The structure and selectivity of the SR protein SRSF2 RRM domain with RNA

Integrative solution structure of PTBP1-IRES complex reveals strong compaction and ordering with residual conformational flexibility

RNA-binding proteins (RBPs) are crucial regulators of gene expression, often composed of defined domains interspersed with flexible, intrinsically disordered regions. Determining the structure of ribonucleoprotein (RNP) complexes involving such RBPs necessitates integrative structural modeling due to their lack of a single stable state. In this study, we integrate magnetic resonance, mass spectrometry, and small-angle scattering data to determine the solution structure of the polypyrimidine-tract binding protein 1 (PTBP1/hnRNP I) bound to an RNA fragment from the internal ribosome entry site (IRES) of the encephalomyocarditis virus (EMCV). This binding, essential for enhancing the translation of viral RNA, leads to a complex structure that demonstrates RNA and protein compaction, while maintaining pronounced conformational flexibility. Acting as an RNA chaperone, PTBP1 orchestrates the IRES RNA into a few distinct conformations, exposing the RNA stems outward. This conformational diversity is likely common among RNP structures and functionally important. Our approach enables atomic-level characterization of heterogeneous RNP structures.

Identification of two short peptide motifs from serine/arginine-rich protein ribonucleic acid recognition motif-1 domain acting as splicing regulators

Background

Serine/arginine-rich (SR) proteins regulate pre-mRNA splicing. However, structurally similar proteins often behave differently in splicing regulation and the underlying mechanisms are largely unknown. Here, using SMN1/2 minigenes we extensively analyzed four SR proteins, SRSF1/5/6/9.

Methods

In this study, the effects of these proteins on SMN1/2 exon 7 splicing when tethered at either intron 6 or 7 were evaluated using an MS2-tethering assay. Deletion analysis in four SR proteins and co-overexpression analysis were performed.

Results

Splicing outcomes varied among all four SR proteins, SRSF1 and SRSF5 function the same at the two sites, acting as repressor and stimulator, respectively; while SRSF6 and SRSF9 promote exon 7 inclusion at only one site. Further, the key domains of each SR proteins were investigated, which identified a potent inhibitory nonapeptide in the C-terminus of SRSF1/9 ribonucleic acid recognition motif-1 (RRM1) and a potent stimulatory heptapeptide at the N-terminus of SRSF5/6 RRM1.

Conclusion

The insight of the four SR proteins and their domains in affecting SMN gene splicing brings a new perspective on the modes of action of SR proteins; and the functional peptides obtained here offers new ideas for developing splice switching-related therapies.

SRSF3/AMOTL1 splicing axis promotes the tumorigenesis of nasopharyngeal carcinoma through regulating the nucleus translocation of YAP1

Dysregulation of serine/arginine splicing factors (SRSFs) and abnormal alternative splicing (AS) have been widely implicated in various cancers but scarcely investigated in nasopharyngeal carcinoma (NPC). Here we examine the expression of 12 classical SRSFs between 87 NPC and 10 control samples, revealing a significant upregulation of SRSF3 and its association with worse prognosis in NPC. Functional assays demonstrate that SRSF3 exerts an oncogenic function in NPC progression. Transcriptome analysis reveals 1,934 SRSF3-regulated AS events in genes related to cell cycle and mRNA metabolism. Among these events, we verify the generation of a long isoform of AMOTL1 (AMOTL1-L) through a direct bond of the SRSF3 RRM domain with the exon 12 of AMOTL1 to promote exon inclusion. Functional studies also reveal that AMOTL1-L promotes the proliferation and migration of NPC cells, while AMOTL1-S does not. Furthermore, overexpression of AMOTL1-L, but not -S, significantly rescues the inhibitory effects of SRSF3 knockdown. Additionally, compared with AMOTL1-S, AMOTL1-L has a localization preference in the intracellular than the cell membrane, leading to a more robust interaction with YAP1 to promote nucleus translocation. Our findings identify SRSF3/AMOTL1 as a novel alternative splicing axis with pivotal roles in NPC development, which could serve as promising prognostic biomarkers and therapeutic targets for NPC.

The rice blast fungus SR protein 1 regulates alternative splicing with unique mechanisms

Serine/arginine-rich (SR) proteins are well known as splicing factors in humans, model animals and plants. However, they are largely unknown in regulating pre-mRNA splicing of filamentous fungi. Here we report that the SR protein MoSrp1 enhances and suppresses alternative splicing in a model fungal plant pathogen Magnaporthe oryzae. Deletion of MoSRP1 caused multiple defects, including reduced virulence and thousands of aberrant alternative splicing events in mycelia, most of which were suppressed or enhanced intron splicing. A GUAG consensus bound by MoSrp1 was identified in more than 94% of the intron or/and proximate exons having the aberrant splicing. The dual functions of regulating alternative splicing of MoSrp1 were exemplified in enhancing and suppressing the consensus-mediated efficient splicing of the introns in MoATF1 and MoMTP1, respectively, which both were important for mycelial growth, conidiation, and virulence. Interestingly, MoSrp1 had a conserved sumoylation site that was essential to nuclear localization and enhancing GUAG binding. Further, we showed that MoSrp1 interacted with a splicing factor and two components of the exon-joining complex via its N-terminal RNA recognition domain, which was required to regulate mycelial growth, development and virulence. In contrast, the C-terminus was important only for virulence and stress responses but not for mycelial growth and development. In addition, only orthologues from Pezizomycotina species could completely rescue defects of the deletion mutants. This study reveals that the fungal conserved SR protein Srp1 regulates alternative splicing in a unique manner.

Splicing factor SRSF2-centric gene regulation

Serine/arginine-rich splicing factor 2 (SRSF2) is a splicing factor that is widely expressed in a variety of mammalian cell types. Increasing evidence has confirmed that SRSF2 plays vital roles in a number of biological and pathological processes. Therefore, it is important to understand how its expression is regulated, and how it regulates the expression of its target genes. Recently, we found that SRSF2 expression could be upregulated by herpes simplex virus-1 (HSV-1) infection, and that altered SRSF2 expression, in turn, epigenetically regulates the transcription of HSV-1 genes. Further studies on T cell exhaustion demonstrated that upregulated SRSF2 in exhausted T cells elevated the levels of multiple immune checkpoint molecules by associating with the acyl-transferases, P300 and CBP, and by altering histone modification near the transcription start sites of these genes, thereby influencing signal transducer and activator of transcription 3 binding to these gene promoters. These findings suggest that SRSF2 acts as an important sensor and effector during disease progression. Here, we discuss the molecules that regulate SRSF2 gene expression and their associated mechanisms, and the mechanisms via which SRSF2 regulates the expression of target genes, thus providing novel insights into the central role of SRSF2 in gene regulation.

Middle-Down Proteomics Reveals Dense Sites of Methylation and Phosphorylation in Arginine-Rich RNA-Binding Proteins

Post-translational modifications (PTMs) within arginine (Arg)-rich RNA-binding proteins, such as phosphorylation and methylation, regulate multiple steps in RNA metabolism. However, the identification of PTMs within Arg-rich domains with complete trypsin digestion is extremely challenging due to the high density of Arg residues within these proteins. Here, we report a middle-down proteomic approach coupled with electron-transfer dissociation (ETD) mass spectrometry to map previously unknown sites of phosphorylation and methylation within the Arg-rich domains of U1-70K and structurally similar RNA-binding proteins from nuclear extracts of human embryonic kidney (HEK)-293T cells. Notably, the Arg-rich domains in RNA-binding proteins are densely modified by methylation and phosphorylation compared with the remainder of the proteome, with methylation and phosphorylation favoring RSRS motifs. Although they favor a common motif, analysis of combinatorial PTMs within RSRS motifs indicates that phosphorylation and methylation do not often co-occur, suggesting that they may functionally oppose one another. Furthermore, we show that phosphorylation may modify interactions between Arg-rich proteins, as serine-arginine splicing factor 2 (SRSF2) has a stronger association with U1-70K and LUC7L3 upon dephosphorylation. Collectively, these findings suggest that the level of PTMs within Arg-rich domains may be among the highest in the proteome and a possible unexplored regulator of RNA-binding protein interactions.

Repeat-associated RNA structure and aberrant splicing

Over 30 hereditary disorders attributed to the expansion of microsatellite repeats have been identified. Despite variant nucleotide content, number of consecutive repeats, and different locations in the genome, many of these diseases have pathogenic RNA gain-of-function mechanisms. The repeat-containing RNAs can form structures in vitro predicted to contribute to the disease through assembly of intracellular RNA aggregates termed foci. The expanded repeat RNAs within these foci sequester RNA binding proteins (RBPs) with important roles in the regulation of RNA metabolism, most notably alternative splicing (AS). These deleterious interactions lead to downstream alterations in transcriptome-wide AS directly linked with disease symptoms. This review summarizes existing knowledge about the association between the repeat RNA structures and RBPs as well as the resulting aberrant AS patterns, specifically in the context of myotonic dystrophy. The connection between toxic, structured RNAs and dysregulation of AS in other repeat expansion diseases is also discussed. This article is part of a Special Issue entitled: RNA structure and splicing regulation edited by Francisco Baralle, Ravindra Singh and Stefan Stamm.

View from an mRNP: The Roles of SR Proteins in Assembly, Maturation and Turnover

Serine- and arginine-rich proteins (SR proteins) are a family of multitasking RNA-binding proteins (RBPs) that are key determinants of messenger ribonucleoprotein (mRNP) formation, identity and fate. Apart from their essential functions in pre-mRNA splicing, SR proteins display additional pre- and post-splicing activities and connect nuclear and cytoplasmic gene expression machineries. Through changes in their post-translational modifications (PTMs) and their subcellular localization, they provide functional specificity and adjustability to mRNPs. Transcriptome-wide UV crosslinking and immunoprecipitation (CLIP-Seq) studies revealed that individual SR proteins are present in distinct mRNPs and act in specific pairs to regulate different gene expression programmes. Adopting an mRNP-centric viewpoint, we discuss the roles of SR proteins in the assembly, maturation, quality control and turnover of mRNPs and describe the mechanisms by which they integrate external signals, coordinate their multiple tasks and couple subsequent mRNA processing steps.

Prognostic significance of SRSF2 mutations in myelodysplastic syndromes and chronic myelomonocytic leukemia: a meta-analysis

OBJECTIVE

Serine/arginine-rich splicing factor 2 (SRSF2) mutations were detected frequently in myelodysplastic syndrome (MDS) and chronic myelomonocytic leukemia (CMML) patients. However, its prognostic value has not yet been fully clarified.

METHODS

In this meta-analysis, Hazard Ratio (HR) and 95% confidence interval (CI) for overall-survival (OS) were chosen to evaluate the prognostic impact of SRSF2 mutations and to compare SRSF2 mutations to those with wild-type.

RESULTS

A total of 2056 patients from 12 studies were obtained. The pooled HRs for OSsuggested that patients with MDS had a poorer prognosis (HR = 1.780, 95% CI (1.410-2.249)), while analysis on SRSF2 mutations revealed no significant effect on the prognosis of CMML patients (HR = 1.091, 95% CI (0.925-1.286)). The frequency of SRSF2 mutations was found to be 11.5% and 39.8% in patients with MDS and CMML, respectively.

DISCUSSION

This meta-analysis suggests that SRSF2 has a poor prognosis in patients with MDS, but no prognosis impact on patients with CMML.

CONCLUSION

In conclusion, SRSF2 mutations were significantly related to the shorter OS in patients with MDS which may consider as an adverse prognostic risk factor. Whereas, analysis did not show any prognostic effect on OS of CMML patients with SRSF2 mutations.

Evolutionarily conserved exon definition interactions with U11 snRNP mediate alternative splicing regulation on U11-48K and U11/U12-65K genes

Many splicing regulators bind to their own pre-mRNAs to induce alternative splicing that leads to formation of unstable mRNA isoforms. This provides an autoregulatory feedback mechanism that regulates the cellular homeostasis of these factors. We have described such an autoregulatory mechanism for two core protein components, U11-48K and U11/U12-65K, of the U12-dependent spliceosome. This regulatory system uses an atypical splicing enhancer element termed USSE (U11 snRNP-binding splicing enhancer), which contains two U12-type consensus 5' splice sites (5'ss). Evolutionary analysis of the USSE element from a large number of animal and plant species indicate that USSE sequence must be located 25-50 nt downstream from the target 3' splice site (3'ss). Together with functional evidence showing a loss of USSE activity when this distance is reduced and a requirement for RS-domain of U11-35K protein for 3'ss activation, our data suggests that U11 snRNP bound to USSE uses exon definition interactions for regulating alternative splicing. However, unlike standard exon definition where the 5'ss bound by U1 or U11 will be subsequently activated for splicing, the USSE element functions similarly as an exonic splicing enhancer and is involved only in upstream splice site activation but does not function as a splicing donor. Additionally, our evolutionary and functional data suggests that the function of the 5'ss duplication within the USSE elements is to allow binding of two U11/U12 di-snRNPs that stabilize each others' binding through putative mutual interactions.

Fragment-based modelling of single stranded RNA bound to RNA recognition motif containing proteins

Protein-RNA complexes are important for many biological processes. However, structural modeling of such complexes is hampered by the high flexibility of RNA. Particularly challenging is the docking of single-stranded RNA (ssRNA). We have developed a fragment-based approach to model the structure of ssRNA bound to a protein, based on only the protein structure, the RNA sequence and conserved contacts. The conformational diversity of each RNA fragment is sampled by an exhaustive library of trinucleotides extracted from all known experimental protein–RNA complexes. The method was applied to ssRNA with up to 12 nucleotides which bind to dimers of the RNA recognition motifs (RRMs), a highly abundant eukaryotic RNA-binding domain. The fragment based docking allows a precise de novo atomic modeling of protein-bound ssRNA chains. On a benchmark of seven experimental ssRNA–RRM complexes, near-native models (with a mean heavy-atom deviation of <3 Å from experiment) were generated for six out of seven bound RNA chains, and even more precise models (deviation < 2 Å) were obtained for five out of seven cases, a significant improvement compared to the state of the art. The method is not restricted to RRMs but was also successfully applied to Pumilio RNA binding proteins.

Characterization of RNA-Protein Interactions: Lessons from Two RNA-Binding Proteins, SRSF1 and SRSF2

SR proteins are a class of RNA-binding proteins whose RNA-binding ability is required for both constitutive and alternative splicing. While members of the SR protein family were once thought to have redundant functions, in-depth biochemical analysis of their RNA-binding abilities has revealed distinct binding profiles for each SR protein, that often lead to either synergistic or antagonistic functions. SR protein family members SRSF1 and SRSF2 are two of the most highly studied RNA-binding proteins. Here we examine the various methods used to differentiate SRSF1 and SRSF2 RNA-binding ability. We discuss the benefits and type of information that can be determined using each method.

Splice factor mutations and alternative splicing as drivers of hematopoietic malignancy

Differential splicing contributes to the vast complexity of mRNA transcripts and protein isoforms that are necessary for cellular homeostasis and response to developmental cues and external signals. The hematopoietic system provides an exquisite example of this. Recently, discovery of mutations in components of the spliceosome in various hematopoietic malignancies (HMs) has led to an explosion in knowledge of the role of splicing and splice factors in HMs and other cancers. A better understanding of the mechanisms by which alternative splicing and aberrant splicing contributes to the leukemogenic process will enable more efficacious targeted approaches to tackle these often difficult to treat diseases. The clinical implications are only just starting to be realized with novel drug targets and therapeutic strategies open to exploitation for patient benefit.

Antisense RNA foci in the motor neurons of C9ORF72-ALS patients are associated with TDP-43 proteinopathy

GGGGCC repeat expansions of C9ORF72 represent the most common genetic variant of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. We and others have proposed that RNA transcribed from the repeat sequence is toxic via sequestration of RNA-binding factors. Both GGGGCC-repeat (sense) and CCCCGG-repeat (antisense) molecules are detectable by fluorescence in situ hybridisation as RNA foci, but their relative expression pattern within the CNS and contribution to disease has not been determined. Blinded examination of CNS biosamples from ALS patients with a repeat expansion of C9ORF72 showed that antisense foci are present at a significantly higher frequency in cerebellar Purkinje neurons and motor neurons, whereas sense foci are present at a significantly higher frequency in cerebellar granule neurons. Consistent with this, inclusions containing sense or antisense derived dipeptide repeat proteins were present at significantly higher frequency in cerebellar granule neurons or motor neurons, respectively. Immunohistochemistry and UV-crosslinking studies showed that sense and antisense RNA molecules share similar interactions with SRSF2, hnRNP K, hnRNP A1, ALYREF, and hnRNP H/F. Together these data suggest that, although sense and antisense RNA molecules might be expected to be equally toxic via their shared protein binding partners, distinct patterns of expression in various CNS neuronal populations could lead to relative differences in their contribution to the pathogenesis of neuronal injury. Moreover in motor neurons, which are the primary target of pathology in ALS, the presence of antisense foci (χ (2), p < 0.00001) but not sense foci (χ (2), p = 0.75) correlated with mislocalisation of TDP-43, which is the hallmark of ALS neurodegeneration. This has implications for translational approaches to C9ORF72 disease, and furthermore interacting RNA-processing factors and transcriptional activators responsible for antisense versus sense transcription might represent novel therapeutic targets.

Conserved proline-directed phosphorylation regulates SR protein conformation and splicing function

The alternative splicing of human genes is dependent on SR proteins, a family of essential splicing factors whose name derives from a signature C-terminal domain rich in arginine-serine dipeptide repeats (RS domains). Although the SRPKs (SR-specific protein kinases) phosphorylate these repeats, RS domains also contain prolines with flanking serines that are phosphorylated by a second family of protein kinases known as the CLKs (Cdc2-like kinases). The role of specific serine-proline phosphorylation within the RS domain has been difficult to assign since CLKs also phosphorylate arginine-serine dipeptides and, thus, display overlapping residue specificities with the SRPKs. In the present study, we address the effects of discrete serine-proline phosphorylation on the conformation and cellular function of the SR protein SRSF1 (SR protein splicing factor 1). Using chemical tagging and dephosphorylation experiments, we show that modification of serine-proline dipeptides broadly amplifies the conformational ensemble of SRSF1. The induction of these new structural forms triggers SRSF1 mobilization in the nucleus and alters its binding mechanism to an exonic splicing enhancer in precursor mRNA. These physical events correlate with changes in the alternative splicing of over 100 human genes based on a global splicing assay. Overall, these studies draw a direct causal relationship between a specific type of chemical modification in an SR protein and the regulation of alternative gene splicing programmes.

N-terminus of the protein kinase CLK1 induces SR protein hyperphosphorylation

SR proteins are essential splicing factors that are regulated through multisite phosphorylation of their RS (arginine/serine-rich) domains by two major families of protein kinases. The SRPKs (SR-specific protein kinases) efficiently phosphorylate the arginine/serine dipeptides in the RS domain using a conserved docking groove in the kinase domain. In contrast, CLKs (Cdc2-like kinases) lack a docking groove and phosphorylate both arginine/serine and serine-proline dipeptides, modifications that generate a hyperphosphorylated state important for unique SR protein-dependent splicing activities. All CLKs contain long flexible N-terminal extensions (140-300 residues) that resemble the RS domains present in their substrate SR proteins. We showed that the N-terminus in CLK1 contacts both the kinase domain and the RS domain of the SR protein SRSF1 (SR protein splicing factor 1). This interaction not only is essential for facilitating hyperphosphorylation, but also induces co-operative binding of SRSF1 to RNA. The N-terminus of CLK1 enhances the total phosphoryl contents of a panel of physiological substrates including SRSF1, SRSF2, SRSF5 and Tra2β1 (transformer 2β1) by 2-3-fold. These findings suggest that CLK1-dependent hyperphosphorylation is the result of a general mechanism in which the N-terminus acts as a bridge connecting the kinase domain and the RS domain of the SR protein.

Thermodynamic and phylogenetic insights into hnRNP A1 recognition of the HIV-1 exon splicing silencer 3 element

Complete expression of the HIV-1 genome requires balanced usage of suboptimal splice sites. The 3′ acceptor site A7 (ssA7) is negatively regulated in part by an interaction between the host hnRNP A1 protein and a viral splicing silencer (ESS3). Binding of hnRNP A1 to ESS3 and other upstream silencers is sufficient to occlude spliceosome assembly. Efforts to understand the splicing repressive properties of hnRNP A1 on ssA7 have revealed hnRNP A1 binds specific sites within the context of a highly folded RNA structure; however, biochemical models assert hnRNP A1 disrupts RNA structure through cooperative spreading. In an effort to improve our understanding of the ssA7 binding properties of hnRNP A1, herein we have performed a combined phylogenetic and biophysical study of the interaction of its UP1 domain with ESS3. Phylogenetic analyses of group M sequences (x̅ = 2860) taken from the Los Alamos HIV database reveal the ESS3 stem loop (SL3^ESS3) structure has been conserved throughout HIV-1 evolution, despite variations in primary sequence. Calorimetric titrations with UP1 clearly show the SL3^ESS3 structure is a critical binding determinant because deletion of the base-paired region reduces the affinity by ∼150-fold (K_d values of 27.8 nM and 4.2 μM). Cytosine substitutions of conserved apical loop nucleobases show UP1 preferentially binds purines over pyrimidines, where site-specific interactions were detected via saturation transfer difference nuclear magnetic resonance. Chemical shift mapping of the UP1–SL3^ESS3 interface by ¹H–¹⁵N heteronuclear single-quantum coherence spectroscopy titrations reveals a broad interaction surface on UP1 that encompasses both RRM domains and the inter-RRM linker. Collectively, our results describe a UP1 binding mechanism that is likely different from current models used to explain the alternative splicing properties of hnRNP A1.