Dustin B. Ritchie

Structural elucidation of a PRP8 core domain from the heart of the spliceosome

The spliceosome is a complex ribonucleoprotein (RNP) particle containing five RNAs and more than 100 associated proteins. One of these proteins, PRP8, has been shown to interact directly with the splice sites and branch region of precursor-mRNAs (pre-mRNAs) and spliceosomal RNAs associated with catalysis of the two steps of splicing. The 1.85-Å X-ray structure of the core of PRP8 domain IV, implicated in key spliceosomal interactions, reveals a bipartite structure that includes the presence of an RNase H fold linked to a five-helix assembly. Analysis of mutant yeast alleles and cross-linking results in the context of this structure, coupled with RNA binding studies, suggests that domain IV forms a surface that interacts directly with the RNA structures at the catalytic core of the spliceosome.

Programmed −1 frameshifting efficiency correlates with RNA pseudoknot conformational plasticity, not resistance to mechanical unfolding

Programmed −1 frameshifting, whereby the reading frame of a ribosome on messenger RNA is shifted in order to generate an alternate gene product, is often triggered by a pseudoknot structure in the mRNA in combination with an upstream slippery sequence. The efficiency of frameshifting varies widely for different sites, but the factors that determine frameshifting efficiency are not yet fully understood. Previous work has suggested that frameshifting efficiency is related to the resistance of the pseudoknot against mechanical unfolding. We tested this hypothesis by studying the mechanical properties of a panel of pseudoknots with frameshifting efficiencies ranging from 2% to 30%: four pseudoknots from retroviruses, two from luteoviruses, one from a coronavirus, and a nonframeshifting bacteriophage pseudoknot. Using optical tweezers to apply tension across the RNA, we measured the distribution of forces required to unfold each pseudoknot. We found that neither the average unfolding force, nor the unfolding kinetics, nor the parameters describing the energy landscape for mechanical unfolding of the pseudoknot (energy barrier height and distance to the transition state) could be correlated to frameshifting efficiency. These results indicate that the resistance of pseudoknots to mechanical unfolding is not a primary determinant of frameshifting efficiency. However, increased frameshifting efficiency was correlated with an increased tendency to form alternate, incompletely folded structures, suggesting a more complex picture of the role of the pseudoknot involving the conformational dynamics.

Spliceosome structure: piece by piece.

Processing of pre-mRNAs by RNA splicing is an essential step in the maturation of protein coding RNAs in eukaryotes. Structural studies of the cellular splicing machinery, the spliceosome, are a major challenge in structural biology due to the size and complexity of the splicing ensemble. Specifically, the structural details of splice site recognition and the architecture of the spliceosome active site are poorly understood. X-ray and NMR techniques have been successfully used to address these questions defining the structure of individual domains, isolated splicing proteins, spliceosomal RNA fragments and recently the U1 snRNP multiprotein.RNA complex. These results combined with extant biochemical and genetic data have yielded important insights as well as posing fresh questions with respect to the regulation and mechanism of this critical gene regulatory process.

Pre-mRNA splicing: a complex picture in higher definition

Intron excision from pre-mRNAs of higher eukaryotes requires a transition from splice-site recognition across short exons to organization of the spliceosome across long introns. Recently, insight into this transition has been provided and, in addition, it has been shown that an alternative splicing factor, the polypyrimidine-tract-binding protein, can exert its control on splice-site choice by blocking this key step in the assembly of the splicing machinery.

Probing the structural dynamics of proteins and nucleic acids with optical tweezers.

Conformational changes are an essential feature of most molecular processes in biology. Optical tweezers have emerged as a powerful tool for probing conformational dynamics at the single-molecule level because of their high resolution and sensitivity, opening new windows on phenomena ranging from folding and ligand binding to enzyme function, molecular machines, and protein aggregation. By measuring conformational changes induced in a molecule by forces applied by optical tweezers, new insight has been gained into the relationship between dynamics and function. We discuss recent advances from studies of how structure forms in proteins and RNA, including non-native structures, fluctuations in disordered proteins, and interactions with chaperones assisting native folding. We also review the development of assays probing the dynamics of complex protein–nucleic acid and protein–protein assemblies that reveal the dynamic interactions between biomolecular machines and their substrates.

Characterization of a U2AF-independent commitment complex (E') in the mammalian spliceosome assembly pathway.

ABSTRACT Early recognition of pre-mRNA during spliceosome assembly in mammals proceeds through the association of U1 small nuclear ribonucleoprotein particle (snRNP) with the 5′ splice site as well as the interactions of the branch binding protein SF1 with the branch region and the U2 snRNP auxiliary factor U2AF with the polypyrimidine tract and 3′ splice site. These factors, along with members of the SR protein family, direct the ATP-independent formation of the early (E) complex that commits the pre-mRNA to splicing. We report here the observation in U2AF-depleted HeLa nuclear extract of a distinct, ATP-independent complex designated E′ which can be chased into E complex and itself commits a pre-mRNA to the splicing pathway. The E′ complex is characterized by a U1 snRNA-5′ splice site base pairing, which follows the actual commitment step, an interaction of SF1 with the branch region, and a close association of the 5′ splice site with the branch region. These results demonstrate that both commitment to splicing and the early proximity of conserved sequences within pre-mRNA substrates can occur in a minimal complex lacking U2AF, which may function as a precursor to E complex in spliceosome assembly.

A conformational switch in PRP8 mediates metal ion coordination that promotes pre-mRNA exon ligation.

Splicing of pre-mRNAs in eukaryotes is catalyzed by the spliceosome, a large RNA-protein metalloenzyme. The catalytic center of the spliceosome involves a structure comprising the U2 and U6 snRNAs and includes a metal bound by U6 snRNA. The precise architecture of the splicesome active site, however, and the question of whether it includes protein components, remains unresolved. A wealth of evidence places the protein PRP8 at the heart of the spliceosome through assembly and catalysis. Here we provide evidence that the RNase H domain of PRP8 undergoes a conformational switch between the two steps of splicing, rationalizing yeast prp8 alleles that promote either the first or second step. We also show that this switch unmasks a metal-binding site involved in the second step. Together, these data establish that PRP8 is a metalloprotein that promotes exon ligation within the spliceosome.

Anti-frameshifting ligand reduces the conformational plasticity of the SARS virus pseudoknot.

Programmed -1 ribosomal frameshifting (-1 PRF) stimulated by mRNA pseudoknots regulates gene expression in many viruses, making pseudoknots potential targets for anti-viral drugs. The mechanism by which pseudoknots trigger -1 PRF, however, remains controversial, with several competing models. Recent work showed that high -1 PRF efficiency was linked to high pseudoknot conformational plasticity via the formation of alternate conformers. We tested whether pseudoknots bound with an anti-frameshifting ligand exhibited a similar correlation between conformational plasticity and -1 PRF efficiency by measuring the effects of a ligand that was found to inhibit -1 PRF in the SARS coronavirus on the conformational dynamics of the SARS pseudoknot. Using single-molecule force spectroscopy to unfold pseudoknots mechanically, we found that the ligand binding effectively abolished the formation of alternate conformers. This result extends the connection between -1 PRF and conformational dynamics and, moreover, suggests that targeting the conformational dynamics of pseudoknots may be an effective strategy for anti-viral drug design.

Context-Dependent Remodeling of Structure in Two Large Protein Fragments

Protein folding involves the formation of secondary structural elements from the primary sequence and their association with tertiary assemblies. The relation of this primary sequence to a specific folded protein structure remains a central question in structural biology. An increasing body of evidence suggests that variations in homologous sequence ranging from point mutations to substantial insertions or deletions can yield stable proteins with markedly different folds. Here we report the structural characterization of domain IV (D4) and ΔD4 (polypeptides with 222 and 160 amino acids, respectively) that differ by virtue of an N-terminal deletion of 62 amino acids (28% of the overall D4 sequence). The high-resolution crystal structures of the monomeric D4 and the dimeric ΔD4 reveal substantially different folds despite an overall conservation of secondary structure. These structures show that the formation of tertiary structures, even in extended polypeptide sequences, can be highly context dependent, and they serve as a model for structural plasticity in protein isoforms.

High-Precision Single-Molecule Characterization of the Folding of an HIV RNA Hairpin by Atomic Force Microscopy

The folding of RNA into a wide range of structures is essential for its diverse biological functions from enzymatic catalysis to ligand binding and gene regulation. The unfolding and refolding of individual RNA molecules can be probed by single-molecule force spectroscopy (SMFS), enabling detailed characterization of the conformational dynamics of the molecule as well as the free-energy landscape underlying folding. Historically, high-precision SMFS studies of RNA have been limited to custom-built optical traps. Although commercial atomic force microscopes (AFMs) are widely deployed and offer significant advantages in ease-of-use over custom-built optical traps, traditional AFM-based SMFS lacks the sensitivity and stability to characterize individual RNA molecules precisely. Here, we developed a high-precision SMFS assay to study RNA folding using a commercial AFM and applied it to characterize a small RNA hairpin from HIV that plays a key role in stimulating programmed ribosomal frameshifting. We achieved rapid data acquisition in a dynamic assay, unfolding and then refolding the same individual hairpin more than 1,100 times in 15 min. In comparison to measurements using optical traps, our AFM-based assay featured a stiffer force probe and a less compliant construct, providing a complementary measurement regime that dramatically accelerated equilibrium folding dynamics. Not only did kinetic analysis of equilibrium trajectories of the HIV RNA hairpin yield the traditional parameters used to characterize folding by SMFS (zero-force rate constants and distances to the transition state), but we also reconstructed the full 1D projection of the folding free-energy landscape comparable to state-of-the-art studies using dual-beam optical traps, a first for this RNA hairpin and AFM studies of nucleic acids in general. Looking forward, we anticipate that the ease-of-use of our high-precision assay implemented on a commercial AFM will accelerate studying folding of diverse nucleic acid structures.