroscopy, dynamic light scattering, and NMR spectroscopy that the yeast spliceosomal disassembly factor Ntr2 is largely intrinsically disordered. Peptide SPOT analyses, analytical size-exclusion chromatography, and surface plasmon resonance measurements revealed that Ntr2 uses an N-terminal region to bind the C-terminal helicase unit of the Brr2 RNA helicase, an enzyme involved in spliceosome activation and implicated in splicing catalysis and spliceosome disassembly. NMR analyses suggested that Ntr2 does not adopt a tertiary structure and likely remains disordered upon complex formation. RNA binding and unwinding studies showed that Ntr2 downregulates Brr2 helicase activity in vitro by modulating the fraction of helicase molecules productively bound to the RNA substrate. Our data clarify the nature of a physical link between Brr2 and Ntr2, and point to the possibility of a functional Ntr2-Brr2 interplay during splicing.

Mutagenesis of Snu114 domain IV identifies a developmental role in meiotic splicing

Snu114, a component of the U5 snRNP, plays a key role in activation of the spliceosome. It controls the action of Brr2, an RNA-stimulated ATPase/RNA helicase that disrupts U4/U6 snRNA base-pairing prior to formation of the spliceosome's catalytic centre. Snu114 has a highly conserved domain structure that resembles that of the GTPase EF-2/EF-G in the ribosome. It has been suggested that the regulatory function of Snu114 in activation of the spliceosome is mediated by its C-terminal region, however, there has been only limited characterisation of the interactions of the C-terminal domains. We show a direct interaction between protein phosphatase PP1 and Snu114 domain 'IVa' and identify sequence 'YGVQYK' as a PP1 binding motif. Interestingly, this motif is also required for Cwc21 binding. We provide evidence for mutually exclusive interaction of Cwc21 and PP1 with Snu114 and show that the affinity of Cwc21 and PP1 for Snu114 is influenced by the different nucleotide-bound states of Snu114. Moreover, we identify a novel mutation in domain IVa that, while not affecting vegetative growth of yeast cells, causes a defect in splicing transcripts of the meiotic genes, SPO22, AMA1 and MER2, thereby inhibiting an early stage of meiosis.

Interaction paths promote module integration and network-level robustness of spliceosome to cascading effects

The functionality of distinct types of protein networks depends on the patterns of protein-protein interactions. A problem to solve is understanding the fragility of protein networks to predict system malfunctioning due to mutations and other errors. Spectral graph theory provides tools to understand the structural and dynamical properties of a system based on the mathematical properties of matrices associated with the networks. We combined two of such tools to explore the fragility to cascading effects of the network describing protein interactions within a key macromolecular complex, the spliceosome. Using S. cerevisiae as a model system we show that the spliceosome network has more indirect paths connecting proteins than random networks. Such multiplicity of paths may promote routes to cascading effects to propagate across the network. However, the modular network structure concentrates paths within modules, thus constraining the propagation of such cascading effects, as indicated by analytical results from the spectral graph theory and by numerical simulations of a minimal mathematical model parameterized with the spliceosome network. We hypothesize that the concentration of paths within modules favors robustness of the spliceosome against failure, but may lead to a higher vulnerability of functional subunits, which may affect the temporal assembly of the spliceosome. Our results illustrate the utility of spectral graph theory for identifying fragile spots in biological systems and predicting their implications.

Deep Profiling of the Aggregated Proteome in Alzheimer's Disease: From Pathology to Disease Mechanisms

Hallmarks of Alzheimer’s disease (AD), a progressive neurodegenerative disease causing dementia, include protein aggregates such as amyloid beta plaques and tau neurofibrillary tangles in a patient’s brain. Understanding the complete composition and structure of protein aggregates in AD can shed light on the as-yet unidentified underlying mechanisms of AD development and progression. Biochemical isolation of aggregates coupled with mass spectrometry (MS) provides a comprehensive proteomic analysis of aggregates in AD. Dissection of these AD-specific aggregate components, such as U1 small nuclear ribonucleoprotein complex (U1 snRNP), provides novel insights into the deregulation of RNA splicing in the disease. In this review, we summarize the methodologies of laser capture microdissection (LCM) and differential extraction to analyze the aggregated proteomes in AD samples, and discuss the derived novel insights that may contribute to AD pathogenesis.

Specification of Drosophila neuropeptidergic neurons by the splicing component brr2

During embryonic development, a number of genetic cues act to generate neuronal diversity. While intrinsic transcriptional cascades are well-known to control neuronal sub-type cell fate, the target cells can also provide critical input to specific neuronal cell fates. Such signals, denoted retrograde signals, are known to provide critical survival cues for neurons, but have also been found to trigger terminal differentiation of neurons. One salient example of such target-derived instructive signals pertains to the specification of the Drosophila FMRFamide neuropeptide neurons, the Tv4 neurons of the ventral nerve cord. Tv4 neurons receive a BMP signal from their target cells, which acts as the final trigger to activate the FMRFa gene. A recent FMRFa-eGFP genetic screen identified several genes involved in Tv4 specification, two of which encode components of the U5 subunit of the spliceosome: brr2 (l(3)72Ab) and Prp8. In this study, we focus on the role of RNA processing during target-derived signaling. We found that brr2 and Prp8 play crucial roles in controlling the expression of the FMRFa neuropeptide specifically in six neurons of the VNC (Tv4 neurons). Detailed analysis of brr2 revealed that this control is executed by two independent mechanisms, both of which are required for the activation of the BMP retrograde signaling pathway in Tv4 neurons: (1) Proper axonal pathfinding to the target tissue in order to receive the BMP ligand. (2) Proper RNA splicing of two genes in the BMP pathway: the thickveins (tkv) gene, encoding a BMP receptor subunit, and the Medea gene, encoding a co-Smad. These results reveal involvement of specific RNA processing in diversifying neuronal identity within the central nervous system.

The nervous system displays daunting cellular diversity, largely generated through complex regulatory input operating on stem cells and their neural lineages during development. Most of the reported mechanisms acting to generate neural diversity pertain to transcriptional regulation. In contrast, little is known regarding the post-transcriptional mechanisms involved. Here, we use a specific group of neurons, Apterous neurons, in the ventral nerve cord of Drosophila melanogaster as our model, to analyze the function of two essential components of the spliceosome; Brr2 and Prp8. Apterous neurons require a BMP retrograde signal for terminal differentiation, and we find that brr2 and Prp8 play crucial roles during this process. brr2 is critical for two independent events; axon pathfinding and BMP signaling, both of which are required for the activation of the retrograde signaling pathway necessary for Apterous neurons. These results identify a post-transcriptional mechanism as key for specifying neuronal identity, by ensuring the execution of a retrograde signal.

Pathway Network Analysis of Complex Diseases Based on Multiple Biological Networks

Biological pathways play important roles in the development of complex diseases, such as cancers, which are multifactorial complex diseases that are usually caused by multiple disorders gene mutations or pathway. It has become one of the most important issues to analyze pathways combining multiple types of high-throughput data, such as genomics and proteomics, to understand the mechanisms of complex diseases. In this paper, we propose a method for constructing the pathway network of gene phenotype and find out disease pathogenesis pathways through the analysis of the constructed network. The specific process of constructing the network includes, firstly, similarity calculation between genes expressing data combined with phenotypic mutual information and GO ontology information, secondly, calculating the correlation between pathways based on the similarity between differential genes and constructing the pathway network, and, finally, mining critical pathways to identify diseases. Experimental results on Breast Cancer Dataset using this method show that our method is better. In addition, testing on an alternative dataset proved that the key pathways we found were more accurate and reliable as biological markers of disease. These results show that our proposed method is effective.

Post-Translational Modifications in Polypyrimidine Tract Binding Proteins PTBP1 and PTBP2

RNA binding proteins play an important role in regulating alternative pre-mRNA splicing and in turn cellular gene expression. Many of these RNA binding proteins occur as gene families with members sharing a high degree of primary structure identity and domain organization yet have tissue-specific expression patterns and regulate different sets of target exons. How highly similar members in a gene family can exert different splicing outcomes is not well understood. We conducted mass spectrometry analysis of post-translational phosphorylation and acetylation modifications for two paralogs of the polypyrimidine tract binding protein family, PTBP1 and PTBP2, to discover modifications that occur in splicing reaction mixtures and to identify discrete modifications that may direct their different splicing activities. We find that PTBP1 and PTBP2 have many distinct phosphate modifications located in the unstructured N-terminal, linker 1, and linker 2 regions. We find that the two proteins have many overlapping acetate modifications in the RNA recognition motifs (RRMs) with a few distinct sites in PTBP1 RRM2 and RRM3. Our data also reveal that lysine residues in the nuclear localization sequence of PTBP2 are acetylated. Collectively, our results highlight important differences in post-translational modifications between the paralogs and suggest a role for them in the differential splicing activity of PTBP1 and PTBP2.

Viruses Seen by Our Cells: The Role of Viral RNA Sensors

The role of the innate immune response in detecting RNA viruses is crucial for the establishment of proper inflammatory and antiviral responses. Different receptors, known as pattern recognition receptors (PRRs), are present in the cytoplasm, endosomes, and on the cellular surface. These receptors have the capacity to sense the presence of viral nucleic acids as pathogen-associated molecular patterns (PAMPs). This recognition leads to the induction of type 1 interferons (IFNs) as well as inflammatory cytokines and chemokines. In this review, we provide an overview of the significant involvement of cellular RNA helicases and Toll-like receptors (TLRs) 3, 7, and 8 in antiviral immune defenses.

Functional and Biochemical Characterization of Dib1's Role in Pre-Messenger RNA Splicing

The spliceosome is a dynamic macromolecular machine that undergoes a series of conformational rearrangements as it transitions between the several states required for accurate splicing. The transition from the B to B^act is a key part of spliceosome assembly and is defined by the departure of several proteins, including essential U5 component Dib1. Recent structural studies suggest that Dib1 has a role in preventing premature spliceosome activation, as it is positioned adjacent to the U6 snRNA ACAGAGA and the U5 loop I, but its mechanism is unknown. Our data indicate that Dib1 is a robust protein that tolerates incorporation of many mutations, even at positions thought to be key for its folding stability. However, we have identified two temperature-sensitive mutants that stall in vitro splicing prior to the first catalytic step and block assembly at the B complex. In addition, Dib1 readily exchanges in splicing extracts despite being a central component of the U5 snRNP, suggesting that the binding site of Dib1 is flexible. Structural analyses show that the overall conformation of Dib1 and the mutants are not affected by temperature, so the temperature sensitive defects most likely result from altered interactions between Dib1 and other spliceosomal components. Together, these data lead to a new understanding of Dib1's role in the B to B^act transition and provide a model for how dynamic protein-RNA interactions contribute to the correct assembly of a complex molecular machine.

Life under the Microscope: Single-Molecule Fluorescence Highlights the RNA World

The emergence of single-molecule (SM) fluorescence techniques has opened up a vast new toolbox for exploring the molecular basis of life. The ability to monitor individual biomolecules in real time enables complex, dynamic folding pathways to be interrogated without the averaging effect of ensemble measurements. In parallel, modern biology has been revolutionized by our emerging understanding of the many functions of RNA. In this comprehensive review, we survey SM fluorescence approaches and discuss how the application of these tools to RNA and RNA-containing macromolecular complexes in vitro has yielded significant insights into the underlying biology. Topics covered include the three-dimensional folding landscapes of a plethora of isolated RNA molecules, their assembly and interactions in RNA-protein complexes, and the relation of these properties to their biological functions. In all of these examples, the use of SM fluorescence methods has revealed critical information beyond the reach of ensemble averages.

Genetics of personalized medicine: cancer and rare diseases

The 21st annual meeting of the Portuguese Society of Human Genetics (SPGH), organized by Luísa Romão, Ana Sousa and Rosário Pinto Leite, was held in Caparica, Portugal, from the 16th to the 18th of November 2017. Having entered an era in which personalized medicine is emerging as a paradigm for disease diagnosis, treatment and prevention, the program of this meeting intended to include lectures by leading national and international scientists presenting exceptional findings on the genetics of personalized medicine. Various topics were discussed, including cancer genetics, transcriptome dynamics and novel therapeutics for cancers and rare disorders that are designed to specifically target molecular alterations in individual patients. Several panel discussions were held to emphasize (ethical) issues associated with personalized medicine, including genetic cancer counseling.

Imaging of pre-mRNA splicing in living subjects using a genetically encoded luciferase reporter

Pre-mRNA splicing is an essential step in gene expression in most eukaryote genes. Here we present the feasibility of a genetically encoded luciferase reporter to monitor the pre-mRNA splicing process in living cells and animals. We showed that the splicing activity change induced by isoginkgetin could be readily visualized in vitro both in a dose and time dependent manner. Moreover, the pre-mRNA splicing process could be also obviously detected in mice by bioluminescence imaging and confirmed by RT-PCR. Our work provided a reporter system that allows high-throughput screening of chemical libraries to identify potential compounds leading to aberrant patterns of splicing.

The RES complex is required for efficient transformation of the precatalytic B spliceosome into an activated Bact complex

The precise function of the trimeric retention and splicing (RES) complex in pre-mRNA splicing remains unclear. Here we dissected the role of RES during the assembly and activation of yeast spliceosomes. The efficiency of pre-mRNA splicing was significantly lower in the absence of the RES protein Snu17, and the recruitment of its binding partners, Pml1 (pre-mRNA leakage protein 1) and Bud13 (bud site selection protein 13), to the spliceosome was either abolished or substantially reduced. RES was not required for the assembly of spliceosomal B complexes, but its absence hindered efficient B^act complex formation. ΔRES spliceosomes were no longer strictly dependent on Prp2 activity for their catalytic activation, suggesting that they are structurally compromised. Addition of Prp2, Spp2, and UTP to affinity-purified ΔRES B or a mixture of B/B^act complexes formed on wild-type pre-mRNA led to their disassembly. However, no substantial disassembly was observed with ΔRES spliceosomes formed on a truncated pre-mRNA that allows Prp2 binding but blocks its activity. Thus, in the absence of RES, Prp2 appears to bind prematurely, leading to the disassembly of the ΔRES B complexes to which it binds. Our data suggest that Prp2 can dismantle B complexes with an aberrant protein composition, suggesting that it may proofread the spliceosome's RNP structure prior to activation.

Yeast Prp2 liberates the 5' splice site and the branch site adenosine for catalysis of pre-mRNA splicing

The RNA helicase Prp2 facilitates the remodeling of the spliceosomal B^act complex to the catalytically activated B* complex just before step one of splicing. As a high-resolution cryo-EM structure of the B* complex is currently lacking, the precise spliceosome remodeling events mediated by Prp2 remain poorly understood. To investigate the latter, we used chemical structure probing to compare the RNA structure of purified yeast B^act and B* complexes. Our studies reveal deviations from conventional RNA helices in the functionally important U6 snRNA internal stem-loop and U2/U6 helix Ib in the activated B^act complex, and to a lesser extent in B*. Interestingly, the N7 of U6-G60 of the catalytic triad becomes accessible to DMS modification in the B* complex, suggesting that the Hoogsteen interaction with U6-A52 is destabilized in B*. Our data show that Prp2 action does not unwind double-stranded RNA, but enhances the flexibility of the first step reactants, the pre-mRNA's 5' splice site and branch site adenosine. Prp2 therefore appears to act primarily as an RNPase to achieve catalytic activation by liberating the first step reactants in preparation for catalysis of the first step of splicing.

Slow molecular recognition by RNA

Molecular recognition is central to biological processes, function, and specificity. Proteins associate with ligands with a wide range of association rate constants, with maximal values matching the theoretical limit set by the rate of diffusional collision. As less is known about RNA association, we compiled association rate constants for all RNA/ligand complexes that we could find in the literature. Like proteins, RNAs exhibit a wide range of association rate constants. However, the fastest RNA association rates are considerably slower than those of the fastest protein associations and fall well below the diffusional limit. The apparently general observation of slow association with RNAs has implications for evolution and for modern-day biology. Our compilation highlights a quantitative molecular property that can contribute to biological understanding and underscores our need to develop a deeper physical understanding of molecular recognition events.

Cryo-electron microscopy snapshots of the spliceosome: structural insights into a dynamic ribonucleoprotein machine

The spliceosome excises introns from pre-messenger RNAs using an RNA-based active site that is cradled by a dynamic protein scaffold. A recent revolution in cryo-electron microscopy (cryo-EM) has led to near-atomic-resolution structures of key spliceosome complexes that provide insight into the mechanism of activation, splice site positioning, catalysis, protein rearrangements and ATPase-mediated dynamics of the active site. The cryo-EM structures rationalize decades of observations from genetic and biochemical studies and provide a molecular framework for future functional studies.

Mutations of RNA splicing factors in hematological malignancies

Systematic large-scale cancer genomic studies have produced numerous significant findings. These studies have not only revealed new cancer-promoting genes, but they also have identified cancer-promoting functions of previously known "housekeeping" genes. These studies have identified numerous mutations in genes which play a fundamental role in nuclear precursor mRNA splicing. Somatic mutations and copy number variation in many of the splicing factors which participate in the formation of multiple spliceosomal complexes appear to play a role in many cancers and in particular in myelodysplastic syndromes (MDS). Mutated proteins seem to interfere with the recognition of the authentic splice sites (SS) leading to utilization of suboptimal alternative splicing sites generating aberrantly spliced mRNA isoforms. This short review is focusing on the function of the splice factors involved in the formation of splicing complexes and potential mechanisms which affect usage of the authentic splice site recognition.

The histone variant H2A.Z promotes splicing of weak introns

In this study, Nissen et al. investigated the function of the highly conserved histone variant H2A.Z in pre-mRNA splicing using the intron-rich model yeast S. pombe. The findings suggest that H2A.Z occupancy promotes cotranscriptional splicing of suboptimal introns that may otherwise be discarded via proofreading ATPases.

Multiple lines of evidence implicate chromatin in the regulation of premessenger RNA (pre-mRNA) splicing. However, the influence of chromatin factors on cotranscriptional splice site usage remains unclear. Here we investigated the function of the highly conserved histone variant H2A.Z in pre-mRNA splicing using the intron-rich model yeast Schizosaccharomyces pombe. Using epistatic miniarray profiles (EMAPs) to survey the genetic interaction landscape of the Swr1 nucleosome remodeling complex, which deposits H2A.Z, we uncovered evidence for functional interactions with components of the spliceosome. In support of these genetic connections, splicing-specific microarrays show that H2A.Z and the Swr1 ATPase are required during temperature stress for the efficient splicing of a subset of introns. Notably, affected introns are enriched for H2A.Z occupancy and more likely to contain nonconsensus splice sites. To test the significance of the latter correlation, we mutated the splice sites in an affected intron to consensus and found that this suppressed the requirement for H2A.Z in splicing of that intron. These data suggest that H2A.Z occupancy promotes cotranscriptional splicing of suboptimal introns that may otherwise be discarded via proofreading ATPases. Consistent with this model, we show that overexpression of splicing ATPase Prp16 suppresses both the growth and splicing defects seen in the absence of H2A.Z.

Identification of a 35S U4/U6.U5 tri-small nuclear ribonucleoprotein (tri-snRNP) complex intermediate in spliceosome assembly

The de novo assembly and post-splicing reassembly of the U4/U6.U5 tri-snRNP remain to be investigated. We report here that ZIP, a protein containing a CCCH-type zinc finger and a G-patch domain, as characterized by us previously, regulates pre-mRNA splicing independent of RNA binding. We found that ZIP physically associates with the U4/U6.U5 tri-small nuclear ribonucleoprotein (tri-snRNP). Remarkably, the ZIP-containing tri-snRNP, which has a sedimentation coefficient of ∼35S, is a tri-snRNP that has not been described previously. We also found that the 35S tri-snRNP contains hPrp24, indicative of a state in which the U4/U6 di-snRNP is integrating with the U5 snRNP. We found that the 35S tri-snRNP is enriched in the Cajal body, indicating that it is an assembly intermediate during 25S tri-snRNP maturation. We showed that the 35S tri-snRNP also contains hPrp43, in which ATPase/RNA helicase activities are stimulated by ZIP. Our study identified, for the first time, a tri-snRNP intermediate, shedding new light on the de novo assembly and recycling of the U4/U6.U5 tri-snRNP.

Quantitative tests of a reconstitution model for RNA folding thermodynamics and kinetics

Decades of study of the architecture and function of structured RNAs have led to the perspective that RNA tertiary structure is modular, made of locally stable domains that retain their structure across RNAs. We formalize a hypothesis inspired by this modularity-that RNA folding thermodynamics and kinetics can be quantitatively predicted from separable energetic contributions of the individual components of a complex RNA. This reconstitution hypothesis considers RNA tertiary folding in terms of ΔG_align, the probability of aligning tertiary contact partners, and ΔG_tert, the favorable energetic contribution from the formation of tertiary contacts in an aligned state. This hypothesis predicts that changes in the alignment of tertiary contacts from different connecting helices and junctions (ΔG_HJH) or from changes in the electrostatic environment (ΔG_+/-) will not affect the energetic perturbation from a mutation in a tertiary contact (ΔΔG_tert). Consistent with these predictions, single-molecule FRET measurements of folding of model RNAs revealed constant ΔΔG_tert values for mutations in a tertiary contact embedded in different structural contexts and under different electrostatic conditions. The kinetic effects of these mutations provide further support for modular behavior of RNA elements and suggest that tertiary mutations may be used to identify rate-limiting steps and dissect folding and assembly pathways for complex RNAs. Overall, our model and results are foundational for a predictive understanding of RNA folding that will allow manipulation of RNA folding thermodynamics and kinetics. Conversely, the approaches herein can identify cases where an independent, additive model cannot be applied and so require additional investigation.

Structural studies of the endogenous spliceosome - The supraspliceosome

Pre-mRNA splicing is executed in mammalian cell nuclei within a huge (21MDa) and highly dynamic molecular machine - the supraspliceosome - that individually package pre-mRNA transcripts of different sizes and number of introns into complexes of a unique structure, indicating their universal nature. Detailed structural analysis of this huge and complex structure requires a stepwise approach using hybrid methods. Structural studies of the supraspliceosome by room temperature electron tomography, cryo-electron tomography, and scanning transmission electron microscope mass measurements revealed that it is composed of four native spliceosomes, each resembling an in vitro assembled spliceosome, which are connected by the pre-mRNA. It also elucidated the arrangement of the native spliceosomes within the intact supraspliceosome. Native spliceosomes and supraspliceosomes contain all five spliceosomal U snRNPs together with other splicing factors, and are active in splicing. The structure of the native spliceosome, at a resolution of 20Å, was determined by cryo-electron microscopy, and a unique spatial arrangement of the spliceosomal U snRNPs within the native spliceosome emerged from in silico studies. The supraspliceosome also harbor components for all pre-mRNA processing activities. Thus the supraspliceosome - the endogenous spliceosome - is a stand-alone complete macromolecular machine capable of performing splicing, alternative splicing, and encompass all nuclear pre-mRNA processing activities that the pre-mRNA has to undergo before it can exit from the nucleus to the cytoplasm to encode for protein. Further high-resolution cryo-electron microscopy studies of the endogenous spliceosome are required to decipher the regulation of alternative splicing, and elucidate the network of processing activities within it.

The role of alternative splicing coupled to nonsense-mediated mRNA decay in human disease

Alternative pre-mRNA splicing (AS) affects gene expression as it generates proteome diversity. Nonsense-mediated mRNA decay (NMD) is a surveillance pathway that recognizes and selectively degrades mRNAs carrying premature translation-termination codons (PTCs), preventing the production of truncated proteins that could result in disease. Several studies have also implicated NMD in the regulation of steady-state levels of physiological mRNAs. In addition, it is known that several regulated AS events do not lead to generation of protein products, as they lead to transcripts that carry PTCs and thus, they are committed to NMD. Indeed, an estimated one-third of naturally occurring, alternatively spliced mRNAs is targeted for NMD, being AS coupled to NMD (AS-NMD) an efficient strategy to regulate gene expression. In this review, we will focus on how AS mechanism operates and how can be coupled to NMD to fine-tune gene expression levels. Furthermore, we will demonstrate the physiological significance of the interplay among AS and NMD in human disease, such as cancer and neurological disorders. The understanding of how AS-NMD orchestrates expression of vital genes is of utmost importance for the advance in diagnosis, prognosis and treatment of many human disorders.

The Spliceosome: A Protein-Directed Metalloribozyme

Pre-mRNA splicing is executed by the ribonucleoprotein machinery spliceosome. Nearly 40 years after the discovery of pre-mRNA splicing, the atomic structure of the spliceosome has finally come to light. Four distinct conformational states of the yeast spliceosome have been captured at atomic or near-atomic resolutions. Two catalytic metal ions at the active site are specifically coordinated by the U6 small nuclear RNA (snRNA) and catalyze both the branching reaction and the exon ligation. Of the three snRNAs in the fully assembled spliceosome, U5 and U6, along with 30 contiguous nucleotides of U2 at its 5'-end, remain structurally rigid throughout the splicing reaction. The rigidity of these RNA elements is safeguarded by Prp8 and 16 core protein components, which maintain the same overall conformation in all structurally characterized spliceosomes during the splicing reaction. Only the sequences downstream of nucleotide 30 of U2 snRNA are mobile; their movement, directed by the protein components, delivers the intron branch site into the close proximity of the 5'-splice site for the branching reaction. A set of additional structural rearrangement is required for exon ligation, and the lariat junction is moved out of the active site for recruitment of the 3'-splice site and 3'-exon. The spliceosome is proven to be a protein-directed metalloribozyme.

Error-Prone Splicing Controlled by the Ubiquitin Relative Hub1

Accurate pre-mRNA splicing is needed for correct gene expression and relies on faithful splice site recognition. Here, we show that the ubiquitin-like protein Hub1 binds to the DEAD-box helicase Prp5, a key regulator of early spliceosome assembly, and stimulates its ATPase activity thereby enhancing splicing and relaxing fidelity. High Hub1 levels enhance splicing efficiency but also cause missplicing by tolerating suboptimal splice sites and branchpoint sequences. Notably, Prp5 itself is regulated by a Hub1-dependent negative feedback loop. Since Hub1-mediated splicing activation induces cryptic splicing of Prp5, it also represses Prp5 protein levels and thus curbs excessive missplicing. Our findings indicate that Hub1 mediates enhanced, but error-prone splicing, a mechanism that is tightly controlled by a feedback loop of PRP5 cryptic splicing activation.

Gene of the month: PRPF31

Pre-mRNA splicing is an essential process in eukaryotic cells where the transcribed intronic sequences are removed, prior to translation into protein. PRPF31 is a ubiquitously expressed splicing factor, which aids in the assembly of the macromolecular spliceosome. Mutations in PRPF31 cause autosomal dominant retinitis pigmentosa (adRP), a form of retinal degeneration that causes progressive visual impairment. Interestingly, mutations in PRPF31 are non-penetrant, with some mutation carriers being phenotypically unaffected. In this review, the gene organisation, protein structure and biological function of PRPF31 are discussed, and the mechanisms of non-penetrance in PRPF31-associated adRP are discussed.

Radical probing of spliceosome assembly

Here we describe the synthesis and use of a directed hydroxyl radical probe, tethered to a pre-mRNA substrate, to map the structure of this substrate during the spliceosome assembly process. These studies indicate an early organization and proximation of conserved pre-mRNA sequences during spliceosome assembly. This methodology may be adapted to the synthesis of a wide variety of modified RNAs for use as probes of RNA structure and RNA-protein interaction.

Discovery of Allosteric Inhibitors Targeting the Spliceosomal RNA Helicase Brr2

Brr2 is an RNA helicase belonging to the Ski2-like subfamily and an essential component of spliceosome. Brr2 catalyzes an ATP-dependent unwinding of the U4/U6 RNA duplex, which is a critical step for spliceosomal activation. An HTS campaign using an RNA-dependent ATPase assay and initial SAR study identified two different Brr2 inhibitors, 3 and 12. Cocrystal structures revealed 3 binds to an unexpected allosteric site between the C-terminal and the N-terminal helicase cassettes, while 12 binds an RNA-binding site inside the N-terminal cassette. Selectivity profiling indicated the allosteric inhibitor 3 is more Brr2-selective than the RNA site binder 12. Chemical optimization of 3 using SBDD culminated in the discovery of the potent and selective Brr2 inhibitor 9 with helicase inhibitory activity. Our findings demonstrate an effective strategy to explore selective inhibitors for helicases, and 9 could be a promising starting point for exploring molecular probes to elucidate biological functions and the therapeutic relevance of Brr2.

quatre-quart1 is an indispensable U12 intron-containing gene that plays a crucial role in Arabidopsis development

Despite increasing understanding of the importance of the splicing of U12-type introns in plant development, the key question of which U12 intron-containing genes are essential for plant development has not yet been explored. Here, we assessed the functional role of the quatre-quart1 (QQT1) gene, one of the ~230 U12 intron-containing genes in Arabidopsis thaliana. Expression of QQT1 in the U11/U12-31K small nuclear ribonucleoprotein mutant (31k) rescued the developmental-defect phenotypes of the 31k mutant, whereas the miRNA-mediated qqt1 knockdown mutants displayed severe defects in growth and development, including severely arrested stem growth, small size, and the formation of serrated leaves. The structures of the shoot apical meristems in the qqt1 mutants were abnormal and disordered. Identification of QQT1-interacting proteins via a yeast two-hybrid screening and a firefly luciferase complementation-imaging assay revealed that a variety of proteins, including many chloroplast-targeted proteins, interacted with QQT1. Importantly, the levels of chloroplast-targeted proteins in the chloroplast were reduced, and the chloroplast structure was abnormal in the qqt1 mutant. Collectively, these results provide clear evidence that QQT1 is an indispensable U12 intron-containing gene whose correct splicing is crucial for the normal development of Arabidopsis.

A central role of Cwc25 in spliceosome dynamics during the catalytic phase of pre-mRNA splicing

Splicing of precursor mRNA occurs via two consecutive steps of transesterification reaction; both require ATP and several proteins. Despite the energy requirement in the catalytic phase, incubation of the purified spliceosome under proper ionic conditions can elicit competitive reversible transesterification, debranching, and spliced-exon-reopening reactions without the necessity for ATP or other factors, suggesting that small changes in the conformational state of the spliceosome can lead to disparate chemical consequences for the substrate. We show here that Cwc25 plays a central role in modulating the conformational state of the catalytic spliceosome during normal splicing reactions. Cwc25 binds tightly to the spliceosome after the reaction and is then removed from the spliceosome, which normally requires DExD/H-box protein Prp16 and ATP hydrolysis, to allow the occurrence of the second reaction. When deprived of Cwc25, the purified first-step spliceosome catalyzes both forward and reverse splicing reactions under normal splicing conditions without requiring energy. Both reactions are inhibited when Cwc25 is added back, presumably due to the stabilization of first-step conformation. Prp16 is dispensable for the second reaction when splicing is carried out under conditions that destabilize Cwc25. We also show that the purified precatalytic spliceosome can catalyze two steps of the reaction at a low efficiency without requiring Cwc25, Slu7, or Prp18 when incubated under proper conditions. Our study reveals conformational modulation of the spliceosome by Cwc25 and Prp16 in stabilization and destabilization of first-step conformation, respectively, to facilitate the splicing process.

Identification of a small molecule inhibitor that stalls splicing at an early step of spliceosome activation

Small molecule inhibitors of pre-mRNA splicing are important tools for identifying new spliceosome assembly intermediates, allowing a finer dissection of spliceosome dynamics and function. Here, we identified a small molecule that inhibits human pre-mRNA splicing at an intermediate stage during conversion of pre-catalytic spliceosomal B complexes into activated B^act complexes. Characterization of the stalled complexes (designated B⁰²⁸) revealed that U4/U6 snRNP proteins are released during activation before the U6 Lsm and B-specific proteins, and before recruitment and/or stable incorporation of Prp19/CDC5L complex and other B^act complex proteins. The U2/U6 RNA network in B⁰²⁸ complexes differs from that of the B^act complex, consistent with the idea that the catalytic RNA core forms stepwise during the B to B^act transition and is likely stabilized by the Prp19/CDC5L complex and related proteins. Taken together, our data provide new insights into the RNP rearrangements and extensive exchange of proteins that occurs during spliceosome activation.

DOI:http://dx.doi.org/10.7554/eLife.23533.001

Genetics and biochemistry remain essential in the structural era of the spliceosome

The spliceosome is not a single macromolecular machine. Rather it is a collection of dynamic heterogeneous subcomplexes that rapidly interconvert throughout the course of a typical splicing cycle. Because of this, for many years the only high resolution structures of the spliceosome available were of smaller, isolated protein or RNA components. Consequently much of our current understanding of the spliceosome derives from biochemical and genetic techniques. Now with the publication of multiple, high resolution structures of the spliceosome, some question the relevance of traditional biochemical and genetic techniques to the splicing field. We argue such techniques are not only relevant, but vital for an in depth mechanistic understanding of pre-mRNA splicing.

SF3B1/Hsh155 HEAT motif mutations affect interaction with the spliceosomal ATPase Prp5, resulting in altered branch site selectivity in pre-mRNA splicing

Mutations in the U2 snRNP component SF3B1 are prominent in myelodysplastic syndromes (MDSs) and other cancers and have been shown recently to alter branch site (BS) or 3' splice site selection in splicing. However, the molecular mechanism of altered splicing is not known. We show here that hsh155 mutant alleles in Saccharomyces cerevisiae, counterparts of SF3B1 mutations frequently found in cancers, specifically change splicing of suboptimal BS pre-mRNA substrates. We found that Hsh155p interacts directly with Prp5p, the first ATPase that acts during spliceosome assembly, and localized the interacting regions to HEAT (Huntingtin, EF3, PP2A, and TOR1) motifs in SF3B1 associated with disease mutations. Furthermore, we show that mutations in these motifs from both human disease and yeast genetic screens alter the physical interaction with Prp5p, alter branch region specification, and phenocopy mutations in Prp5p. These and other data demonstrate that mutations in Hsh155p and Prp5p alter splicing because they change the direct physical interaction between Hsh155p and Prp5p. This altered physical interaction results in altered loading (i.e., "fidelity") of the BS-U2 duplex into the SF3B complex during prespliceosome formation. These results provide a mechanistic framework to explain the consequences of intron recognition and splicing of SF3B1 mutations found in disease.

Structure of a yeast step II catalytically activated spliceosome

Each cycle of precursor messenger RNA (pre-mRNA) splicing comprises two sequential reactions, first freeing the 5' exon and generating an intron lariat-3' exon and then ligating the two exons and releasing the intron lariat. The second reaction is executed by the step II catalytically activated spliceosome (known as the C* complex). Here, we present the cryo-electron microscopy structure of a C* complex from Saccharomyces cerevisiae at an average resolution of 4.0 angstroms. Compared with the preceding spliceosomal complex (C complex), the lariat junction has been translocated by 15 to 20 angstroms to vacate space for the incoming 3'-exon sequences. The step I splicing factors Cwc25 and Yju2 have been dissociated from the active site. Two catalytic motifs from Prp8 (the 1585 loop and the β finger of the ribonuclease H-like domain), along with the step II splicing factors Prp17 and Prp18 and other surrounding proteins, are poised to assist the second transesterification. These structural features, together with those reported for other spliceosomal complexes, yield a near-complete mechanistic picture on the splicing cycle.

Interplay of cis- and trans-regulatory mechanisms in the spliceosomal RNA helicase Brr2

RNA helicase Brr2 is implicated in multiple phases of pre-mRNA splicing and thus requires tight regulation. Brr2 can be auto-inhibited via a large N-terminal region folding back onto its helicase core and auto-activated by a catalytically inactive C-terminal helicase cassette. Furthermore, it can be regulated in trans by the Jab1 domain of the Prp8 protein, which can inhibit Brr2 by intermittently inserting a C-terminal tail in the enzyme's RNA-binding tunnel or activate the helicase after removal of this tail. Presently it is unclear, whether these regulatory mechanisms functionally interact and to which extent they are evolutionarily conserved. Here, we report crystal structures of Saccharomyces cerevisiae and Chaetomium thermophilum Brr2-Jab1 complexes, demonstrating that Jab1-based inhibition of Brr2 presumably takes effect in all eukaryotes but is implemented via organism-specific molecular contacts. Moreover, the structures show that Brr2 auto-inhibition can act in concert with Jab1-mediated inhibition, and suggest that the N-terminal region influences how the Jab1 C-terminal tail interacts at the RNA-binding tunnel. Systematic RNA binding and unwinding studies revealed that the N-terminal region and the Jab1 C-terminal tail specifically interfere with accommodation of double-stranded and single-stranded regions of an RNA substrate, respectively, mutually reinforcing each other. Additionally, such analyses show that regulation based on the N-terminal region requires the presence of the inactive C-terminal helicase cassette. Together, our results outline an intricate system of regulatory mechanisms, which control Brr2 activities during snRNP assembly and splicing.

The RNA helicase Mtr4p is a duplex-sensing translocase

The conserved Saccharomyces cerevisiae Ski2-like RNA helicase Mtr4p plays essential roles in eukaryotic nuclear RNA processing. RNA helicase activity of Mtr4p is critical for biological functions of the enzyme, but the molecular basis for RNA unwinding is not understood. Here, single-molecule high-resolution optical trapping measurements reveal that Mtr4p unwinds RNA duplexes by 3'-to-5' translocation on the loading strand, that strand separation occurs in discrete steps of 6 base pairs and that a single Mtr4p molecule performs consecutive unwinding steps. We further show that RNA unwinding by Mtr4p requires interaction with upstream RNA duplex. Inclusion of Mtr4p within the TRAMP complex increases the rate constant for unwinding initiation but does not change the characteristics of Mtr4p's helicase mechanism. Our data indicate that Mtr4p utilizes a previously unknown unwinding mode that combines aspects of canonical translocating helicases and non-canonical duplex-sensing helicases, thereby restricting directional translocation to duplex regions.

Functions and regulation of the Brr2 RNA helicase during splicing

Pre-mRNA splicing entails the stepwise assembly of an inactive spliceosome, its catalytic activation, splicing catalysis and spliceosome disassembly. Transitions in this reaction cycle are accompanied by compositional and conformational rearrangements of the underlying RNA-protein interaction networks, which are driven and controlled by 8 conserved superfamily 2 RNA helicases. The Ski2-like helicase, Brr2, provides the key remodeling activity during spliceosome activation and is additionally implicated in the catalytic and disassembly phases of splicing, indicating that Brr2 needs to be tightly regulated during splicing. Recent structural and functional analyses have begun to unravel how Brr2 regulation is established via multiple layers of intra- and inter-molecular mechanisms. Brr2 has an unusual structure, including a long N-terminal region and a catalytically inactive C-terminal helicase cassette, which can auto-inhibit and auto-activate the enzyme, respectively. Both elements are essential, also serve as protein-protein interaction devices and the N-terminal region is required for stable Brr2 association with the tri-snRNP, tri-snRNP stability and retention of U5 and U6 snRNAs during spliceosome activation in vivo. Furthermore, a C-terminal region of the Prp8 protein, comprising consecutive RNase H-like and Jab1/MPN-like domains, can both up- and down-regulate Brr2 activity. Biochemical studies revealed an intricate cross-talk among the various cis- and trans-regulatory mechanisms. Comparison of isolated Brr2 to electron cryo-microscopic structures of yeast and human U4/U6•U5 tri-snRNPs and spliceosomes indicates how some of the regulatory elements exert their functions during splicing. The various modulatory mechanisms acting on Brr2 might be exploited to enhance splicing fidelity and to regulate alternative splicing.

Putting Humpty-Dumpty Together: Clustering the Functional Dynamics of Single Biomolecular Machines Such as the Spliceosome

The spliceosome is a biomolecular machine that, in all eukaryotes, accomplishes site-specific splicing of introns from precursor messenger RNAs (pre-mRNAs) with high fidelity. Operating at the nanometer scale, where inertia and friction have lost the dominant role they play in the macroscopic realm, the spliceosome is highly dynamic and assembles its active site around each pre-mRNA anew. To understand the structural dynamics underlying the molecular motors, clocks, and ratchets that achieve functional accuracy in the yeast spliceosome (a long-standing model system), we have developed single-molecule fluorescence resonance energy transfer (smFRET) approaches that report changes in intra- and intermolecular interactions in real time. Building on our work using hidden Markov models (HMMs) to extract kinetic and conformational state information from smFRET time trajectories, we recognized that HMM analysis of individual state transitions as independent stochastic events is insufficient for a biomolecular machine as complex as the spliceosome. In this chapter, we elaborate on the recently developed smFRET-based Single-Molecule Cluster Analysis (SiMCAn) that dissects the intricate conformational dynamics of a pre-mRNA through the splicing cycle in a model-free fashion. By leveraging hierarchical clustering techniques developed for Bioinformatics, SiMCAn efficiently analyzes large datasets to first identify common molecular behaviors. Through a second level of clustering based on the abundance of dynamic behaviors exhibited by defined functional intermediates that have been stalled by biochemical or genetic tools, SiMCAn then efficiently assigns pre-mRNA FRET states and transitions to specific splicing complexes, with the potential to find heretofore undescribed conformations. SiMCAn thus arises as a general tool to analyze dynamic cellular machines more broadly.

ATPγS stalls splicing after B complex formation but prior to spliceosome activation

The ATP analog ATPγS inhibits pre-mRNA splicing in vitro, but there have been conflicting reports as to which step of splicing is inhibited by this small molecule and its inhibitory mechanism remains unclear. Here we have dissected the effect of ATPγS on pre-mRNA splicing in vitro. Addition of ATPγS to splicing extracts depleted of ATP inhibited both catalytic steps of splicing. At ATPγS concentrations ≥0.5 mM, precatalytic B complexes accumulate, demonstrating a block prior to or during the spliceosome activation stage. Affinity purification of the ATPγS-stalled B complexes (B(ATPγS)) and subsequent characterization of their abundant protein components by 2D gel electrophoresis revealed that B(ATPγS) complexes are compositionally more homogeneous than B complexes previously isolated in the presence of ATP. In particular, they contain little or no Prp19/CDC5L complex proteins, indicating that these proteins are recruited after assembly of the precatalytic spliceosome. Under the electron microscope, B(ATPγS) complexes exhibit a morphology highly similar to B complexes, indicating that the ATPγS-induced block in the transformation of the B to B(act) complex is not due to a major structural defect. Likely mechanisms whereby ATPγS blocks spliceosome assembly at the activation stage, including inhibition of the RNA helicase Brr2, are discussed. Given their more homogeneous composition, B complexes stalled by ATPγS may prove highly useful for both functional and structural analyses of the precatalytic spliceosome and its conversion into an activated B(act) spliceosomal complex.

Lights, camera, action! Capturing the spliceosome and pre-mRNA splicing with single-molecule fluorescence microscopy

The process of removing intronic sequences from a precursor to messenger RNA (pre-mRNA) to yield a mature mRNA transcript via splicing is an integral step in eukaryotic gene expression. Splicing is carried out by a cellular nanomachine called the spliceosome that is composed of RNA components and dozens of proteins. Despite decades of study, many fundamentals of spliceosome function have remained elusive. Recent developments in single-molecule fluorescence microscopy have afforded new tools to better probe the spliceosome and the complex, dynamic process of splicing by direct observation of single molecules. These cutting-edge technologies enable investigators to monitor the dynamics of specific splicing components, whole spliceosomes, and even cotranscriptional splicing within living cells. WIREs RNA 2016, 7:683-701. doi: 10.1002/wrna.1358 For further resources related to this article, please visit the WIREs website.

An exon skipping in a SEPALLATA-Like gene is associated with perturbed floral and fruits development in cucumber

We isolated a mutant showing perturbations in the development of male and female floral organs and fruits. Analysis of the single nucleotide polymorphisms from bulked F2 pools identified the causative variant occurring in Csa4G126690. Csa4G126690 shows high homology to Arabidopsis SEPALLATA2 (SEP2) thus being designated CsSEP2. The causative variant was located on the splicing site of CsSEP2, resulting in the skipping of exon 6 and abolishment of the transcriptional activity. Our data suggest that CsSEP2 is involved in the floral organ and fruits development by conferring transcriptional activity.

Molecular architecture of the Saccharomyces cerevisiae activated spliceosome

The activated spliceosome (B^act) is in a catalytically inactive state and is remodeled into a catalytically active machine by the RNA helicase Prp2, but the mechanism is unclear. Here, we describe a 3D electron cryomicroscopy structure of the Saccharomyces cerevisiae B^act complex at 5.8-angstrom resolution. Our model reveals that in B^act, the catalytic U2/U6 RNA-Prp8 ribonucleoprotein core is already established, and the 5' splice site (ss) is oriented for step 1 catalysis but occluded by protein. The first-step nucleophile-the branchsite adenosine-is sequestered within the Hsh155 HEAT domain and is held 50 angstroms away from the 5'ss. Our structure suggests that Prp2 adenosine triphosphatase-mediated remodeling leads to conformational changes in Hsh155's HEAT domain that liberate the first-step reactants for catalysis.

U6atac snRNA stem-loop interacts with U12 p65 RNA binding protein and is functionally interchangeable with the U12 apical stem-loop III

Formation of catalytic core of the U12-dependent spliceosome involves U6atac and U12 interaction with the 5′ splice site and branch site regions of a U12-dependent intron, respectively. Beyond the formation of intermolecular helix I region between U6atac and U12 snRNAs, several other regions within these RNA molecules are predicted to form stem-loop structures. Our previous work demonstrated that the 3′ stem-loop region of U6atac snRNA contains a U12-dependent spliceosome-specific targeting activity. Here, we show a detailed structure-function analysis and requirement of a substructure of U6atac 3′ stem-loop in U12-dependent in vivo splicing. We show that the C-terminal RNA recognition motif of p65, a U12 snRNA binding protein, also binds to the distal 3′ stem-loop of U6atac. By using a binary splice site mutation suppressor assay we demonstrate that p65 protein-binding apical stem-loop of U12 snRNA can be replaced by this U6atac distal 3′ stem-loop. Furthermore, we tested the compatibility of the U6atac 3′ end from phylogenetically distant species in a human U6atac background, to establish the evolutionary relatedness of these structures and in vivo function. In summary, we demonstrate that RNA-RNA and RNA-protein interactions in the minor spliceosome are highly plastic as compared to the major spliceosome.