Force-producing ADP state of myosin bound to actin.

Molecular motors produce force when they interact with their cellular tracks. For myosin motors, the primary force-generating state has MgADP tightly bound, whereas myosin is strongly bound to actin. We have generated an 8-Å cryoEM reconstruction of this state for myosin V and used molecular dynamics flexed fitting for model building. We compare this state to the subsequent state on actin (Rigor). The ADP-bound structure reveals that the actin-binding cleft is closed, even though MgADP is tightly bound. This state is accomplished by a previously unseen conformation of the ß-sheet underlying the nucleotide pocket. The transition from the force-generating ADP state to Rigor requires a 9.5° rotation of the myosin lever arm, coupled to a ß-sheet rearrangement. Thus, the structure reveals the detailed rearrangements underlying myosin force generation as well as the basis of strain-dependent ADP release that is essential for processive myosins, such as myosin V.

Molecular dissection of human Argonaute proteins by DNA shuffling.

A paramount task in RNA interference research is to decipher the complex biology of cellular effectors, exemplified in humans by four pleiotropic Argonaute proteins (Ago1-Ago4). Here, we exploited DNA family shuffling, a molecular evolution technology, to generate chimeric Ago protein libraries for dissection of intricate phenotypes independently of prior structural knowledge. Through shuffling of human Ago2 and Ago3, we discovered two N-terminal motifs that govern RNA cleavage in concert with the PIWI domain. Structural modeling predicts an impact on protein rigidity and/or RNA-PIWI alignment, suggesting new mechanistic explanations for Ago3's slicing deficiency. Characterization of hybrids including Ago1 and Ago4 solidifies that slicing requires the juxtaposition and combined action of multiple disseminated modules. We also present a Gateway library of codon-optimized chimeras of human Ago1-Ago4 and molecular evolution analysis software as resources for future investigations into RNA interference sequence-structure-function relationships.

Negative protein-protein interaction datasets derived from large-scale two-hybrid experiments.

Negative protein-protein interaction datasets are needed for training and evaluation of interaction prediction methods, as well as validation of high-throughput interaction discovery experiments. In large-scale two-hybrid assays, the direct interaction of a large number of protein pairs is systematically probed. We present a simple method to harness two-hybrid data to obtain negative protein-protein interaction datasets, which we validated using other available experimental data. The method identifies interactions that were likely tested but not observed in a two-hybrid screen. For each negative interaction, a confidence score is defined as the shortest-path length between the two proteins in the interaction network derived from the two-hybrid experiment. We show that these high-quality negative datasets are particularly important when a specific biological context is considered, such as in the study of protein interaction specificity. We also illustrate the use of a negative dataset in the evaluation of the InterPreTS interaction prediction method.

Cryo-electron microscopy modeling by the molecular dynamics flexible fitting method.

The increasing power and popularity of cryo-electron microscopy (cryo-EM) in structural biology brought about the development of so-called hybrid methods, which permit the interpretation of cryo-EM density maps beyond their nominal resolution in terms of atomic models. The Cryo-EM Modeling Challenge 2010 is the first community effort to bring together developers of hybrid methods as well as cryo-EM experimentalists. Participating in the challenge, the molecular dynamics flexible fitting (MDFF) method was applied to a number of cryo-EM density maps. The results are described here with special emphasis on the use of symmetry-based restraints to improve the quality of atomic models derived from density maps of symmetric complexes; on a comparison of the stereochemical quality of atomic models resulting from different hybrid methods; and on application of MDFF to electron crystallography data.

PepSite: prediction of peptide-binding sites from protein surfaces.

Complex biological functions emerge through intricate protein-protein interaction networks. An important class of protein-protein interaction corresponds to peptide-mediated interactions, in which a short peptide stretch from one partner interacts with a large protein surface from the other partner. Protein-peptide interactions are typically of low affinity and involved in regulatory mechanisms, dynamically reshaping protein interaction networks. Due to the relatively small interaction surface, modulation of protein-peptide interactions is feasible and highly attractive for therapeutic purposes. Unfortunately, the number of available 3D structures of protein-peptide interfaces is very limited. For typical cases where a protein-peptide structure of interest is not available, the PepSite web server can be used to predict peptide-binding spots from protein surfaces alone. The PepSite method relies on preferred peptide-binding environments calculated from a set of known protein-peptide 3D structures, combined with distance constraints derived from known peptides. We present an updated version of the web server that is orders of magnitude faster than the original implementation, returning results in seconds instead of minutes or hours. The PepSite web server is available at http://pepsite2.russelllab.org.

Stereochemical errors and their implications for molecular dynamics simulations.

BACKGROUND: Biological molecules are often asymmetric with respect to stereochemistry, and correct stereochemistry is essential to their function. Molecular dynamics simulations of biomolecules have increasingly become an integral part of biophysical research. However, stereochemical errors in biomolecular structures can have a dramatic impact on the results of simulations. RESULTS: Here we illustrate the effects that chirality and peptide bond configuration flips may have on the secondary structure of proteins throughout a simulation. We also analyze the most common sources of stereochemical errors in biomolecular structures and present software tools to identify, correct, and prevent stereochemical errors in molecular dynamics simulations of biomolecules. CONCLUSIONS: Use of the tools presented here should become a standard step in the preparation of biomolecular simulations and in the generation of predicted structural models for proteins and nucleic acids.

Molecular dynamics of EF-G during translocation.

Elongation factor G (EF-G) plays a crucial role in two stages of mRNA-(tRNA)(2) translocation. First, EF-G•GTP enters the pre-translocational ribosome in its intersubunit-rotated state, with tRNAs in their hybrid (P/E and A/P) positions. Second, a conformational change in EF-G's Domain IV induced by GTP hydrolysis disengages the mRNA-anticodon stem-loops of the tRNAs from the decoding center to advance relative to the small subunit when the ribosome undergoes a backward inter-subunit rotation. These events take place as EF-G undergoes a series of large conformational changes as visualized by cryo-EM and X-ray studies. The number and variety of these structures leave open many questions on how these different configurations form during the dynamic translocation process. To understand the molecular mechanism of translocation, we examined the molecular motions of EF-G in solution by means of molecular dynamics simulations. Our results show: (1) rotations of the super-domain formed by Domains III-V with respect to the super-domain formed by I-II, and rotations of Domain IV with respect to Domain III; (2) flexible conformations of both 503- and 575-loops; (3) large conformational variability in the bound form caused by the interaction between Domain V and the GTPase-associated center; (4) after GTP hydrolysis, the Switch I region seems to be instrumental for effecting the conformational change at the end of Domain IV implicated in the disengagement of the codon-anticodon helix from the decoding center.

Structural insights into cognate versus near-cognate discrimination during decoding.

The structural basis of the tRNA selection process is investigated by cryo-electron microscopy of ribosomes programmed with UGA codons and incubated with ternary complex (TC) containing the near-cognate Trp-tRNA(Trp) in the presence of kirromycin. Going through more than 350 000 images and employing image classification procedures, we find ∼8% in which the TC is bound to the ribosome. The reconstructed 3D map provides a means to characterize the arrangement of the near-cognate aa-tRNA with respect to elongation factor Tu (EF-Tu) and the ribosome, as well as the domain movements of the ribosome. One of the interesting findings is that near-cognate tRNA's acceptor stem region is flexible and CCA end becomes disordered. The data bring direct structural insights into the induced-fit mechanism of decoding by the ribosome, as the analysis of the interactions between small and large ribosomal subunit, aa-tRNA and EF-Tu and comparison with the cognate case (UGG codon) offers clues on how the conformational signals conveyed to the GTPase differ in the two cases.

Applications of the molecular dynamics flexible fitting method.

In recent years, cryo-electron microscopy (cryo-EM) has established itself as a key method in structural biology, permitting the structural characterization of large biomolecular complexes in various functional states. The data obtained through single-particle cryo-EM has recently seen a leap in resolution thanks to landmark advances in experimental and computational techniques, resulting in sub-nanometer resolution structures being obtained routinely. The remaining gap between these data and revealing the mechanisms of molecular function can be closed through hybrid modeling tools that incorporate known atomic structures into the cryo-EM data. One such tool, molecular dynamics flexible fitting (MDFF), uses molecular dynamics simulations to combine structures from X-ray crystallography with cryo-EM density maps to derive atomic models of large biomolecular complexes. The structures furnished by MDFF can be used subsequently in computational investigations aimed at revealing the dynamics of the complexes under study. In the present work, recent applications of MDFF are presented, including the interpretation of cryo-EM data of the ribosome at different stages of translation and the structure of a membrane-curvature-inducing photosynthetic complex.

The role of L1 stalk-tRNA interaction in the ribosome elongation cycle.

The ribosomal L1 stalk is a mobile structure implicated in directing tRNA movement during translocation through the ribosome. This article investigates three aspects of L1 stalk-tRNA interaction. First, by combining data from cryo electron microscopy, X-ray crystallography, and molecular dynamics simulations through the molecular dynamics flexible fitting method, we obtained atomic models of different tRNAs occupying the hybrid P/E state interacting with the L1 stalk. These models confirm the assignment of fluorescence resonance energy transfer states from previous single-molecule investigations of L1 stalk dynamics. Second, the models reconcile how initiator tRNA(fMet) interacts less strongly with the L1 stalk compared to elongator tRNA(Phe), as seen in previous single-molecule experiments. Third, results from a simulation of the entire ribosome in which the L1 stalk is moved from a half-closed conformation to its open conformation are found to support the hypothesis that L1 stalk opening is involved in tRNA release from the ribosome.

Recognition of the regulatory nascent chain TnaC by the ribosome.

Regulatory nascent chains interact with the ribosomal exit tunnel and modulate their own translation. To characterize nascent chain recognition by the ribosome at the atomic level, extensive molecular dynamics simulations of TnaC, the leader peptide of the tryptophanase operon, inside the exit tunnel were performed for an aggregate time of 2.1 mus. The simulations, complemented by quantum chemistry calculations, suggest that the critical TnaC residue W12 is recognized by the ribosome via a cation-pi interaction, whereas TnaC's D16 forms salt bridges with ribosomal proteins. The simulations also show that TnaC-mediated translational arrest does not involve a swinging of ribosomal protein L22, as previously proposed. Furthermore, bioinformatic analyses and simulations suggest nascent chain elements that may prevent translational arrest in various organisms. Altogether, the current study unveils atomic-detail interactions that explain the role of elements of TnaC and the ribosome essential for translational arrest.

Structural insight into nascent polypeptide chain-mediated translational stalling.

Expression of the Escherichia coli tryptophanase operon depends on ribosome stalling during translation of the upstream TnaC leader peptide, a process for which interactions between the TnaC nascent chain and the ribosomal exit tunnel are critical. We determined a 5.8 angstrom-resolution cryo-electron microscopy and single-particle reconstruction of a ribosome stalled during translation of the tnaC leader gene. The nascent chain was extended within the exit tunnel, making contacts with ribosomal components at distinct sites. Upon stalling, two conserved residues within the peptidyltransferase center adopted conformations that preclude binding of release factors. We propose a model whereby interactions within the tunnel are relayed to the peptidyltransferase center to inhibit translation. Moreover, we show that nascent chains adopt distinct conformations within the ribosomal exit tunnel.

Regulation of the protein-conducting channel by a bound ribosome.

During protein synthesis, it is often necessary for the ribosome to form a complex with a membrane-bound channel, the SecY/Sec61 complex, in order to translocate nascent proteins across a cellular membrane. Structural data on the ribosome-channel complex are currently limited to low-resolution cryo-electron microscopy maps, including one showing a bacterial ribosome bound to a monomeric SecY complex. Using that map along with available atomic-level models of the ribosome and SecY, we have determined, through molecular dynamics flexible fitting (MDFF), an atomic-resolution model of the ribosome-channel complex. We characterized computationally the sites of ribosome-SecY interaction within the complex and determined the effect of ribosome binding on the SecY channel. We also constructed a model of a ribosome in complex with a SecY dimer by adding a second copy of SecY to the MDFF-derived model. The study involved 2.7-million-atom simulations over altogether nearly 50 ns.

Protein-induced membrane curvature investigated through molecular dynamics flexible fitting.

In the photosynthetic purple bacterium Rhodobacter (Rba.) sphaeroides, light is absorbed by membrane-bound light-harvesting (LH) proteins LH1 and LH2. LH1 directly surrounds the reaction center (RC) and, together with PufX, forms a dimeric (RC-LH1-PufX)2 protein complex. In LH2-deficient Rba. sphaeroides mutants, RC-LH1-PufX dimers aggregate into tubular vesicles with a radius of approximately 250-550 A, making RC-LH1-PufX one of the few integral membrane proteins known to actively induce membrane curvature. Recently, a three-dimensional electron microscopy density map showed that the Rba. sphaeroides RC-LH1-PufX dimer exhibits a prominent bend at its dimerizing interface. To investigate the curvature properties of this highly bent protein, we employed molecular dynamics simulations to fit an all-atom structural model of the RC-LH1-PufX dimer within the electron microscopy density map. The simulations reveal how the dimer produces a membrane with high local curvature, even though the location of PufX cannot yet be determined uniquely. The resulting membrane curvature agrees well with the size of RC-LH1-PufX tubular vesicles, and demonstrates how the local curvature properties of the RC-LH1-PufX dimer propagate to form the observed long-range organization of the Rba. sphaeroides tubular vesicles.

Molecular dynamics flexible fitting: a practical guide to combine cryo-electron microscopy and X-ray crystallography.

Hybrid computational methods for combining structural data from different sources and resolutions are becoming an essential part of structural biology, especially as the field moves toward the study of large macromolecular assemblies. We have developed the molecular dynamics flexible fitting (MDFF) method for combining high-resolution atomic structures with cryo-electron microscopy (cryo-EM) maps, that results in atomic models representing the conformational state captured by cryo-EM. The method has been applied successfully to the ribosome, a ribonucleoprotein complex responsible for protein synthesis. MDFF involves a molecular dynamics simulation in which a guiding potential, based on the cryo-EM map, is added to the standard force field. Forces proportional to the gradient of the density map guide an atomic structure, available from X-ray crystallography, into high-density regions of a cryo-EM map. In this paper we describe the necessary steps to set up, run, and analyze MDFF simulations and the software packages that implement the corresponding functionalities.

Ribosome-induced changes in elongation factor Tu conformation control GTP hydrolysis.

In translation, elongation factor Tu (EF-Tu) molecules deliver aminoacyl-tRNAs to the mRNA-programmed ribosome. The GTPase activity of EF-Tu is triggered by ribosome-induced conformational changes of the factor that play a pivotal role in the selection of the cognate aminoacyl-tRNAs. We present a 6.7-A cryo-electron microscopy map of the aminoacyl-tRNA x EF-Tu x GDP x kirromycin-bound Escherichia coli ribosome, together with an atomic model of the complex obtained through molecular dynamics flexible fitting. The model reveals the conformational changes in the conserved GTPase switch regions of EF-Tu that trigger hydrolysis of GTP, along with key interactions, including those between the sarcin-ricin loop and the P loop of EF-Tu, and between the effector loop of EF-Tu and a conserved region of the 16S rRNA. Our data suggest that GTP hydrolysis on EF-Tu is controlled through a hydrophobic gate mechanism.

Structural model and excitonic properties of the dimeric RC-LH1-PufX complex from Rhodobacter sphaeroides.

The light-harvesting apparatus of the purple bacterial photosynthetic unit consists of a pool of peripheral light-harvesting complexes that transfer excitation energy to a reaction center (RC) via the surrounding pigment-protein complex LH1. Recent electron microscopy and atomic force microscopy studies have revealed that RC-LH1 units of Rhodobacter sphaeroides form membrane-bending dimeric complexes together with the polypeptide PufX. We present a structural model for these RC-LH1-PufX dimeric complexes constructed using the molecular dynamics flexible fitting method based on an EM density map. The arrangement of the LH1 BChls displays a distortion near the proposed location of the PufX polypeptide. The resulting atomic model for BChl arrays is used to compute the excitonic properties of the dimeric RC-LH1 complex. A comparison is presented between the structural and excitonic features of the S-shaped dimeric BChl array of Rhodobacter sphaeroides and the circular BChl arrangement found in other purple bacteria.

Flexible fitting of atomic structures into electron microscopy maps using molecular dynamics.

A novel method to flexibly fit atomic structures into electron microscopy (EM) maps using molecular dynamics simulations is presented. The simulations incorporate the EM data as an external potential added to the molecular dynamics force field, allowing all internal features present in the EM map to be used in the fitting process, while the model remains fully flexible and stereochemically correct. The molecular dynamics flexible fitting (MDFF) method is validated for available crystal structures of protein and RNA in different conformations; measures to assess and monitor the fitting process are introduced. The MDFF method is then used to obtain high-resolution structures of the E. coli ribosome in different functional states imaged by cryo-EM.

Dynamics of Recognition between tRNA and elongation factor Tu.

Elongation factor Tu (EF-Tu) binds to all standard aminoacyl transfer RNAs (aa-tRNAs) and transports them to the ribosome while protecting the ester linkage between the tRNA and its cognate amino acid. We use molecular dynamics simulations to investigate the dynamics of the EF-Tu.guanosine 5'-triphosphate.aa-tRNA(Cys) complex and the roles played by Mg2+ ions and modified nucleosides on the free energy of protein.RNA binding. Individual modified nucleosides have pronounced effects on the structural dynamics of tRNA and the EF-Tu.Cys-tRNA(Cys) interface. Combined energetic and evolutionary analyses identify the coevolution of residues in EF-Tu and aa-tRNAs at the binding interface. Highly conserved EF-Tu residues are responsible for both attracting aa-tRNAs as well as providing nearby nonbonded repulsive energies that help fine-tune molecular attraction at the binding interface. In addition to the 3' CCA end, highly conserved tRNA nucleotides G1, G52, G53, and U54 contribute significantly to EF-Tu binding energies. Modification of U54 to thymine affects the structure of the tRNA common loop resulting in a change in binding interface contacts. In addition, other nucleotides, conserved within certain tRNA specificities, may be responsible for tuning aa-tRNA binding to EF-Tu. The trend in EF-Tu.Cys-tRNA(Cys) binding energies observed as the result of mutating the tRNA agrees with experimental observation. We also predict variations in binding free energies upon misacylation of tRNA(Cys) with d-cysteine or O-phosphoserine and upon changing the protonation state of l-cysteine. Principal components analysis in each case reveals changes in the communication network across the protein.tRNA interface and is the basis for the entropy calculations.

Accelerating molecular modeling applications with graphics processors.

Molecular mechanics simulations offer a computational approach to study the behavior of biomolecules at atomic detail, but such simulations are limited in size and timescale by the available computing resources. State-of-the-art graphics processing units (GPUs) can perform over 500 billion arithmetic operations per second, a tremendous computational resource that can now be utilized for general purpose computing as a result of recent advances in GPU hardware and software architecture. In this article, an overview of recent advances in programmable GPUs is presented, with an emphasis on their application to molecular mechanics simulations and the programming techniques required to obtain optimal performance in these cases. We demonstrate the use of GPUs for the calculation of long-range electrostatics and nonbonded forces for molecular dynamics simulations, where GPU-based calculations are typically 10-100 times faster than heavily optimized CPU-based implementations. The application of GPU acceleration to biomolecular simulation is also demonstrated through the use of GPU-accelerated Coulomb-based ion placement and calculation of time-averaged potentials from molecular dynamics trajectories. A novel approximation to Coulomb potential calculation, the multilevel summation method, is introduced and compared with direct Coulomb summation. In light of the performance obtained for this set of calculations, future applications of graphics processors to molecular dynamics simulations are discussed.