RNA tertiary structure determination: NOE pathways construction by tabu search

MOTIVATION: Liquid state nuclear magnetic resonance (NMR) spectroscopy has now been well established as a method for RNA tertiary structure determination. Most of the steps involved in the determination of RNA molecules are performed using computer programs. They however, do not apply to resonance assignment being the starting point of the whole procedure. We propose a tabu search algorithm as a tool for automating this step. Nuclear overhause effect (NOE) pathway, which determines the assignment, is constructed during an analysis of possible connections between resonances within aromatic/anomeric region of two-dimensional NOESY spectrum resulting from appropriate NMR experiment. RESULTS: Computational tests demonstrate the superior performance of the tabu search algorithm as compared with the exact enumerative approach and genetic procedure applied to the experimental and simulated spectral data for RNA molecules. AVAILABILITY: The software package can be obtained upon request from Marta Szachniuk.     

RNAPack: an integrated NMR approach to RNA structure determination

Over the last decade, a vast number of useful nuclear magnetic resonance (NMR) experiments have been developed and successfully employed to determine the structure and dynamics of RNA oligonucleotides. Despite this progress, high-resolution RNA structure determination by NMR spectroscopy still remains a lengthy process and requires programming and extensive calibrations to perform NMR experiments successfully. To accelerate RNA structure determination by NMR spectroscopy, we have designed and programmed a package of RNA NMR experiments, called RNAPack. The user-friendly package contains a set of semiautomated single, double, and triple resonance NMR experiments, which are fully optimized for high-resolution RNA solution structure determination on Varian NMR spectrometers. RNAPack provides an autocalibration feature that allows rapid calibration of all NMR experiments in a single step and thereby speeds up the NMR data collection and eliminates user errors. In our laboratory, we have successfully employed this technology to solve RNA solution structures of domains of the internal ribosome entry site of the genomic hepatitis C viral RNA in less than 3 months. RNAPack therefore makes NMR spectroscopy an attractive and rapid structural tool and allows integration of atomic resolution structural information into biochemical studies of large RNA systems.     

Secondary structure determination of conserved SARS-CoV-2 RNA elements by NMR spectroscopy

The current pandemic situation caused by the Betacoronavirus SARS-CoV-2 (SCoV2) highlights the need for coordinated research to combat COVID-19. A particularly important aspect is the development of medication. In addition to viral proteins, structured RNA elements represent a potent alternative as drug targets. The search for drugs that target RNA requires their high-resolution structural characterization. Using nuclear magnetic resonance (NMR) spectroscopy, a worldwide consortium of NMR researchers aims to characterize potential RNA drug targets of SCoV2. Here, we report the characterization of 15 conserved RNA elements located at the 5′ end, the ribosomal frameshift segment and the 3′-untranslated region (3′-UTR) of the SCoV2 genome, their large-scale production and NMR-based secondary structure determination. The NMR data are corroborated with secondary structure probing by DMS footprinting experiments. The close agreement of NMR secondary structure determination of isolated RNA elements with DMS footprinting and NMR performed on larger RNA regions shows that the secondary structure elements fold independently. The NMR data reported here provide the basis for NMR investigations of RNA function, RNA interactions with viral and host proteins and screening campaigns to identify potential RNA binders for pharmaceutical intervention.     

Conjoined Use of EM and NMR in RNA Structure Refinement

More than 40% of the RNA structures have been determined using nuclear magnetic resonance (NMR) technique. NMR mainly provides local structural information of protons and works most effectively on relatively small biomacromolecules. Hence structural characterization of large RNAs can be difficult for NMR alone. Electron microscopy (EM) provides global shape information of macromolecules at nanometer resolution, which should be complementary to NMR for RNA structure determination. Here we developed a new energy term in Xplor-NIH against the density map obtained by EM. We conjointly used NMR and map restraints for the structure refinement of three RNA systems — U2/U6 small-nuclear RNA, genome-packing motif (Ψ^CD)₂ from Moloney murine leukemia virus, and ribosome-binding element from turnip crinkle virus. In all three systems, we showed that the incorporation of a map restraint, either experimental or generated from known PDB structure, greatly improves structural precision and accuracy. Importantly, our method does not rely on an initial model assembled from RNA duplexes, and allows full torsional freedom for each nucleotide in the torsion angle simulated annealing refinement. As increasing number of macromolecules can be characterized by both NMR and EM, the marriage between the two techniques would enable better characterization of RNA three-dimensional structures.     

A polynomial-time algorithm for de novo protein backbone structure determination from nuclear magnetic resonance data.

We describe an efficient algorithm for protein backbone structure determination from solution Nuclear Magnetic Resonance (NMR) data. A key feature of our algorithm is that it finds the conformation and orientation of secondary structure elements as well as the global fold in polynomial time. This is the first polynomial-time algorithm for de novo high-resolution biomacromolecular structure determination using experimentally recorded data from either NMR spectroscopy or X-ray crystallography. Previous algorithmic formulations of this problem focused on using local distance restraints from NMR (e.g., nuclear Overhauser effect [NOE] restraints) to determine protein structure. This approach has been shown to be NP-hard, essentially due to the local nature of the constraints. In practice, approaches such as molecular dynamics and simulated annealing, which lack both combinatorial precision and guarantees on running time and solution quality, are used routinely for structure determination. We show that residual dipolar coupling (RDC) data, which gives global restraints on the orientation of internuclear bond vectors, can be used in conjunction with very sparse NOE data to obtain a polynomial-time algorithm for structure determination. Furthermore, an implementation of our algorithm has been applied to six different real biological NMR data sets recorded for three proteins. Our algorithm is combinatorially precise, polynomialtime, and uses much less NMR data to produce results that are as good or better than previous approaches in terms of accuracy of the computed structure as well as running time.     

Structural and dynamic studies of two antigenic loops from haemagglutinin: A relaxation matrix approach

Summary We have investigated the dynamics and structural behaviour of two antigenic peptides using ¹H NMR. The two cyclic peptides mimic the antigenic site A of influenza haemagglutinin protein; they only differ in the way they were cyclized and in the size of their respective linkers. Homonuclear relaxation parameters extracted from a complete NOE matrix were interpreted in terms of local dynamics. A set of distance constraints was deduced from these parameters which allowed 3D models to be constructed using distance geometry. NOE back-calculation was used to check the validity of the final models. Strong variations of internal motion amplitude have been found in both peptides along their backbone. Motions with high amplitudes have been localized in the Gly-Pro-Gly sequence which forms a -turn in both structures.Abbreviations DSS 3-(trimethylsilyl)-1-propanesulfonic acid - D-loop aspartic acid loop - ELISA enzyme-linked immunoabsorbent assay - f.i.d free induction decay - HOHAHA homonuclear Hartmann-Hahn spectroscopy - HPLC high pressure liquid chromatography - K-loop lysine loop - NMR nuclear magnetic resonance - NOE nuclear Overhauser enhancement - NOESY nuclear Overhauser enhancement spectroscopy - r.m.s.d. root-mean-square deviation of atomic positions     

Solution structure of a PAN module from the apicomplexan parasite Eimeria tenella

Micronemes, specialised organelles found in all apicomplexan parasites, secrete molecules that are essential for parasite attachment and invasion of host cells. EtMIC5 is one such microneme protein that contains eleven tandemly repeating modules. These modules have homology with the PAN module superfamily. Members of this family are found in blood clotting proteins, some growth factors and some nematode proteins. This paper presents the structure of the 9^th PAN module in EtMIC5, determined using high resolution NMR. The structure shows similarities to and some differences from the N-terminal module of hepatocyte growth factor (HGF), the only previous member of the PAN family with known structure. AbbreviationsNMR – nuclear magnetic resonance; NOE – nuclear Overhauser enhancement; NOESY – NOE spectroscopy; COSY – correlated spectroscopy; TOCSY – total correlated spectroscopy; HSQC – hetero nuclear single quantum coherence; HMQC-J – hetero nuclear multiple quantum coherence-J coupling; MICs – microneme proteins; EtMIC5 – a microneme protein from Eimeria tenella; Apple9 – the ninth Apple repeat of EtMIC5; FXI – blood coagulation factor XI; PK – plasma prekallikrein; HGF – hepatocyte growth factor.     

Influence of the completeness of chemical shift assignments on NMR structures obtained with automated NOE assignment

Reliable automated NOE assignment and structure calculation on the basis of a largely complete, assigned input chemical shift list and a list of unassigned NOESY cross peaks has recently become feasible for routine NMR protein structure calculation and has been shown to yield results that are equivalent to those of the conventional, manual approach. However, these algorithms rely on the availability of a virtually complete list of the chemical shifts. This paper investigates the influence of incomplete chemical shift assignments on the reliability of NMR structures obtained with automated NOESY cross peak assignment. The program CYANA was used for combined automated NOESY assignment with the CANDID algorithm and structure calculations with torsion angle dynamics at various degrees of completeness of the chemical shift assignment which was simulated by random omission of entries in the experimental ¹H chemical shift lists that had been used for the earlier, conventional structure determinations of two proteins. Sets of structure calculations were performed choosing the omitted chemical shifts randomly among all assigned hydrogen atoms, or among aromatic hydrogen atoms. For comparison, automated NOESY assignment and structure calculations were performed with the complete experimental chemical shift but under random omission of NOESY cross peaks. When heteronuclear-resolved three-dimensional NOESY spectra are available the current CANDID algorithm yields in the absence of up to about 10% of the experimental ¹H chemical shifts reliable NOE assignments and three-dimensional structures that deviate by less than 2 Å from the reference structure obtained using all experimental chemical shift assignments. In contrast, the algorithm can accommodate the omission of up to 50% of the cross peaks in heteronuclear- resolved NOESY spectra without producing structures with a RMSD of more than 2 Å to the reference structure. When only homonuclear NOESY spectra are available, the algorithm is slightly more susceptible to missing data and can tolerate the absence of up to about 7% of the experimental ¹H chemical shifts or of up to 30% of the NOESY peaks.Abbreviations: BmPBP^A – Bombyx mori pheromone binding protein form A; CYANA – combined assignment and dynamics algorithm for NMR applications; NMR – nuclear magnetic resonance; NOE – nuclear Overhauser effect; NOESY – NOE spectroscopy; RMSD – root-mean-square deviation; WmKT – Williopsis mrakii killer toxin     

Structural features of influenza A virus panhandle RNA enabling the activation of RIG-I independently of 5′-triphosphate

Retinoic acid-inducible gene I (RIG-I) recognizes specific molecular patterns of viral RNAs for inducing type I interferon. The C-terminal domain (CTD) of RIG-I binds to double-stranded RNA (dsRNA) with the 5′-triphosphate (5′-PPP), which induces a conformational change in RIG-I to an active form. It has been suggested that RIG-I detects infection of influenza A virus by recognizing the 5′-triphosphorylated panhandle structure of the viral RNA genome. Influenza panhandle RNA has a unique structure with a sharp helical bending. In spite of extensive studies of how viral RNAs activate RIG-I, whether the structural elements of the influenza panhandle RNA confer the ability to activate RIG-I signaling has been poorly explored. Here, we investigated the dynamics of the influenza panhandle RNA in complex with RIG-I CTD using NMR spectroscopy and showed that the bending structure of the panhandle RNA negates the requirement of a 5′-PPP moiety for RIG-I activation.     

Determination of three-dimensional solution structure of waglerin I,a toxin from Trimeresurus wagleri,using 2D-NMR and molecular dynamics simulation

The solution conformation of a synthetic snake venom toxin waglerin I, has been determined by using proton nuclear magnetic resonance spectroscopy. By y a combination of various two-dimensional NMR techniques, the ¹H-NMR spectrum of waglerin I was completely assigned. A set of 247 interproton distance restraints was derived from nuclear Overhauser enhancement (NOE) measurements. These NOE constraints, in addition to the 2 dihedral angle restraints (from coupling constant measurements) and 7 ω torsion angle restraints for prolines, formed the basis of three-dimensional structure determined by molecular dynamics techniques. The 19 structures that were obtained satisfy the experimental restraints, and display small deviation from idealized covalent geometry. Analysis of converged structures indicates that the toxin has no special secondary structure. In the solution structure of waglerin I, the central ring region is well defined but the N- and C-termini possesses more disorder.     

Quantifying the energetic interplay of RNA tertiary and secondary structure interactions

To understand the RNA-folding problem, we must know the extent to which RNA structure formation is hierarchical (tertiary folding of preformed secondary structure). Recently, nuclear magnetic resonance (NMR) spectroscopy was used to show that Mg2+-dependent tertiary interactions force secondary structure rearrangement in the 56-nt tP5abc RNA, a truncated subdomain of the Tetrahymena group I intron. Here we combine mutagenesis with folding computations, nondenaturing gel electrophoresis, high-resolution NMR spectroscopy, and chemical-modification experiments to probe further the energetic interplay of tertiary and secondary interactions in tP5abc. Point mutations predicted to destabilize the secondary structure of folded tP5abc greatly disrupt its Mg2+-dependent folding, as monitored by nondenaturing gels. Imino proton assignments and sequential NOE walks of the two-dimensional NMR spectrum of one of the tP5abc mutants confirm the predicted secondary structure, which does not change in the presence of Mg2+. In contrast to these data on tP5abc, the same point mutations in the context of the P4-P6 domain (of which P5abc is a subdomain) shift the Mg2+ dependence of P4-P6 folding only moderately, and dimethyl sulfate (DMS) modification experiments demonstrate that Mg2+ does cause secondary structure rearrangement of the P4-P6 mutants' P5abc subdomains. Our data provide experimental support for two simple conclusions: (1) Even single point mutations at bases involved only in secondary structure can be enough to tip the balance between RNA tertiary and secondary interactions. (2) Domain context must be considered in evaluating the relative importance of tertiary and secondary contributions. This tertiary/secondary interplay is likely relevant to the folding of many large RNA and to bimolecular snRNA-snRNA and snRNA-intron RNA interactions.     

The structure of ColE1 rop in solution

Summary The structure of the ColE1 repressor of primer (rop) protein in solution was determined from the proton nuclear magnetic resonance data by a combined use of distance geometry and restrained molecular dynamics calculations. A set of structures was determined with low internal energy and virtually no violations of the experimental distance restraints. Rop forms homodimers: Two helical hairpins are arranged as an antiparallel four helix bundle with a left-handed rope-like twist of the helix axes and with left-handed bundle topology. The very compact packing of the side chains in the helix interfaces of the rop coiled-coil structure may well account for its high stability. Overall, the solution structure is highly similar to the recently determined X-ray structure (Banner, D.W., Kokkinidis, M. and Tsernoglou, D. (1987)J. Mol. Biol.,196, 657–675), although there are minor differences in regions where packing forces appear to influence the crystal structure.Abbreviations rop repressor of primer - NMR nuclear magnetic resonance - NOE nuclear Overhauser enhancement - NOESY NOE spectroscopy - RAN Set Structures generated from random choice of the dihedrai angles - HEL Set Structures generated from random choice of the dihedral angles restricted to ranges allowed for helices - MD molecular dynamics - EM energy minimization - RMSD root-mean-square deviation of atomic positions     

Towards structural genomics of RNA: rapid NMR resonance assignment and simultaneous RNA tertiary structure determination using residual dipolar couplings

We report a new residual dipolar couplings (RDCs) based NMR procedure for rapidly determining RNA tertiary structure demonstrated on a uniformly (15)N/(13)C-labeled 27 nt variant of the trans-activation response element (TAR) RNA from HIV-I. In this procedure, the time-consuming nuclear Overhauser enhancement (NOE)-based sequential assignment step is replaced by a fully automated RDC-based assignment strategy. This approach involves examination of all allowed sequence-specific resonance assignment permutations for best-fit agreement between measured RDCs and coordinates for sub-structures in a target RNA. Using idealized A-form geometries to model Watson-Crick helices and coordinates from a previous X-ray structure to model a hairpin loop in TAR, the best-fit RDC assignment solutions are determined very rapidly (相似文献   

MOTOR: Model assisted software for NMR structure determination

Eukaryotic proteins with important biological function can be partially unstructured, conformational flexible, or heterogenic. Crystallization trials often fail for such proteins. In NMR spectroscopy, parts of the polypeptide chain undergoing dynamics in unfavorable time regimes cannot be observed. De novo NMR structure determination is seriously hampered when missing signals lead to an incomplete chemical shift assignment resulting in an information content of the NOE data insufficient to determine the structure ab initio. We developed a new protein structure determination strategy for such cases based on a novel NOE assignment strategy utilizing a number of model structures but no explicit reference structure as it is used for bootstrapping like algorithms. The software distinguishes in detail between consistent and mutually exclusive pairs of possible NOE assignments on the basis of different precision levels of measured chemical shifts searching for a set of maximum number of consistent NOE assignments in agreement with 3D space. Validation of the method using the structure of the low molecular‐weight‐protein tyrosine phosphatase A (MptpA) showed robust results utilizing protein structures with 30–45% sequence identity and 70% of the chemical shift assignments. About 60% of the resonance assignments are sufficient to identify those structural models with highest conformational similarity to the real structure. The software was benchmarked by de novo solution structures of fibroblast growth factor 21 (FGF21) and the extracellular fibroblast growth factor receptor domain FGFR4 D2, which both failed in crystallization trials and in classical NMR structure determination. Proteins 2013; 81:2007–2022. © 2013 Wiley Periodicals, Inc.     

Solution structure of a LewisX analogue by off-resonance 1H NMR spectroscopy without use of an internal distance reference

Summary A recent ¹H NMR method has been applied to the determination of the solution structure and internal dynamics of a synthetic mixed C/O trisaccharide related to sialyl Lewis^x. Varying the rf field offset in ROESY-type experiments enabled the measurement of longitudinal and transverse dipolar cross-relaxation rates with high accuracy. Assuming that for each proton pair the motion could be represented by a single exponential autocorrelation function, it was possible to derive geometrical parameters (r) and dynamic parameters _cp. With this assumption, 224 cross-relaxation rates have been transformed into 30 interproton distance constraints and 30 dipolar correlation times. The distance constraints have been used in a simulated-annealing procedure. This trisaccharide exhibits a structure close to the O-glycosidic analogue, but its flexibility seems highly reduced. On the basis of the determined structure and dynamics, it is shown that no conformational exchange occurs, the molecule existing in the form of a unique family in aqueous solution. In order to assess the quality of the resulting structures and to validate this new experimental procedure of distance extraction, we finally compare these solution structures to the ones obtained using three different sets of distances deduced from three choices of internal reference. It appears that this procedure allows the determination of the most precise and accurate solution.Abbreviations COSY correlation spectroscopy - NOE nuclear Overhauser enhancement - NOESY nuclear Overhauser enhancement spectroscopy; rmsd, root-mean-square deviation - ROESY rotating frame Overhauser enhancement spectroscopy - SLe^x sialyl Lewis^x - TOCSY total correlation spectroscopy     

Characterizing the relative orientation and dynamics of RNA A-form helices using NMR residual dipolar couplings

We present a protocol for determining the relative orientation and dynamics of A-form helices in 13C/15N isotopically enriched RNA samples using NMR residual dipolar couplings (RDCs). Non-terminal Watson-Crick base pairs in helical stems are experimentally identified using NOE and trans-hydrogen bond connectivity and modeled using the idealized A-form helix geometry. RDCs measured in the partially aligned RNA are used to compute order tensors describing average alignment of each helix relative to the applied magnetic field. The order tensors are translated into Euler angles defining the average relative orientation of helices and order parameters describing the amplitude and asymmetry of interhelix motions. The protocol does not require complete resonance assignments and therefore can be implemented rapidly to RNAs much larger than those for which complete high-resolution NMR structure determination is feasible. The protocol is particularly valuable for exploring adaptive changes in RNA conformation that occur in response to biologically relevant signals. Following resonance assignments, the procedure is expected to take no more than 2 weeks of acquisition and data analysis time.     

Solving the structure of PTB in complex with pyrimidine tracts: an NMR study of protein-RNA complexes of weak affinities

NMR spectroscopy has proven to be a powerful tool for the structure determination of protein/RNA complexes. However, the quality of these structures depends critically on the number of unambiguous intermolecular and intra-RNA nuclear Overhauser effect (NOE) constraints that can be derived. This number is often limited due to exchange phenomena that can cause signal line broadening and the fact that unambiguous NOE assignments are challenging in systems that exchange between different conformations in the intermediate to fast exchange limit. These exchange processes can include exchange between free and bound form, as well as exchange of the ligand between different binding sites on the protein. Furthermore, for the large class of RNA metabolizing proteins that bind repetitive low-complexity RNA sequences in multiple register, exchange of the protein between these overlapping binding sites introduces additional exchange pathways. Here, we describe the strategy we used to overcome these exchange processes and to reduce significantly the line width of the RNA resonances in complexes of the RNA recognition motifs (RRMs) of the polypyrimidine tract-binding protein (PTB) in complex with pyrimidine tracts and hence allowed a highly precise structure determination. This method could be employed to derive structures of other protein/single-stranded nucleic acid complexes by NMR spectroscopy. Furthermore, we have determined the affinities of the individual RRMs of PTB for pyrimidine tracts of different length and sequence. These measurements show that PTB binds preferentially to long pyrimidine tracts that contain cytosine and hence confirm the structure of PTB in complex with RNA. Furthermore, they provide quantitative insight into the question of which pyrimidine sequences within alternatively spliced pre-mRNAs will be preferentially bound by PTB.     

Nuclear Magnetic Resonance Spectroscopy for the Study of B-Cell Epitopes

High-resolution nuclear magnetic resonance (NMR) spectroscopy is a structural technique that is finding increasing use in the study of antibody–antigen interactions. In this review we describe how the dynamic structural parameters obtained from NMR spectroscopy can further our understanding of B-cell epitopes and their function. Specific applications of NMR spectroscopy to examine the residues on peptides and proteins that contact the antibody combining site are also described. These include “footprinting” techniques using H–D exchange–COSY NMR spectroscopy, which are particularly useful for epitope mapping of protein antigens. For smaller systems, such as Fab–or Fv–peptide complexes, nuclear magnetization transfer difference NMR spectroscopy, transferred nuclear Overhauser effect spectroscopy, double-quantum-filtered NOE spectroscopy, and isotope editing techniques have been applied. The interpretation and limitations of the data obtained from these procedures and anticipated improvements in these applications in the future are discussed.