The "spliceosome": yeast pre-messenger RNA associates with a 40S complex in a splicing-dependent reaction

Structures of aberrant spliceosome intermediates on their way to disassembly

Intron removal during pre-mRNA splicing is of extraordinary complexity and its disruption causes a vast number of genetic diseases in humans. While key steps of the canonical spliceosome cycle have been revealed by combined structure-function analyses, structural information on an aberrant spliceosome committed to premature disassembly is not available. Here, we report two cryo-electron microscopy structures of post-Bact spliceosome intermediates from Schizosaccharomyces pombe primed for disassembly. We identify the DEAH-box helicase-G-patch protein pair (Gih35-Gpl1, homologous to human DHX35-GPATCH1) and show how it maintains catalytic dormancy. In both structures, Gpl1 recognizes a remodeled active site introduced by an overstabilization of the U5 loop I interaction with the 5' exon leading to a single-nucleotide insertion at the 5' splice site. Remodeling is communicated to the spliceosome surface and the Ntr1 complex that mediates disassembly is recruited. Our data pave the way for a targeted analysis of splicing quality control.

Defenders of the Transcriptome: Guard Protein-Mediated mRNA Quality Control in Saccharomyces cerevisiae

Cell survival depends on precise gene expression, which is controlled sequentially. The guard proteins surveil mRNAs from their synthesis in the nucleus to their translation in the cytoplasm. Although the proteins within this group share many similarities, they play distinct roles in controlling nuclear mRNA maturation and cytoplasmic translation by supporting the degradation of faulty transcripts. Notably, this group is continuously expanding, currently including the RNA-binding proteins Npl3, Gbp2, Hrb1, Hrp1, and Nab2 in Saccharomyces cerevisiae. Some of the human serine-arginine (SR) splicing factors (SRSFs) show remarkable similarities to the yeast guard proteins and may be considered as functional homologues. Here, we provide a comprehensive summary of their crucial mRNA surveillance functions and their implications for cellular health.

Study of the RNA splicing kinetics via in vivo 5-EU labeling

Splicing is an important step of gene expression in all eukaryotes. Splice sites might be used with different efficiency, giving rise to alternative splicing products. At the same time, splice sites might be used at a variable rate. We used 5-ethynyl uridine labeling to sequence a nascent transcriptome of HeLa cells and deduced the rate of splicing for each donor and acceptor splice site. The following correlation analysis showed a correspondence of primary transcript features with the rate of splicing. Some dependencies we revealed were anticipated, such as a splicing rate decrease with a decreased complementarity of the donor splice site to U1 and acceptor sites to U2 snRNAs. Other dependencies were more surprising, like a negative influence of a distance to the 5' end on the rate of the acceptor splicing site utilization, or the differences in splicing rate between long, short, and RBM17-dependent introns. We also observed a deceleration of last intron splicing with an increase of the distance to the poly(A) site, which might be explained by the cooperativity of the splicing and polyadenylation. Additional analysis of splicing kinetics of SF3B4 knockdown cells suggested the impairment of a U2 snRNA recognition step. As a result, we deconvoluted the effects of several examined features on the splicing rate into a single regression model. The data obtained here are useful for further studies in the field, as they provide general splicing rate dependencies as well as help to justify the existence of slowly removed splice sites.

THUMPD2 catalyzes the N2-methylation of U6 snRNA of the spliceosome catalytic center and regulates pre-mRNA splicing and retinal degeneration

The mechanisms by which the relatively conserved spliceosome manages the enormously large number of splicing events that occur in humans (∼200 000 versus ∼300 in yeast) are poorly understood. Here, we show deposition of one RNA modification-N2-methylguanosine (m2G) on the G72 of U6 snRNA (the catalytic center of the spliceosome) promotes efficient pre-mRNA splicing activity in human cells. This modification was identified to be conserved among vertebrates. Further, THUMPD2 was demonstrated as the methyltransferase responsible for U6 m2G72 by explicitly recognizing the U6-specific sequences and structural elements. The knock-out of THUMPD2 eliminated U6 m2G72 and impaired the pre-mRNA splicing activity, resulting in thousands of changed alternative splicing events of endogenous pre-mRNAs in human cells. Notably, the aberrantly spliced pre-mRNA population elicited the nonsense-mediated mRNA decay pathway. We further show that THUMPD2 was associated with age-related macular degeneration and retinal function. Our study thus demonstrates how an RNA epigenetic modification of the major spliceosome regulates global pre-mRNA splicing and impacts physiology and disease.

Loss-of-function mutation in PRMT9 causes abnormal synapse development by dysregulation of RNA alternative splicing

Protein arginine methyltransferase 9 (PRMT9) is a recently identified member of the PRMT family, yet its biological function remains largely unknown. Here, by characterizing an intellectual disability associated PRMT9 mutation (G189R) and establishing a Prmt9 conditional knockout (cKO) mouse model, we uncover an important function of PRMT9 in neuronal development. The G189R mutation abolishes PRMT9 methyltransferase activity and reduces its protein stability. Knockout of Prmt9 in hippocampal neurons causes alternative splicing of ~1900 genes, which likely accounts for the aberrant synapse development and impaired learning and memory in the Prmt9 cKO mice. Mechanistically, we discover a methylation-sensitive protein-RNA interaction between the arginine 508 (R508) of the splicing factor 3B subunit 2 (SF3B2), the site that is exclusively methylated by PRMT9, and the pre-mRNA anchoring site, a cis-regulatory element that is critical for RNA splicing. Additionally, using human and mouse cell lines, as well as an SF3B2 arginine methylation-deficient mouse model, we provide strong evidence that SF3B2 is the primary methylation substrate of PRMT9, thus highlighting the conserved function of the PRMT9/SF3B2 axis in regulating pre-mRNA splicing.

Photochemically Controlled Release of the Glucose Transporter 1 Inhibitor for Glucose Deprivation Responses and Cancer Suppression Research

Cancer cells need a greater supply of glucose mainly due to their aerobic glycolysis, known as the Warburg effect. Glucose transport by glucose transporter 1 (GLUT1) is the rate-limiting step for glucose uptake, making it a potential cancer therapeutic target. However, GLUT1 is widely expressed and performs crucial functions in a variety of cells, and its indiscriminate inhibition will cause serious side effects. In this study, we designed and synthesized a photocaged GLUT1 inhibitor WZB117-PPG to suppress the growth of cancer cells in a spatiotemporally controllable manner. WZB117-PPG exhibited remarkable photolysis efficiency and substantial cytotoxicity toward cancer cells under visible light illumination with minimal side effects, ensuring its safety as a potential cancer therapy. Furthermore, our quantitative proteomics data delineated a comprehensive portrait of responses in cancer cells under glucose deprivation, underlining the mechanism of cell death via necrosis rather than apoptosis. We reason that our study provides a potentially reliable cancer treatment strategy and can be used as a spatiotemporally controllable trigger for studying nutrient deprivation-related stress responses.

Molecular Basis of RNA-Driven ATP Hydrolysis in DExH-Box Helicases

The spliceosome machinery catalyzes precursor messenger (pre-m)RNA splicing. In each cycle, the spliceosome experiences massive compositional and conformational remodeling fueled by the concerted action of specific RNA-dependent ATPases/helicases. Intriguingly, these enzymes are allosterically activated to perform ATP hydrolysis and trigger helicase activity only upon pre-mRNA binding. Yet, the molecular mechanism underlying the RNA-driven regulation of their ATPase function remains elusive. Here, we focus on the Prp2 ATPase/helicase which contributes to reshaping the spliceosome into its catalytic competent state. By performing classical and quantum-classical molecular dynamics simulations, we unprecedentedly unlock the molecular terms governing the Prp2 ATPase/helicase function. Namely, we dissect the molecular mechanism of ATP hydrolysis, and we disclose that RNA binding allosterically triggers the formation of a set of interactions linking the RNA binding tunnel to the catalytic site. This activates the Prp2's ATPase function by optimally placing the nucleophilic water and the general base of the enzymatic process to perform ATP hydrolysis. The key structural motifs, mechanically coupling RNA gripping and the ATPase/helicase functions, are conserved across all DExH-box helicases. This mechanism could thus be broadly applicable to all DExH-box helicase family.

Emerging Therapies in CLL in the Era of Precision Medicine

Over the past decade, the treatment landscape of CLL has vastly changed from the conventional FC (fludarabine and cyclophosphamide) and FCR (FC with rituximab) chemotherapies to targeted therapies, including inhibitors of Bruton tyrosine kinase (BTK) and phosphatidylinositol 3-kinase (PI3K) as well as inhibitors of BCL2. These treatment options dramatically improved clinical outcomes; however, not all patients respond well to these therapies, especially high-risk patients. Clinical trials of immune checkpoint inhibitors (PD-1, CTLA4) and chimeric antigen receptor T (CAR T) or NK (CAR NK) cell treatment have shown some efficacy; still, long-term outcomes and safety issues have yet to be determined. CLL remains an incurable disease. Thus, there are unmet needs to discover new molecular pathways with targeted or combination therapies to cure the disease. Large-scale genome-wide whole-exome and whole-genome sequencing studies have discovered genetic alterations associated with disease progression, refined the prognostic markers in CLL, identified mutations underlying drug resistance, and pointed out critical targets to treat the disease. More recently, transcriptome and proteome landscape characterization further stratified the disease and revealed novel therapeutic targets in CLL. In this review, we briefly summarize the past and present available single or combination therapies, focusing on potential emerging therapies to address the unmet clinical needs in CLL.

Mechanisms of the RNA helicases DDX42 and DDX46 in human U2 snRNP assembly

Three RNA helicases - DDX42, DDX46 and DHX15 - are found to be associated with human U2 snRNP, but their roles and mechanisms in U2 snRNP and spliceosome assembly are insufficiently understood. Here we report the cryo-electron microscopy (cryo-EM) structures of the DDX42-SF3b complex and a putative assembly precursor of 17S U2 snRNP that contains DDX42 (DDX42-U2 complex). DDX42 is anchored on SF3B1 through N-terminal sequences, with its N-plug occupying the RNA path of SF3B1. The binding mode of DDX42 to SF3B1 is in striking analogy to that of DDX46. In the DDX42-U2 complex, the N-terminus of DDX42 remains anchored on SF3B1, but the helicase domain has been displaced by U2 snRNA and TAT-SF1. Through in vitro assays, we show DDX42 and DDX46 are mutually exclusive in terms of binding to SF3b. Cancer-driving mutations of SF3B1 target the residues in the RNA path that directly interact with DDX42 and DDX46. These findings reveal the distinct roles of DDX42 and DDX46 in assembly of 17S U2 snRNP and provide insights into the mechanisms of SF3B1 cancer mutations.

An Arabidopsis pre-RNA processing8a (prp8a) missense allele restores splicing of a subset of mis-spliced mRNAs

Eukaryotic precursor mRNAs often harbor noncoding introns that must be removed prior to translation. Accurate splicing of precursor messenger RNA depends on placement and assembly of small nuclear ribonucleoprotein (snRNP) sub-complexes of the spliceosome. Yeast (Saccharomyces cerevisiae) studies established a role in splice-site selection for PRE-RNA PROCESSING8 (PRP8), a conserved spliceosome scaffolding protein of the U5 snRNP. However, analogous splice-site selection studies in multicellular eukaryotes are lacking. Such studies are crucial for a comprehensive understanding of alternative splicing, which is extensive in plants and animals but limited in yeast. In this work, we describe an Arabidopsis (Arabidopsis thaliana) prp8a mutant that modulates splice-site selection. We isolated prp8a-14 from a screen for suppressors of pex14-6, which carries a splice-site mutation in the PEROXIN14 (PEX14) peroxisome biogenesis gene. To elucidate Arabidopsis PRP8A function in spliceosome fidelity, we combined prp8a-14 with various pex14 splice-site mutations and monitored the double mutants for physiological and molecular consequences of dysfunctional and functional peroxisomes that correspond to impaired and recovered splicing, respectively. prp8a-14 restored splicing and PEX14 function to alleles with mutations in the exonic guanine of the 5'-splice site but did not restore splicing or function to alleles with mutations in the intronic guanine of 5'- or 3'-splice sites. We used RNA-seq to reveal the systemic impact of prp8a-14 and found hundreds of differentially spliced transcripts and thousands of transcripts with significantly altered levels. Among differentially spliced transcripts, prp8a-14 significantly altered 5'- and 3'-splice-site utilization to favor sites resulting in shorter introns. This study provides a genetic platform for probing splicing in plants and hints at a role for plant PRP8 in splice-site selection.

Dysregulation and therapeutic targeting of RNA splicing in cancer

High-throughput sequencing and functional characterization of the cancer transcriptome have uncovered cancer-specific dysregulation of RNA splicing across a variety of cancers. Alterations in the cancer genome and dysregulation of RNA splicing factors lead to missplicing, splicing alteration-dependent gene expression and, in some cases, generation of novel splicing-derived proteins. Here, we review recent advances in our understanding of aberrant splicing in cancer pathogenesis and present strategies to harness cancer-specific aberrant splicing for therapeutic intent.

Co-transcriptional splicing efficiencies differ within genes and between cell types

Pre-mRNA splicing is carried out by the spliceosome and involves splice site recognition, removal of introns, and ligation of exons. Components of the spliceosome have been shown to interact with the elongating RNA polymerase II (RNAPII) which is thought to allow splicing to occur concurrently with transcription. However, little is known about the regulation and efficiency of co-transcriptional splicing in human cells. In this study, we used Bru-seq and BruChase-seq to determine the co-transcriptional splicing efficiencies of 17,000 introns expressed across 6 human cell lines. We found that less than half of all introns across these 6 cell lines were co-transcriptionally spliced. Splicing efficiencies for individual introns showed variations across cell lines, suggesting that splicing may be regulated in a cell-type specific manner. Moreover, the splicing efficiency of introns varied within genes. The efficiency of co-transcriptional splicing did not correlate with gene length, intron position, splice site strengths, or the intron/neighboring exons GC content. However, we identified binding signals from multiple RNA binding proteins (RBPs) that correlated with splicing efficiency, including core spliceosomal machinery components-such as SF3B4, U2AF1 and U2AF2 showing higher binding signals in poorly spliced introns. In addition, multiple RBPs, such as BUD13, PUM1 and SND1, showed preferential binding in exons that flank introns with high splicing efficiencies. The nascent RNA splicing patterns presented here across multiple cell types add to our understanding of the complexity in RNA splicing, wherein RNA-binding proteins may play important roles in determining splicing outcomes in a cell type- and intron-specific manner.

DEAH-Box RNA Helicases in Pre-mRNA Splicing

In eukaryotic cells, pre-mRNA splicing is catalyzed by the spliceosome, a highly dynamic molecular machinery that undergoes dramatic conformational and compositional rearrangements throughout the splicing cycle. These crucial rearrangements are largely driven by eight DExD/H-box RNA helicases. Interestingly, the four helicases participating in the late stages of splicing are all DEAH-box helicases that share structural similarities. This review aims to provide an overview of the structure and function of these DEAH-box helicases, including new information provided by recent cryo-electron microscopy structures of the spliceosomal complexes.

Structure of the activated human minor spliceosome

The minor spliceosome mediates splicing of the rare but essential U12-type precursor messenger RNA. Here, we report the atomic features of the activated human minor spliceosome determined by cryo-electron microscopy at 2.9-angstrom resolution. The 5' splice site and branch point sequence of the U12-type intron are recognized by the U6atac and U12 small nuclear RNAs (snRNAs), respectively. Five newly identified proteins stabilize the conformation of the catalytic center: The zinc finger protein SCNM1 functionally mimics the SF3a complex of the major spliceosome, the RBM48-ARMC7 complex binds the γ-monomethyl phosphate cap at the 5' end of U6atac snRNA, the U-box protein PPIL2 coordinates loop I of U5 snRNA and stabilizes U5 small nuclear ribonucleoprotein (snRNP), and CRIPT stabilizes U12 snRNP. Our study provides a framework for the mechanistic understanding of the function of the human minor spliceosome.

U1 snRNP telescripting: molecular mechanisms and beyond

U1 snRNP is one of the most abundant ribonucleoprotein (RNP) complexes in eukaryotic cells and is estimated to be approximately 1 million copies per cell. Apart from its canonical role in mRNA splicing, this complex has emerged as a key regulator of eukaryotic mRNA length via inhibition of mRNA 3'-end processing at numerous intronic polyadenylation sites, in a process that is also termed 'U1 snRNP telescripting'. Several reviews have extensively described the concept of U1 telescripting and subsequently highlighted its potential impacts in mRNA metabolism. Here, we review what is currently known regarding the underlying mechanisms of this important phenomenon and discuss open questions and future challenges.

Mechanism of spliceosome remodeling by the ATPase/helicase Prp2 and its coactivator Spp2

Spliceosome remodeling, executed by conserved adenosine triphosphatase (ATPase)/helicases including Prp2, enables precursor messenger RNA (pre-mRNA) splicing. However, the structural basis for the function of the ATPase/helicases remains poorly understood. Here, we report atomic structures of Prp2 in isolation, Prp2 complexed with its coactivator Spp2, and Prp2-loaded activated spliceosome and the results of structure-guided biochemical analysis. Prp2 weakly associates with the spliceosome and cannot function without Spp2, which stably associates with Prp2 and anchors on the spliceosome, thus tethering Prp2 to the activated spliceosome and allowing Prp2 to function. Pre-mRNA is loaded into a featured channel between the N and C halves of Prp2, where Leu⁵³⁶ from the N half and Arg⁸⁴⁴ from the C half prevent backward sliding of pre-mRNA toward its 5'-end. Adenosine 5'-triphosphate binding and hydrolysis trigger interdomain movement in Prp2, which drives unidirectional stepwise translocation of pre-mRNA toward its 3'-end. These conserved mechanisms explain the coupling of spliceosome remodeling to pre-mRNA splicing.

How Is Precursor Messenger RNA Spliced by the Spliceosome?

Splicing of the precursor messenger RNA, involving intron removal and exon ligation, is mediated by the spliceosome. Together with biochemical and genetic investigations of the past four decades, structural studies of the intact spliceosome at atomic resolution since 2015 have led to mechanistic delineation of RNA splicing with remarkable insights. The spliceosome is proven to be a protein-orchestrated metalloribozyme. Conserved elements of small nuclear RNA (snRNA) constitute the splicing active site with two catalytic metal ions and recognize three conserved intron elements through duplex formation, which are delivered into the splicing active site for branching and exon ligation. The protein components of the spliceosome stabilize the conformation of the snRNA, drive spliceosome remodeling, orchestrate the movement of the RNA elements, and facilitate the splicing reaction. The overall organization of the spliceosome and the configuration of the splicing active site are strictly conserved between human and yeast.

In or Out? New Insights on Exon Recognition through Splice-Site Interdependency

Noncanonical splice-site mutations are an important cause of inherited diseases. Based on in vitro and stem-cell-based studies, some splice-site variants show a stronger splice defect than expected based on their predicted effects, suggesting that other sequence motifs influence the outcome. We investigated whether splice defects due to human-inherited-disease-associated variants in noncanonical splice-site sequences in ABCA4, DMD, and TMC1 could be rescued by strengthening the splice site on the other side of the exon. Noncanonical 5′- and 3′-splice-site variants were selected. Rescue variants were introduced based on an increase in predicted splice-site strength, and the effects of these variants were analyzed using in vitro splice assays in HEK293T cells. Exon skipping due to five variants in noncanonical splice sites of exons in ABCA4, DMD, and TMC1 could be partially or completely rescued by increasing the predicted strengths of the other splice site of the same exon. We named this mechanism “splicing interdependency”, and it is likely based on exon recognition by splicing machinery. Awareness of this interdependency is of importance in the classification of noncanonical splice-site variants associated with disease and may open new opportunities for treatments.

The SF3b complex: splicing and beyond

The SF3b complex is an intrinsic component of the functional U2 small nuclear ribonucleoprotein (snRNP). As U2 snRNP enters nuclear pre-mRNA splicing, SF3b plays key roles in recognizing the branch point sequence (BPS) and facilitating spliceosome assembly and activation. Since the discovery of SF3b, substantial progress has been made in elucidating its molecular mechanism during splicing. In addition, numerous recent studies indicate that SF3b and its components are engaged in various molecular and cellular events that are beyond the canonical role in splicing. This review summarizes the current knowledge on the SF3b complex and highlights its multiple roles in splicing and beyond.

RNA Splicing by the Spliceosome

The spliceosome removes introns from messenger RNA precursors (pre-mRNA). Decades of biochemistry and genetics combined with recent structural studies of the spliceosome have produced a detailed view of the mechanism of splicing. In this review, we aim to make this mechanism understandable and provide several videos of the spliceosome in action to illustrate the intricate choreography of splicing. The U1 and U2 small nuclear ribonucleoproteins (snRNPs) mark an intron and recruit the U4/U6.U5 tri-snRNP. Transfer of the 5' splice site (5'SS) from U1 to U6 snRNA triggers unwinding of U6 snRNA from U4 snRNA. U6 folds with U2 snRNA into an RNA-based active site that positions the 5'SS at two catalytic metal ions. The branch point (BP) adenosine attacks the 5'SS, producing a free 5' exon. Removal of the BP adenosine from the active site allows the 3'SS to bind, so that the 5' exon attacks the 3'SS to produce mature mRNA and an excised lariat intron.

Molecular choreography of pre-mRNA splicing by the spliceosome

The spliceosome executes eukaryotic precursor messenger RNA (pre-mRNA) splicing to remove noncoding introns through two sequential transesterification reactions, branching and exon ligation. The fidelity of this process is based on the recognition of the conserved sequences in the intron and dynamic compositional and structural rearrangement of this multi-megadalton machinery. Since atomic visualization of the splicing active site in an endogenous Schizosaccharomyces pombe spliceosome in 2015, high-resolution cryoelectron microscopy (cryo-EM) structures of other spliceosome intermediates began to uncover the molecular mechanism. Recent advances in the structural biology of the spliceosome make it clearer the mechanisms of its assembly, activation, disassembly and exon ligation. Together, these discrete structural images give rise to a molecular choreography of the spliceosome.

Structural Basis of Nuclear pre-mRNA Splicing: Lessons from Yeast

Noncoding introns are removed from nuclear precursor messenger RNA (pre-mRNA) in a two-step phosphoryl transfer reaction by the spliceosome, a dynamic multimegadalton enzyme. Cryo-electron microscopy (cryo-EM) structures of the Saccharomyces cerevisiae spliceosome were recently determined in eight key states. Combined with the wealth of available genetic and biochemical data, these structures have revealed new insights into the mechanisms of spliceosome assembly, activation, catalysis, and disassembly. The structures show how a single RNA catalytic center forms during activation and accomplishes both steps of the splicing reaction. The structures reveal how spliceosomal helicases remodel the spliceosome for active site formation, substrate docking, reaction product undocking, and spliceosome disassembly and how they facilitate splice site proofreading. Although human spliceosomes contain additional proteins, their cryo-EM structures suggest that the underlying mechanism is conserved across all eukaryotes. In this review, we summarize the current structural understanding of pre-mRNA splicing.

Frontiers in Cryo Electron Microscopy of Complex Macromolecular Assemblies

In recent years, cryo electron microscopy (cryo-EM) technology has been transformed with the development of better instrumentation, direct electron detectors, improved methods for specimen preparation, and improved software for data analysis. Analyses using single-particle cryo-EM methods have enabled determination of structures of proteins with sizes smaller than 100 kDa and resolutions of ∼2 Å in some cases. The use of electron tomography combined with subvolume averaging is beginning to allow the visualization of macromolecular complexes in their native environment in unprecedented detail. As a result of these advances, solutions to many intractable challenges in structural and cell biology, such as analysis of highly dynamic soluble and membrane-embedded protein complexes or partially ordered protein aggregates, are now within reach. Recent reports of structural studies of G protein-coupled receptors, spliceosomes, and fibrillar specimens illustrate the progress that has been made using cryo-EM methods, and are the main focus of this review.

Structures of the Catalytically Activated Yeast Spliceosome Reveal the Mechanism of Branching

Pre-mRNA splicing is executed by the spliceosome. Structural characterization of the catalytically activated complex (B^∗) is pivotal for understanding the branching reaction. In this study, we assembled the B^∗ complexes on two different pre-mRNAs from Saccharomyces cerevisiae and determined the cryo-EM structures of four distinct B^∗ complexes at overall resolutions of 2.9-3.8 Å. The duplex between U2 small nuclear RNA (snRNA) and the branch point sequence (BPS) is discretely away from the 5'-splice site (5'SS) in the three B^∗ complexes that are devoid of the step I splicing factors Yju2 and Cwc25. Recruitment of Yju2 into the active site brings the U2/BPS duplex into the vicinity of 5'SS, with the BPS nucleophile positioned 4 Å away from the catalytic metal M2. This analysis reveals the functional mechanism of Yju2 and Cwc25 in branching. These structures on different pre-mRNAs reveal substrate-specific conformations of the spliceosome in a major functional state.

Molecular Mechanisms of pre-mRNA Splicing through Structural Biology of the Spliceosome

Precursor messenger RNA (pre-mRNA) splicing is executed by the spliceosome. In the past 3 years, cryoelectron microscopy (cryo-EM) structures have been elucidated for a majority of the yeast spliceosomal complexes and for a few human spliceosomes. During the splicing reaction, the dynamic spliceosome has an immobile core of about 20 protein and RNA components, which are organized around a conserved splicing active site. The divalent metal ions, coordinated by U6 small nuclear RNA (snRNA), catalyze the branching reaction and exon ligation. The spliceosome also contains a mobile but compositionally stable group of about 13 proteins and a portion of U2 snRNA, which facilitate substrate delivery into the splicing active site. The spliceosomal transitions are driven by the RNA-dependent ATPase/helicases, resulting in the recruitment and dissociation of specific splicing factors that enable the reaction. In summary, the spliceosome is a protein-directed metalloribozyme.

Pathogenicity and selective constraint on variation near splice sites

Mutations that perturb normal pre-mRNA splicing are significant contributors to human disease. We used exome sequencing data from 7833 probands with developmental disorders (DDs) and their unaffected parents, as well as more than 60,000 aggregated exomes from the Exome Aggregation Consortium, to investigate selection around the splice sites and quantify the contribution of splicing mutations to DDs. Patterns of purifying selection, a deficit of variants in highly constrained genes in healthy subjects, and excess de novo mutations in patients highlighted particular positions within and around the consensus splice site of greater functional relevance. By using mutational burden analyses in this large cohort of proband–parent trios, we could estimate in an unbiased manner the relative contributions of mutations at canonical dinucleotides (73%) and flanking noncanonical positions (27%), and calculate the positive predictive value of pathogenicity for different classes of mutations. We identified 18 patients with likely diagnostic de novo mutations in dominant DD-associated genes at noncanonical positions in splice sites. We estimate 35%–40% of pathogenic variants in noncanonical splice site positions are missing from public databases.

Folding, Assembly, and Persistence: The Essential Nature and Origins of Biopolymers

Life as we know it requires three basic types of polymers: polypeptide, polynucleotide, and polysaccharide. Here we evaluate both universal and idiosyncratic characteristics of these biopolymers. We incorporate this information into a model that explains much about their origins, selection, and early evolution. We observe that all three biopolymer types are pre-organized, conditionally self-complementary, chemically unstable in aqueous media yet persistent because of kinetic trapping, with chiral monomers and directional chains. All three biopolymers are synthesized by dehydration reactions that are catalyzed by molecular motors driven by hydrolysis of phosphorylated nucleosides. All three biopolymers can access specific states that protect against hydrolysis. These protected states are folded, using self-complementary interactions among recurrent folding elements within a given biopolymer, or assembled, in associations between the same or different biopolymer types. Self-association in a hydrolytic environment achieves self-preservation. Heterogeneous association achieves partner-preservation. These universal properties support a model in which life's polymers emerged simultaneously and co-evolved in a common hydrolytic milieu where molecular persistence depended on folding and assembly. We believe that an understanding of the structure, function, and origins of any given type of biopolymer requires the context of other biopolymers.

Structural studies of the spliceosome: past, present and future perspectives

The spliceosome is a multi-subunit RNA-protein complex involved in the removal of non-coding segments (introns) from between the coding regions (exons) in precursors of messenger RNAs (pre-mRNAs). Intron removal proceeds via two transesterification reactions, occurring between conserved sequences at intron-exon junctions. A tightly regulated, hierarchical assembly with a multitude of structural and compositional rearrangements posed a great challenge for structural studies of the spliceosome. Over the years, X-ray crystallography dominated the field, providing valuable high-resolution structural information that was mostly limited to individual proteins and smaller sub-complexes. Recent developments in the field of cryo-electron microscopy allowed the visualisation of fully assembled yeast and human spliceosomes, providing unprecedented insights into substrate recognition, catalysis, and active site formation. This has advanced our mechanistic understanding of pre-mRNA splicing enormously.

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.

Structure of the Post-catalytic Spliceosome from Saccharomyces cerevisiae

Removal of an intron from a pre-mRNA by the spliceosome results in the ligation of two exons in the post-catalytic spliceosome (known as the P complex). Here, we present a cryo-EM structure of the P complex from Saccharomyces cerevisiae at an average resolution of 3.6 Å. The ligated exon is held in the active site through RNA-RNA contacts. Three bases at the 3' end of the 5' exon remain anchored to loop I of U5 small nuclear RNA, and the conserved AG nucleotides of the 3'-splice site (3'SS) are specifically recognized by the invariant adenine of the branch point sequence, the guanine base at the 5' end of the 5'SS, and an adenine base of U6 snRNA. The 3'SS is stabilized through an interaction with the 1585-loop of Prp8. The P complex structure provides a view on splice junction formation critical for understanding the complete splicing cycle.

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.

Mechanistic Insights Into Catalytic RNA-Protein Complexes Involved in Translation of the Genetic Code

The contemporary world is an "RNA-protein world" rather than a "protein world" and tracing its evolutionary origins is of great interest and importance. The different RNAs that function in close collaboration with proteins are involved in several key physiological processes, including catalysis. Ribosome-the complex megadalton cellular machinery that translates genetic information encoded in nucleotide sequence to amino acid sequence-epitomizes such an association between RNA and protein. RNAs that can catalyze biochemical reactions are known as ribozymes. They usually employ general acid-base catalytic mechanism, often involving the 2'-OH of RNA that activates and/or stabilizes a nucleophile during the reaction pathway. The protein component of such RNA-protein complexes (RNPCs) mostly serves as a scaffold which provides an environment conducive for the RNA to function, or as a mediator for other interacting partners. In this review, we describe those RNPCs that are involved at different stages of protein biosynthesis and in which RNA performs the catalytic function; the focus of the account is on highlighting mechanistic aspects of these complexes. We also provide a perspective on such associations in the context of proofreading during translation of the genetic code. The latter aspect is not much appreciated and recent works suggest that this is an avenue worth exploring, since an understanding of the subject can provide useful insights into how RNAs collaborate with proteins to ensure fidelity during these essential cellular processes. It may also aid in comprehending evolutionary aspects of such associations.

A close-up look at the spliceosome, at last

Major developments in cryo-electron microscopy in the past three or four years have led to the solution of a number of spliceosome structures at high resolution, e.g., the fully assembled but not yet active spliceosome (Bact), the spliceosome just after the first step of splicing (C), and the spliceosome activated for the second step (C*). Therefore 30 years of genetics and biochemistry of the spliceosome can now be interpreted at the structural level. I have closely examined the RNase H domain of Prp8 in each of the structures. Interestingly, the RNase H domain has different and unexpected roles in each of the catalytic steps of splicing.

Alternative splicing: the pledge, the turn, and the prestige : The key role of alternative splicing in human biological systems

Alternative pre-mRNA splicing is a tightly controlled process conducted by the spliceosome, with the assistance of several regulators, resulting in the expression of different transcript isoforms from the same gene and increasing both transcriptome and proteome complexity. The differences between alternative isoforms may be subtle but enough to change the function or localization of the translated proteins. A fine control of the isoform balance is, therefore, needed throughout developmental stages and adult tissues or physiological conditions and it does not come as a surprise that several diseases are caused by its deregulation. In this review, we aim to bring the splicing machinery on stage and raise the curtain on its mechanisms and regulation throughout several systems and tissues of the human body, from neurodevelopment to the interactions with the human microbiome. We discuss, on one hand, the essential role of alternative splicing in assuring tissue function, diversity, and swiftness of response in these systems or tissues, and on the other hand, what goes wrong when its regulatory mechanisms fail. We also focus on the possibilities that splicing modulation therapies open for the future of personalized medicine, along with the leading techniques in this field. The final act of the spliceosome, however, is yet to be fully revealed, as more knowledge is needed regarding the complex regulatory network that coordinates alternative splicing and how its dysfunction leads to disease.

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.

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.

Splice-switching antisense oligonucleotides as therapeutic drugs

Splice-switching oligonucleotides (SSOs) are short, synthetic, antisense, modified nucleic acids that base-pair with a pre-mRNA and disrupt the normal splicing repertoire of the transcript by blocking the RNA–RNA base-pairing or protein–RNA binding interactions that occur between components of the splicing machinery and the pre-mRNA. Splicing of pre-mRNA is required for the proper expression of the vast majority of protein-coding genes, and thus, targeting the process offers a means to manipulate protein production from a gene. Splicing modulation is particularly valuable in cases of disease caused by mutations that lead to disruption of normal splicing or when interfering with the normal splicing process of a gene transcript may be therapeutic. SSOs offer an effective and specific way to target and alter splicing in a therapeutic manner. Here, we discuss the different approaches used to target and alter pre-mRNA splicing with SSOs. We detail the modifications to the nucleic acids that make them promising therapeutics and discuss the challenges to creating effective SSO drugs. We highlight the development of SSOs designed to treat Duchenne muscular dystrophy and spinal muscular atrophy, which are currently being tested in clinical trials.

Design, Synthesis, and Properties of Phosphoramidate 2',5'-Linked Branched RNA: Toward the Rational Design of Inhibitors of the RNA Lariat Debranching Enzyme

Two RNA fragments linked by means of a 2',5' phosphodiester bridge (2' hydroxyl of one fragment connected to the 5' hydroxyl of the other) constitute a class of nucleic acids known as 2'-5' branched RNAs (bRNAs). In this report we show that bRNA analogues containing 2'-5' phosphoramidate linkages (bN-RNAs) inhibit the lariat debranching enzyme, a 2',5'-phosphodiesterase that has recently been implicated in neurodegenerative diseases associated with aging. bN-RNAs were efficiently generated using automated solid-phase synthesis and suitably protected branchpoint building blocks. Two orthogonally removable groups, namely the 4-monomethoxytrityl (MMTr) group and the fluorenylmethyl-oxycarbonyl (Fmoc) groups, were evaluated as protecting groups of the 2' amino functionality. The 2'-N-Fmoc methodology was found to successfully produce bN-RNAs on solid-phase oligonucleotide synthesis. The synthesized bN-RNAs resisted hydrolysis by the lariat debranching enzyme (Dbr1) and, in addition, were shown to attenuate the Dbr1-mediated hydrolysis of native bRNA.

Recognizing the 35th anniversary of the proposal that snRNPs are involved in splicing

Thirty-five years ago, as young graduate students, we had the pleasure and privilege of being in Joan Steitz's laboratory at a pivotal point in the history of RNA molecular biology. Introns had recently been discovered in the laboratories of Philip Sharp and Richard Roberts, but the machinery for removing them from mRNA precursors was entirely unknown. This Retrospective describes our hypothesis that recently discovered snRNPs functioned in pre-mRNA splicing. The proposal was proven correct, as has Joan's intuition that small RNAs provide specificity to RNA processing reactions through base pairing in diverse settings. However, research over the intervening years has revealed that both splice site selection and splicing itself are much more complex and dynamic than we imagined.

Single Molecule Cluster Analysis dissects splicing pathway conformational dynamics

We report Single Molecule Cluster Analysis (SiMCAn), which utilizes hierarchical clustering of hidden Markov modeling-fitted single-molecule fluorescence resonance energy transfer (smFRET) trajectories to dissect the complex conformational dynamics of biomolecular machines. We used this method to study the conformational dynamics of a precursor mRNA during the splicing cycle as carried out by the spliceosome. By clustering common dynamic behaviors derived from selectively blocked splicing reactions, SiMCAn was able to identify the signature conformations and dynamic behaviors of multiple ATP-dependent intermediates. In addition, it identified an open conformation adopted late in splicing by a 3' splice-site mutant, invoking a mechanism for substrate proofreading. SiMCAn enables rapid interpretation of complex single-molecule behaviors and should prove useful for the comprehensive analysis of a plethora of dynamic cellular machines.

Functional roles of DExD/H-box RNA helicases in Pre-mRNA splicing

Splicing of precursor mRNA takes place via two consecutive steps of transesterification catalyzed by a large ribonucleoprotein complex called the spliceosome. The spliceosome is assembled through ordered binding to the pre-mRNA of five small nuclear RNAs and numerous protein factors, and is disassembled after completion of the reaction to recycle all components. Throughout the splicing cycle, the spliceosome changes its structure, rearranging RNA-RNA, RNA-protein and protein-protein interactions, for positioning and repositioning of splice sites. DExD/H-box RNA helicases play important roles in mediating structural changes of the spliceosome by unwinding of RNA duplexes or disrupting RNA-protein interactions. DExD/H-box proteins are also implicated in the fidelity control of the splicing process at various steps. This review summarizes the functional roles of DExD/H-box proteins in pre-mRNA splicing according to studies conducted mostly in yeast and will discuss the concept of the complicated splicing reaction based on recent findings.

RNA-Redesign: a web server for fixed-backbone 3D design of RNA

RNA is rising in importance as a design medium for interrogating fundamental biology and for developing therapeutic and bioengineering applications. While there are several online servers for design of RNA secondary structure, there are no tools available for the rational design of 3D RNA structure. Here we present RNA-Redesign (http://rnaredesign.stanford.edu), an online 3D design tool for RNA. This resource utilizes fixed-backbone design to optimize the sequence identity and nucleobase conformations of an RNA to match a desired backbone, analogous to fundamental tools that underlie rational protein engineering. The resulting sequences suggest thermostabilizing mutations that can be experimentally verified. Further, sequence preferences that differ between natural and computationally designed sequences can suggest whether natural sequences possess functional constraints besides folding stability, such as cofactor binding or conformational switching. Finally, for biochemical studies, the designed sequences can suggest experimental tests of 3D models, including concomitant mutation of base triples. In addition to the designs generated, detailed graphical analysis is presented through an integrated and user-friendly environment.

Mechanisms of spliceosomal assembly

Pre-mRNA splicing is a key step for generating mature protein-coding mRNA. An RNA-protein complex known as the spliceosome carries out the chemistry of pre-mRNA splicing. However, several pre-spliceosomal intermediates are assembled on the pre-mRNA before the formation of the catalytically activated spliceosome. The progression to the activated spliceosome involves a cascade of the rearrangement events of the RNA-RNA, RNA-protein, and protein-protein interactions within the pre-spliceosomal intermediates. These rearrangements generate multiple combinatorial interactions of the spliceosome with the substrate, which enhances the accuracy of the splice site selection. Each rearrangement also represents a step at which splicing can potentially be subjected to regulation. The aim of this chapter is to provide an overview of the components of the spliceosome and their rearrangements along the spliceosome assembly pathway.

Biased Brownian ratcheting leads to pre-mRNA remodeling and capture prior to first-step splicing

The spliceosome is a dynamic ribonucleoprotein (RNP) machine that catalyzes the removal of introns in the two transesterification steps of eukaryotic pre-mRNA splicing. Here we used single molecule fluorescence resonance energy transfer to monitor the distance of the 5′ splice site (5′SS) and branchpoint (BP) of pre-mRNA in affinity-purified spliceosomes stalled by a mutation in the DExD/H-box helicase Prp2 immediately prior to the first splicing step. Addition of recombinant Prp2 together with NTP and protein cofactor Spp2 rearranges the spliceosome-substrate complex to reversibly explore conformations with proximal 5′SS and BP that accommodate chemistry. Addition of Cwc25 then strongly biases this equilibrium towards the proximal conformation, promoting efficient first-step splicing. The spliceosome thus functions as a biased Brownian ratchet machine where a helicase unlocks thermal fluctuations subsequently rectified by a cofactor “pawl”, a principle possibly widespread among the many helicase-driven RNPs.

Sixty years of genome biology

Sixty years after Watson and Crick published the double helix model of DNA's structure, thirteen members of Genome Biology's Editorial Board select key advances in the field of genome biology subsequent to that discovery.

Reflections on the history of pre-mRNA processing and highlights of current knowledge: a unified picture

Several strong conclusions emerge concerning pre-mRNA processing from both old and newer experiments. The RNAPII complex is involved with pre-mRNA processing through binding of processing proteins to the CTD (carboxyl terminal domain) of the largest RNAPII subunit. These interactions are necessary for efficient processing, but whether factor binding to the CTD and delivery to splicing sites is obligatory or facilitatory is unsettled. Capping, addition of an m(7)Gppp residue (cap) to the initial transcribed residue of a pre-mRNA, occurs within seconds. Splicing of pre-mRNA by spliceosomes at particular sites is most likely committed during transcription by the binding of initiating processing factors and ∼50% of the time is completed in mammalian cells before completion of the primary transcript. This fact has led to an outpouring in the literature about "cotranscriptional splicing." However splicing requires several minutes for completion and can take longer. The RNAPII complex moves through very long introns and also through regions dense with alternating exons and introns at an average rate of ∼3 kb per min and is, therefore, not likely detained at each splice site for more than a few seconds, if at all. Cleavage of the primary transcript at the 3' end and polyadenylation occurs within 30 sec or less at recognized polyA sites, and the majority of newly polyadenylated pre-mRNA molecules are much larger than the average mRNA. Finally, it seems quite likely that the nascent RNA most often remains associated with the chromosomal locus being transcribed until processing is complete, possibly acquiring factors related to the transport of the new mRNA to the cytoplasm.

Probing nucleic acid interactions and pre-mRNA splicing by Förster Resonance Energy Transfer (FRET) microscopy

Förster resonance energy transfer (FRET) microscopy is a powerful technique routinely used to monitor interactions between biomolecules. Here, we focus on the techniques that are used for investigating the structure and interactions of nucleic acids (NAs). We present a brief overview of the most commonly used FRET microscopy techniques, their advantages and drawbacks. We list experimental approaches recently used for either in vitro or in vivo studies. Next, we summarize how FRET contributed to the understanding of pre-mRNA splicing and spliceosome assembly.

Functional roles of protein splicing factors

RNA splicing is one of the fundamental processes in gene expression in eukaryotes. Splicing of pre-mRNA is catalysed by a large ribonucleoprotein complex called the spliceosome, which consists of five small nuclear RNAs and numerous protein factors. The spliceosome is a highly dynamic structure, assembled by sequential binding and release of the small nuclear RNAs and protein factors. DExD/H-box RNA helicases are required to mediate structural changes in the spliceosome at various steps in the assembly pathway and have also been implicated in the fidelity control of the splicing reaction. Other proteins also play key roles in mediating the progression of the spliceosome pathway. In this review, we discuss the functional roles of the protein factors involved in the spliceosome pathway primarily from studies in the yeast system.