Sequence-specific RNA recognition by an RGG motif connects U1 and U2 snRNP for spliceosome assembly.

In mammals, the structural basis for the interaction between U1 and U2 small nuclear ribonucleoproteins (snRNPs) during the early steps of splicing is still elusive. The binding of the ubiquitin-like (UBL) domain of SF3A1 to the stem-loop 4 of U1 snRNP (U1-SL4) contributes to this interaction. Here, we determined the 3D structure of the complex between the UBL of SF3A1 and U1-SL4 RNA. Our crystallography, NMR spectroscopy, and cross-linking mass spectrometry data show that SF3A1-UBL recognizes, sequence specifically, the GCG/CGC RNA stem and the apical UUCG tetraloop of U1-SL4. In vitro and in vivo mutational analyses support the observed intermolecular contacts and demonstrate that the carboxyl-terminal arginine-glycine-glycine-arginine (RGGR) motif of SF3A1-UBL binds sequence specifically by inserting into the RNA major groove. Thus, the characterization of the SF3A1-UBL/U1-SL4 complex expands the repertoire of RNA binding domains and reveals the capacity of RGG/RG motifs to bind RNA in a sequence-specific manner.

Altered expression levels of long non-coding natural antisense transcripts overlapping the UGT73C6 gene affect rosette size in Arabidopsis thaliana.

Natural antisense long non-coding RNAs (lncNATs) are involved in the regulation of gene expression in plants, modulating different relevant developmental processes and responses to various stimuli. We have identified and characterized two lncNATs (NAT1UGT73C6 and NAT2UGT73C6 , collectively NATsUGT73C6 ) from Arabidopsis thaliana that are transcribed from a gene fully overlapping UGT73C6, a member of the UGT73C subfamily of genes encoding UDP-glycosyltransferases (UGTs). Expression of both NATsUGT73C6 is developmentally controlled and occurs independently of the transcription of UGT73C6 in cis. Downregulation of NATsUGT73C6 levels through artificial microRNAs results in a reduction of the rosette area, while constitutive overexpression of NAT1UGT73C6 or NAT2UGT73C6 leads to the opposite phenotype, an increase in rosette size. This activity of NATsUGT73C6 relies on its RNA sequence and, although modulation of UGT73C6 in cis cannot be excluded, the observed phenotypes are not a consequence of the regulation of UGT73C6 in trans. The NATsUGT73C6 levels were shown to affect cell proliferation and thus individual leaf size. Consistent with this concept, our data suggest that the NATsUGT73C6 influence the expression levels of key transcription factors involved in regulating leaf growth by modulating cell proliferation. These findings thus reveal an additional regulatory layer on the process of leaf growth. In this work, we characterized at the molecular level two long non-coding RNAs (NATsUGT73C6 ) that are transcribed in the opposite direction to UGT73C6, a gene encoding a glucosyltransferase involved in brassinosteroid homeostasis in A. thaliana. Our results indicate that NATsUGT73C6 expression influences leaf growth by acting in trans and by modulating the levels of transcription factors that are involved in the regulation of cell proliferation.

NMR and EPR reveal a compaction of the RNA-binding protein FUS upon droplet formation.

Many RNA-binding proteins undergo liquid-liquid phase separation, which underlies the formation of membraneless organelles, such as stress granules and P-bodies. Studies of the molecular mechanism of phase separation in vitro are hampered by the coalescence and sedimentation of organelle-sized droplets interacting with glass surfaces. Here, we demonstrate that liquid droplets of fused in sarcoma (FUS)-a protein found in cytoplasmic aggregates of amyotrophic lateral sclerosis and frontotemporal dementia patients-can be stabilized in vitro using an agarose hydrogel that acts as a cytoskeleton mimic. This allows their spectroscopic characterization by liquid-phase NMR and electron paramagnetic resonance spectroscopy. Protein signals from both dispersed and condensed phases can be observed simultaneously, and their respective proportions can be quantified precisely. Furthermore, the agarose hydrogel acts as a cryoprotectant during shock-freezing, which facilitates pulsed electron paramagnetic resonance measurements at cryogenic temperatures. Surprisingly, double electron-electron resonance measurements revealed a compaction of FUS in the condensed phase.

An in vitro reconstituted U1 snRNP allows the study of the disordered regions of the particle and the interactions with proteins and ligands.

U1 small nuclear ribonucleoparticle (U1 snRNP) plays a central role during RNA processing. Previous structures of U1 snRNP revealed how the ribonucleoparticle is organized and recognizes the pre-mRNA substrate at the exon-intron junction. As with many other ribonucleoparticles involved in RNA metabolism, U1 snRNP contains extensions made of low complexity sequences. Here, we developed a protocol to reconstitute U1 snRNP in vitro using mostly full-length components in order to perform liquid-state NMR spectroscopy. The accuracy of the reconstitution was validated by probing the shape and structure of the particle by SANS and cryo-EM. Using an NMR spectroscopy-based approach, we probed, for the first time, the U1 snRNP tails at atomic detail and our results confirm their high degree of flexibility. We also monitored the labile interaction between the splicing factor PTBP1 and U1 snRNP and validated the U1 snRNA stem loop 4 as a binding site for the splicing regulator on the ribonucleoparticle. Altogether, we developed a method to probe the intrinsically disordered regions of U1 snRNP and map the interactions controlling splicing regulation. This approach could be used to get insights into the molecular mechanisms of alternative splicing and screen for potential RNA therapeutics.

Single Nucleotide Resolution RNA-Protein Cross-Linking Mass Spectrometry: A Simple Extension of the CLIR-MS Workflow.

RNA-protein interactions mediate many intracellular processes. CLIR-MS (cross-linking of isotope-labeled RNA and tandem mass spectrometry) allows the identification of RNA-protein interaction sites at single nucleotide/amino acid resolution in a single experiment. Using isotopically labeled RNA segments for UV-light-induced cross-linking generates characteristic isotope patterns that constrain the sequence database searches, increasing spatial resolution. Whereas the use of segmentally isotopically labeled RNA is effective, it is technically involved and not applicable in some settings, e.g., in cell or tissue samples. Here we introduce an extension of the CLIR-MS workflow that uses unlabeled RNA during cross-linking and subsequently adds an isotopic label during sample preparation for MS analysis. After RNase and protease digests of a cross-linked complex, the nucleic acid part of a peptide-RNA conjugate is labeled using the enzyme T4 polynucleotide kinase and a 1:1 mixture of heavy 18O4-γ-ATP and light ATP. In this simple, one-step reaction, three heavy oxygen atoms are transferred from the γ-phosphate to the 5'-end of the RNA, introducing an isotopic shift of 6.01 Da that is detectable by mass spectrometry. We applied this approach to the RNA recognition motif (RRM) of the protein FOX1 in complex with its cognate binding substrate, FOX-binding element (FBE) RNA. We also labeled a single phosphate within an RNA and unambiguously determined the cross-linking site of the FOX1-RRM binding to FBE at single residue resolution on the RNA and protein level and used differential ATP labeling for relative quantification based on isotope dilution. Data are available via ProteomeXchange with the identifier PXD024010.

Specific protein-RNA interactions are mostly preserved in biomolecular condensates.

Many biomolecular condensates are enriched in and depend on RNAs and RNA binding proteins (RBPs). So far, only a few studies have addressed the characterization of the intermolecular interactions responsible for liquid-liquid phase separation (LLPS) and the impact of condensation on RBPs and RNAs. Here, we present an approach to study protein-RNA interactions inside biomolecular condensates by applying cross-linking of isotope labeled RNA and tandem mass spectrometry to phase-separating systems (LLPS-CLIR-MS). LLPS-CLIR-MS enables the characterization of intermolecular interactions present within biomolecular condensates at residue-specific resolution and allows a comparison with the same complexes in the dispersed phase. We observe that sequence-specific RBP-RNA interactions present in the dispersed phase are generally maintained inside condensates. In addition, LLPS-CLIR-MS identifies structural alterations at the protein-RNA interfaces, including additional unspecific contacts in the condensed phase. Our approach offers a procedure to derive structural information of protein-RNA complexes within biomolecular condensates that could be critical for integrative structural modeling of ribonucleoproteins (RNPs) in this form.

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.

Probing the Interactions of Splicing Regulatory Small Molecules and Proteins with U1 snRNP Using NMR Spectroscopy.

Alternative RNA splicing is an essential part of gene expression that not only increases the protein diversity of metazoan but also provides an additional layer of gene expression regulation. The U1 small ribonucleoparticle (U1 snRNP) plays an essential role in seeding spliceosome assembly and its binding on weak 5'-splice sites is regulated by transient interactions with splicing factors. Recent progress in allele specific splicing correction has shown the therapeutic potential offered by small molecule splicing modifiers that specifically promotes the recruitment of U1 snRNP to modulate alternative splicing and gene expression. Here, we described a method to reconstitute U1 snRNP in vitro and to study labile interactions with protein or synthetic splicing factors using solution state NMR spectroscopy. This approach allowed us to validate direct interactions between splicing regulators and U1 snRNP and could also be useful for the screening of small molecules acting on splicing regulation.

Nucleotide-amino acid π-stacking interactions initiate photo cross-linking in RNA-protein complexes.

Photo-induced cross-linking is a mainstay technique to characterize RNA-protein interactions. However, UV-induced cross-linking between RNA and proteins at "zero-distance" is poorly understood. Here, we investigate cross-linking of the RBFOX alternative splicing factor with its hepta-ribonucleotide binding element as a model system. We examine the influence of nucleobase, nucleotide position and amino acid composition using CLIR-MS technology (crosslinking-of-isotope-labelled-RNA-and-tandem-mass-spectrometry), that locates cross-links on RNA and protein with site-specific resolution. Surprisingly, cross-linking occurs only at nucleotides that are π-stacked to phenylalanines. Notably, this π-stacking interaction is also necessary for the amino-acids flanking phenylalanines to partake in UV-cross-linking. We confirmed these observations in several published datasets where cross-linking sites could be mapped to a high resolution structure. We hypothesize that π-stacking to aromatic amino acids activates cross-linking in RNA-protein complexes, whereafter nucleotide and peptide radicals recombine. These findings will facilitate interpretation of cross-linking data from structural studies and from genome-wide datasets generated using CLIP (cross-linking-and-immunoprecipitation) methods.