Somatic mutations of pre-mRNA splicing factors recur among patients with myelodysplastic syndrome (MDS) and related malignancies. Although these MDS-relevant mutations alter the splicing of a subset of transcripts, the mechanisms by which these single amino acid substitutions change gene expression remain controversial. New structures of spliceosome intermediates and associated protein complexes shed light on the molecular interactions mediated by ‘hotspots’ of the SF3B1 and U2AF1 pre-mRNA splicing factors. The frequently mutated SF3B1 residues contact the pre-mRNA splice site. Based on structural homology with other spliceosome subunits, and recent findings of altered RNA binding by mutant U2AF1 proteins, we suggest that affected U2AF1 residues also contact pre-mRNA. Altered pre-mRNA recognition emerges as a molecular theme among MDS-relevant mutations of pre-mRNA splicing factors.

Purine interruptions of polypyrimidine (Py) tract splice site signals contribute to human genetic diseases. The essential splicing factor U2AF65 normally recognizes a Py tract consensus sequence preceding the major class of 3′ splice sites. We found that neurofibromatosis- or retinitis pigmentosa-causing mutations in the 5′ regions of Py tracts severely reduce U2AF65 affinity. Conversely, we identified a preferred binding site of U2AF65 for purine substitutions in the 3′ regions of Py tracts. Based on a comparison of new U2AF65 structures bound to either A- or G-containing Py tracts with previously identified pyrimidine-containing structures, we expected to find that a D231V amino acid change in U2AF65 would specify U over other nucleotides. We found that the crystal structure of the U2AF65-D231V variant confirms favorable packing between the engineered valine and a target uracil base. The D231V amino acid change restores U2AF65 affinity for two mutated splice sites that cause human genetic diseases and successfully promotes splicing of a defective retinitis pigmentosa-causing transcript. We conclude that reduced U2AF65 binding is a molecular consequence of disease-relevant mutations, and that a structure-guided U2AF65 variant is capable of manipulating gene expression in eukaryotic cells.

U2AF65 is essential for pre-mRNA splicing in most eukaryotes. Two consecutive RNA recognition motifs (RRM) of U2AF65 recognize a polypyrimidine tract at the 3′ splice site. Here, we use small-angle X-ray scattering to demonstrate that the tandem U2AF65 RRMs exhibit a broad range of conformations in the solution ensemble. The majority of U2AF65 conformations exhibit few contacts between the RRMs, such as observed in the crystal structure. A subpopulation adopts tight inter-RRM contacts, such as independently reported based on paramagnetic relaxation enhancements. These complementary structural methods demonstrate that diverse splice sites have the opportunity to select compact or extended inter-RRM proximities from the U2AF65 conformational pool.

The RNA recognition motif (RRM) is a prevalent class of RNA binding domains. Although a number of RRM/RNA structures have been determined, thermodynamic analyses are relatively uncommon. Here, we use isothermal titration calorimetry to characterize single-stranded (ss)RNA binding by four representative RRM-containing proteins: (i) U2AF65, (ii) SXL, (iii) TIA-1, and (iv) PAB. In all cases, ssRNA binding is accompanied by remarkably large favorable enthalpy changes (−30 to −60 kcal mol−1) and unfavorable entropy changes. Alterations of key RRM residues and binding sites indicate that under the nearly physiological conditions of these studies, large thermodynamic changes represent a signature of specific ssRNA recognition by RRMs.

A pseudouridine-modified region of the U2 small nuclear (sn)RNA anneals with the intronic branchpoint sequence and positions a bulged adenosine to serve as the nucleophile in the first chemical step of pre-mRNA splicing. We have determined three X-ray structures of RNA oligonucleotides containing the pseudouridylated U2 snRNA and the branchpoint consensus sequences. The expected adenosine branchpoint is extrahelical in a 1.65 Å resolution structure containing the mammalian consensus sequence variant and in a 2.10 Å resolution structure containing a shortened Saccharomyces cerevisiae consensus sequence. The adenosine adjacent to the expected branchpoint is extrahelical in a third structure, which contains the intact yeast consensus sequence at 1.57 Å resolution. The hydration and base stacking interactions mediated by the U2 snRNA pseudouridines correlate with the identity of the unpaired adenosine. The expected adenosine bulge is associated with a well-stacked pseudouridine, which is linked via an ordered water molecule to a neighboring nucleotide. In contrast, the bulge of the adjacent adenosine shifts the base stacking and disrupts the water-mediated interactions of the pseudouridine. These structural differences may contribute to the ability of the pseudouridine modification to promote the bulged conformation of the branch site adenosine and to enhance catalysis by snRNAs. Furthermore, iodide binding sites are identified adjacent to the unconventional bulged adenosine, and the structure of the mammalian consensus sequence variant provides a high-resolution view of a hydrated magnesium ion bound in a similar manner to a divalent cation binding site of the group II intron.

The essential splicing factors U2AF65 and SF1 cooperatively bind consensus sequences at the 3′ end of introns. Phosphorylation of SF1 on a highly conserved “SPSP” motif enhances its interaction with U2AF65 and the pre-mRNA. Here, we reveal that phosphorylation induces essential conformational changes in SF1 and in the SF1/U2AF65/3′ splice site complex. Crystal structures of the phosphorylated (P)SF1 domain bound to the C-terminal domain of U2AF65 at 2.29 Å resolution and of the unphosphorylated SF1 domain at 2.48 Å resolution demonstrate that phosphorylation induces a disorder-to-order transition within a previously unknown SF1/U2AF65 interface. We find by small-angle X-ray scattering that the local folding of the SPSP motif transduces into global conformational changes in the nearly full-length (P)SF1/U2AF65/3′ splice site assembly. We further determine that SPSP phosphorylation and the SF1/U2AF65 interface are essential in vivo. These results offer a structural prototype for phosphorylation-dependent control of pre-mRNA splicing factors.Graphical AbstractDownload high-res image (401KB)Download full-size imageHighlights► Splicing factor SF1 phosphorylation on a conserved SPSP motif is required in vivo ► SPSP phosphorylation (P) induces local folding within an SF1/U2AF65 interface ► Phosphorylation promotes an acutely bent (P)SF1/U2AF65/RNA conformation

Spliceosomes assemble on pre-mRNA splice sites through a series of dynamic ribonucleoprotein complexes, yet the nature of the conformational changes remains unclear. Splicing factor 1 (SF1) and U2 auxiliary factor (U2AF65) cooperatively recognize the 3′ splice site during the initial stages of pre-mRNA splicing. Here, we used small-angle X-ray scattering to compare the molecular dimensions and ab initio shape restorations of SF1 and U2AF65 splicing factors, as well as the SF1/U2AF65 complex in the absence and presence of AdML (adenovirus major late) splice site RNAs. The molecular dimensions of the SF1/U2AF65/RNA complex substantially contracted by 15 Å in the maximum dimension, relative to the SF1/U2AF65 complex in the absence of RNA ligand. In contrast, no detectable changes were observed for the isolated SF1 and U2AF65 splicing factors or their individual complexes with RNA, although slight differences in the shapes of their molecular envelopes were apparent. We propose that the conformational changes that are induced by assembly of the SF1/U2AF65/RNA complex serve to position the pre-mRNA splice site optimally for subsequent stages of splicing.Graphical AbstractDownload high-res image (56KB)Download full-size imageResearch Highlights► SAXS analyses of SF1, U2AF65, and their complexes with splice site RNAs. ► Dimensions of apo- SF1 or U2AF65 remain similar when bound to their RNA sites. ► Dimensions of the SF1/U2AF65 complex contract when bound to a 3’ splice site RNA.

Nicotinamide adenine dinucleotides have emerged as key signals of the cellular redox state. Yet the structural basis for allosteric gene regulation by the ratio of reduced NADH to oxidized NAD+ is poorly understood. A key sensor among Gram-positive bacteria, Rex represses alternative respiratory gene expression until a limited oxygen supply elevates the intracellular NADH:NAD+ ratio. Here we investigate the molecular mechanism for NADH/NAD+ sensing among Rex family members by determining structures of Thermus aquaticus Rex bound to (1) NAD+, (2) DNA operator, and (3) without ligand. Comparison with the Rex/NADH complex reveals that NADH releases Rex from the DNA site following a 40° closure between the dimeric subunits. Complementary site-directed mutagenesis experiments implicate highly conserved residues in NAD-responsive DNA-binding activity. These rare views of a redox sensor in action establish a means for slight differences in the nicotinamide charge, pucker, and orientation to signal the redox state of the cell.Graphical AbstractDownload high-res image (106KB)Download full-size imageHighlights▸ A structural comparison of Rex in the NAD+/DNA-bound, apo-, and NADH-bound states ▸ A rigid-body rotation of Rex subunits permits DNA binding after NADH dissociation ▸ A single NAD+ binds the Rex dimer/DNA complex ▸ A triad of conserved residues are required for NADH/NAD+ sensing by Rex

T-cell intracellular antigen-1 (TIA-1) regulates developmental and stress-responsive pathways through distinct activities at the levels of alternative pre-mRNA splicing and mRNA translation. The TIA-1 polypeptide contains three RNA recognition motifs (RRMs). The central RRM2 and C-terminal RRM3 associate with cellular mRNAs. The N-terminal RRM1 enhances interactions of a C-terminal Q-rich domain of TIA-1 with the U1-C splicing factor, despite linear separation of the domains in the TIA-1 sequence. Given the expanded functional repertoire of the RRM family, it was unknown whether TIA-1 RRM1 contributes to RNA binding as well as documented protein interactions. To address this question, we used isothermal titration calorimetry and small-angle X-ray scattering to dissect the roles of the TIA-1 RRMs in RNA recognition. Notably, the fas RNA exhibited two binding sites with indistinguishable affinities for TIA-1. Analyses of TIA-1 variants established that RRM1 was dispensable for binding AU-rich fas sites, yet all three RRMs were required to bind a polyU RNA with high affinity. Small-angle X-ray scattering analyses demonstrated a “V” shape for a TIA-1 construct comprising the three RRMs and revealed that its dimensions became more compact in the RNA-bound state. The sequence-selective involvement of TIA-1 RRM1 in RNA recognition suggests a possible role for RNA sequences in regulating the distinct functions of TIA-1. Further implications for U1-C recruitment by the adjacent TIA-1 binding sites of the fas pre-mRNA and the bent TIA-1 shape, which organizes the N- and C-termini on the same side of the protein, are discussed.Download high-res image (83KB)Download full-size imageHighlights► Three tandem RRMs are required for TIA-1 to bind a polyU RNA with high affinity. ► The N-terminal RRM is not required for TIA-1 to bind a fas splice site RNA. ► The average solution conformation of three TIA-1 RRMs is an obtuse V shape. ► The tandem TIA-1 RRMs become more compact when bound to RNA.