Exit strategies for charged tRNA from GluRS

Allostery in Protein Tyrosine Phosphatases is Enabled by Divergent Dynamics

Dynamics-driven allostery provides important insights into the working mechanics of proteins, especially enzymes. In this study, we employ this paradigm to answer a basic question: in enzyme superfamilies, where the catalytic mechanism, active sites, and protein fold are conserved, what accounts for the difference in the catalytic prowess of the individual members? We show that when subtle changes in sequence do not translate to changes in structure, they do translate to changes in dynamics. We use sequentially diverse PTP1B, TbPTP1, and YopH as representatives of the conserved protein tyrosine phosphatase (PTP) superfamily. Using amino acid network analysis of group behavior (community analysis) and influential node dominance on networks (eigenvector centrality), we explain the dynamic basis of the catalytic variations seen between the three proteins. Importantly, we explain how a dynamics-based blueprint makes PTP1B amenable to allosteric control and how the same is abstracted in TbPTP1 and YopH.

Allostery in Protein Tyrosine Phosphatases is Enabled by Divergent Dynamics

Dynamics-driven allostery provides important insights into the working mechanics of proteins, especially enzymes. In this study we employ this paradigm to answer a basic question: in enzyme superfamilies where the catalytic mechanism, active sites and protein fold are conserved, what accounts for the difference in the catalytic prowess of the individual members? We show that when subtle changes in sequence do not translate to changes in structure, they do translate to changes in dynamics. We use sequentially diverse PTP1B, TbPTP1, and YopH as the representatives of the conserved Protein Tyrosine Phosphatase (PTP) superfamily. Using amino acid network analysis of group behavior (community analysis) and influential node dominance on networks (eigenvector centrality), we explain the dynamic basis of catalytic variations seen between the three proteins. Importantly, we explain how a dynamics-based blueprint makes PTP1B amenable to allosteric control and how the same is abstracted in TbPTP1 and YopH.

Turning up the heat mimics allosteric signaling in imidazole-glycerol phosphate synthase

Allosteric drugs have the potential to revolutionize biomedicine due to their enhanced selectivity and protection against overdosage. However, we need to better understand allosteric mechanisms in order to fully harness their potential in drug discovery. In this study, molecular dynamics simulations and nuclear magnetic resonance spectroscopy are used to investigate how increases in temperature affect allostery in imidazole glycerol phosphate synthase. Results demonstrate that temperature increase triggers a cascade of local amino acid-to-amino acid dynamics that remarkably resembles the allosteric activation that takes place upon effector binding. The differences in the allosteric response elicited by temperature increase as opposed to effector binding are conditional to the alterations of collective motions induced by either mode of activation. This work provides an atomistic picture of temperature-dependent allostery, which could be harnessed to more precisely control enzyme function.

Cooperation of structural motifs controls drug selectivity in cyclin-dependent kinases: an advanced theoretical analysis

Understanding drug selectivity mechanism is a long-standing issue for helping design drugs with high specificity. Designing drugs targeting cyclin-dependent kinases (CDKs) with high selectivity is challenging because of their highly conserved binding pockets. To reveal the underlying general selectivity mechanism, we carried out comprehensive analyses from both the thermodynamics and kinetics points of view on a representative CDK12 inhibitor. To fully capture the binding features of the drug-target recognition process, we proposed to use kinetic residue energy analysis (KREA) in conjunction with the community network analysis (CNA) to reveal the underlying cooperation effect between individual residues/protein motifs to the binding/dissociating process of the ligand. The general mechanism of drug selectivity in CDKs can be summarized as that the difference of structural cooperation between the ligand and the protein motifs leads to the difference of the energetic contribution of the key residues to the ligand. The proposed mechanisms may be prevalent in drug selectivity issues, and the insights may help design new strategies to overcome/attenuate the drug selectivity associated problems.

Prediction of allosteric druggable pockets of cyclin-dependent kinases

Cyclin-dependent kinase (Cdk) proteins play crucial roles in the cell cycle progression and are thus attractive drug targets for therapy against such aberrant cell cycle processes as cancer. Since most of the available Cdk inhibitors target the highly conserved catalytic ATP pocket and their lack of specificity often lead to side effects, it is imperative to identify and characterize less conserved non-catalytic pockets capable of interfering with the kinase activity allosterically. However, a systematic analysis of these allosteric druggable pockets is still in its infancy. Here, we summarize the existing Cdk pockets and their selectivity. Then, we outline a network-based pocket prediction approach (NetPocket) and illustrate its utility for systematically identifying the allosteric druggable pockets with case studies. Finally, we discuss potential future directions and their challenges.

Probing biased activation of mu-opioid receptor by the biased agonist PZM21 using all atom molecular dynamics simulation

Morphine is a commonly used opioid drug to treat acute pain by binding to the mu-opioid receptor (MOR), but its effective analgesic efficacy via triggering of the heterotrimeric G_i protein pathway is accompanied by a series of adverse side effects via triggering of the β-arrestin pathway. Recently, PZM21, a recently developed MOR biased agonist, shows preferentially activating the G protein pathway over β-arrestin pathway. However, there is no high-resolution receptor structure in complex with PZM21 and its action mechanism remains elusive. In this study, PZM21 and Morphine were docked to the active human MOR-1 homology structure and then subjected to the molecular dynamics (MD) simulations in two different situations (i.e., one situation includes the crystal waters but another does not). Detailed comparisons between the two systems were made to characterize the differences in protein-ligand interactions, protein secondary and tertiary structures and dynamics networks. PZM21 could strongly interact with Y328^7.43 of TM7, besides the residues (Asp149^3.32 and Tyr150^3.33) of TM3. The two systems' network paths to the intracellular end of TM6 were roughly similar but the paths to the end of TM7 were different. The PZM21-bound MOR's intracellular ends of TM5-7 bent outward more along with the distance changes of the three key molecular switches (ionic lock, transmission and Tyr toggle) and the distance increase of some conserved inter-helical residue pairs. The larger intracellular opening of the receptor could potentially facilitate G protein binding.

Dynamic Structural Differences between Human and Mouse STING Lead to Differing Sensitivity to DMXAA

The stimulator-of-interferon-genes (STING) protein is involved in innate immunity. It has recently been shown that modulation of STING can lead to an aggressive antitumor response. DMXAA is an antitumor agent that had shown great promise in murine models but failed in human clinical trials. The molecular target of DMXAA was subsequently shown to be murine STING (mSTING); however, human STING (hSTING) is insensitive to DMXAA. Molecular dynamics simulations were employed to investigate the differences between hSTING and mSTING that could influence DMXAA binding. An initial set of simulations was performed to investigate a single lid region mutation G230I in hSTING (corresponding residue in mSTING is an Ile), which rendered the protein sensitive to DMXAA. The simulations found that an Ile side chain was enough to form a steric barrier that prevents exit of DMXAA, whereas in WT hSTING, the Gly residue that lacks a side chain formed a porous lid region that allowed DMXAA to exit. A second set of molecular dynamics simulations compared the tendency of STING to be in an open-inactive conformation or a closed-active conformation. The results show that hSTING prefers to be in an open-inactive conformation even with cGAMP, the native ligand, bound. On the other hand, mSTING prefers a closed-active conformation even without a ligand bound. These results highlight the challenges in translating a mouse active STING compound into a human active compound, while also providing avenues to pursue for designing a small-molecule drug targeting human STING.

Conformational stability and dynamics of the cancer-associated isoform Δ133p53β are modulated by p53 peptides and p53-specific DNA

p53 is a tumor suppressor protein that maintains genome stability, but its Δ133p53β and Δ160p53β isoforms promote breast cancer cell invasion. The sequence truncations in the p53 core domain raise key questions related to their physicochemical properties, including structural stabilities, interaction mechanisms, and DNA-binding abilities. Herein, we investigated the conformational dynamics of Δ133p53β and Δ160p53β with and without binding to p53-specific DNA by using molecular dynamics simulations. We observed that the core domains of the 2 truncated isoforms are much less stable than wild-type (wt) p53β, and the increased solvent exposure of their aggregation-triggering segment indicates their higher aggregation propensities than wt p53. We also found that Δ133p53β stability is modulable by peptide or DNA interactions. Adding a p53 peptide (derived from truncated p53 sequence 107-129) may help stabilize Δ133p53. Most importantly, our simulations of p53 isomer-DNA complexes indicate that Δ133p53β dimer, but not Δ160p53β dimer, could form a stable complex with p53-specific DNA, which is consistent with recent experiments. This study provides physicochemical insight into Δ133p53β, Δ133p53β-DNA complexes, Δ133p53β's pathologic mechanism, and peptide-based inhibitor design against p53-related cancers.-Lei, J., Qi, R., Tang, Y., Wang, W., Wei, G., Nussinov, R., Ma, B. Conformational stability and dynamics of the cancer-associated isoform Δ133p53β are modulated by p53 peptides and p53-specific DNA.

Assessment of Conformational State Transitions of Class B GPCRs Using Molecular Dynamics

Class B G protein-coupled receptors (GPCRs) comprise a family of 15 peptide-binding members, which are crucial targets for endocrine, metabolic, and stress-related disorders. While their protein structures and dynamics remain largely unclear, computer modeling and simulations represent a promising means to help solve such puzzles. Herein, we present a basic introduction to the methodology of molecular dynamics (MD) simulations and two analytical methods to assess the conformational ensembles and transitions of Class B GPCRs, using our recent studies of the human pituitary adenylate cyclase activating polypeptide (PAC₁) receptor as an example. From long MD simulations, conformational ensembles with different roles in ligand binding and receptor activation are sampled to establish four states identified as either "open" or "closed" for the PAC₁ receptor. Next, the dynamical network can be applied to analyze the simulations and identify key features within each conformational ensemble, which help distinguish the ligand-bound states of the PAC₁ receptor from the ligand-free one. Further, the Markov State Model has emerged as a key approach to construct the transition network and connect the GPCR ensembles, providing detailed information for the transition pathways and kinetics. For the ligand-free PAC₁ receptor, the transitions within the closed states are near 10-30 times faster than the open-closed transitions, which is likely related to the activation mechanism of the receptor. Overall, long MD simulations and analyses are useful to assess conformational transitions for the Class B GPCRs and to gain mechanistic insight, which is difficult to obtain using other methods.

Structural organization of a major neuronal G protein regulator, the RGS7-Gβ5-R7BP complex

Signaling by the G-protein-coupled receptors (GPCRs) plays fundamental role in a vast number of essential physiological functions. Precise control of GPCR signaling requires action of regulators of G protein signaling (RGS) proteins that deactivate heterotrimeric G proteins. RGS proteins are elaborately regulated and comprise multiple domains and subunits, yet structural organization of these assemblies is poorly understood. Here, we report a crystal structure and dynamics analyses of the multisubunit complex of RGS7, a major regulator of neuronal signaling with key roles in controlling a number of drug target GPCRs and links to neuropsychiatric disease, metabolism, and cancer. The crystal structure in combination with molecular dynamics and mass spectrometry analyses reveals unique organizational features of the complex and long-range conformational changes imposed by its constituent subunits during allosteric modulation. Notably, several intermolecular interfaces in the complex work in synergy to provide coordinated modulation of this key GPCR regulator.

PAC1 Receptors: Shapeshifters in Motion

Shapeshifters, in common mythology, are entities that can undergo multiple physical transformations. As our understanding of G protein-coupled receptors (GPCRs) has accelerated and been refined over the last two decades, we now understand that GPCRs are not static proteins, but rather dynamic structures capable of moving from one posture to the next, and adopting unique functional characteristics at each transition. This model of GPCR dynamics underlies our current understanding of biased agonism-how different ligands to the same receptor can generate different intracellular signals-and constitutive receptor activity, or the level of unbound basal receptor signaling that can be attenuated by inverse agonists. From information derived from related class B receptors, we have recently modeled the structure and molecular dynamics of the full-length pituitary adenylate cyclase activating polypeptide (PACAP, Adcyap1)-selective PAC1 receptor (PAC1R, Adcyap1r1). The class B receptors are different from the class A GPCRs in part from the presence of a large extracellular domain (ECD); the transitions of the ECD along with the dynamics of the transmembrane domains (TMD or 7TM) of the PAC1R describes a series of open- and closed-state conformations that appear to identify the mechanisms for receptor activation. The PAC1R shapeshifts also have the ability of delineating the mechanisms and the design of reagents that may direct biased agonism (or antagonism) for potential therapeutics.

NAMD goes quantum: an integrative suite for hybrid simulations

Hybrid methods that combine quantum mechanics (QM) and molecular mechanics (MM) can be applied to studies of reaction mechanisms in locations ranging from active sites of small enzymes to multiple sites in large bioenergetic complexes. By combining the widely used molecular dynamics and visualization programs NAMD and VMD with the quantum chemistry packages ORCA and MOPAC, we created an integrated, comprehensive, customizable, and easy-to-use suite (http://www.ks.uiuc.edu/Research/qmmm). Through the QwikMD interface, setup, execution, visualization, and analysis are streamlined for all levels of expertise.

Spatio-temporal coordination among functional residues in protein

The microscopic basis of communication among the functional sites in bio-macromolecules is a fundamental challenge in uncovering their functions. We study the communication through temporal cross-correlation among the binding sites. We illustrate via Molecular Dynamics simulations the properties of the temporal cross-correlation between the dihedrals of a small protein, ubiquitin which participates in protein degradation in eukaryotes. We show that the dihedral angles of the residues possess non-trivial temporal cross-correlations with asymmetry with respect to exchange of the dihedrals, having peaks at low frequencies with time scales in nano-seconds and an algebraic tail with a universal exponent for large frequencies. We show the existence of path for temporally correlated degrees of freedom among the functional residues. We explain the qualitative features of the cross-correlations through a general mathematical model. The generality of our analysis suggests that temporal cross-correlation functions may provide convenient theoretical framework to understand bio-molecular functions on microscopic basis.

A water-mediated and substrate-assisted aminoacylation mechanism in the discriminating aminoacyl-tRNA synthetase GlnRS and non-discriminating GluRS

Computational methods have been used to elucidate key differences and similarities between the distinct aminoacyl-tRNA synthetases (aaRS) GlnRS and non-discriminating-GluRS.

A Distal Disulfide Bridge in OXA-1 β-Lactamase Stabilizes the Catalytic Center and Alters the Dynamics of the Specificity Determining Ω Loop

Widespread antibiotic resistance, particularly when mediated by broad-spectrum β-lactamases, has major implications for public health. Substitutions in the active site often allow broad-spectrum enzymes to accommodate diverse types of β-lactams. Substitutions observed outside the active site are thought to compensate for the loss of thermal stability. The OXA-1 clade of class D β-lactamases contains a pair of conserved cysteines located outside the active site that forms a disulfide bond in the periplasm. Here, the effect of the distal disulfide bond on the structure and dynamics of OXA-1 was investigated via 4 μs molecular dynamics simulations. The results reveal that the disulfide promotes the preorganized orientation of the catalytic residues and affects the conformation of the functionally important Ω loop. Furthermore, principal component analysis reveals differences in the global dynamics between the oxidized and reduced forms, especially in the motions involving the Ω loop. A dynamical network analysis indicates that, in the oxidized form, in addition to its role in ligand binding, the KTG family motif is a central hub of the global dynamics. As activity of OXA-1 has been measured only in the reduced form, we suggest that accurate assessment of its functional profile would require oxidative conditions mimicking periplasm.

Aminoacetylation Reaction Catalyzed by Leucyl-tRNA Synthetase Operates via a Self-Assisted Mechanism Using a Conserved Residue and the Aminoacyl Substrate

Leucyl-tRNA synthetase catalyzes attachment of leucine amino acid to its cognate tRNA. During the second, aminoacetylation, step of the reaction, the leucyl moiety is transferred from leucyl-adenylate to the terminal A76 adenosine of tRNA. In this work, we have investigated the aminoacetylation step catalyzed by leucyl-tRNA synthase, using ab initio quantum chemical/molecular mechanical hybrid potentials in conjunction with reaction-path-location algorithms and molecular dynamics free energy simulations. We have modeled reaction mechanisms arising from both crystallographic studies and computational work. We invoke various groups as potential proton acceptors-namely, the phosphate and leucyl amino groups of leucyl-adenylate, the A76 base of tRNA, and the Asp80 and Glu532 residues of the protein-and consider both metal-assisted and metal-free reactions. Free energy calculations indicate that both the phosphate group of leucyl adenylate and Glu532 are not strong bases. This agrees with the results of the quantum chemical/molecular mechanical reaction path calculations which give high free energy barriers for the studied pathways involving these groups. A self-assisted mechanism with the leucyl amino group and Asp80 as proton acceptors is the most likely. Furthermore, in this mechanism the presence of a metal ion coordinated by the phosphate group and Glu532 strongly activates the reaction.

Aminoacyl-tRNA Synthetases in the Bacterial World

Aminoacyl-tRNA synthetases (aaRSs) are modular enzymes globally conserved in the three kingdoms of life. All catalyze the same two-step reaction, i.e., the attachment of a proteinogenic amino acid on their cognate tRNAs, thereby mediating the correct expression of the genetic code. In addition, some aaRSs acquired other functions beyond this key role in translation. Genomics and X-ray crystallography have revealed great structural diversity in aaRSs (e.g., in oligomery and modularity, in ranking into two distinct groups each subdivided in 3 subgroups, by additional domains appended on the catalytic modules). AaRSs show huge structural plasticity related to function and limited idiosyncrasies that are kingdom or even species specific (e.g., the presence in many Bacteria of non discriminating aaRSs compensating for the absence of one or two specific aaRSs, notably AsnRS and/or GlnRS). Diversity, as well, occurs in the mechanisms of aaRS gene regulation that are not conserved in evolution, notably between distant groups such as Gram-positive and Gram-negative Bacteria. The review focuses on bacterial aaRSs (and their paralogs) and covers their structure, function, regulation, and evolution. Structure/function relationships are emphasized, notably the enzymology of tRNA aminoacylation and the editing mechanisms for correction of activation and charging errors. The huge amount of genomic and structural data that accumulated in last two decades is reviewed, showing how the field moved from essentially reductionist biology towards more global and integrated approaches. Likewise, the alternative functions of aaRSs and those of aaRS paralogs (e.g., during cell wall biogenesis and other metabolic processes in or outside protein synthesis) are reviewed. Since aaRS phylogenies present promiscuous bacterial, archaeal, and eukaryal features, similarities and differences in the properties of aaRSs from the three kingdoms of life are pinpointed throughout the review and distinctive characteristics of bacterium-like synthetases from organelles are outlined.

MD Simulations of tRNA and Aminoacyl-tRNA Synthetases: Dynamics, Folding, Binding, and Allostery

While tRNA and aminoacyl-tRNA synthetases are classes of biomolecules that have been extensively studied for decades, the finer details of how they carry out their fundamental biological functions in protein synthesis remain a challenge. Recent molecular dynamics (MD) simulations are verifying experimental observations and providing new insight that cannot be addressed from experiments alone. Throughout the review, we briefly discuss important historical events to provide a context for how far the field has progressed over the past few decades. We then review the background of tRNA molecules, aminoacyl-tRNA synthetases, and current state of the art MD simulation techniques for those who may be unfamiliar with any of those fields. Recent MD simulations of tRNA dynamics and folding and of aminoacyl-tRNA synthetase dynamics and mechanistic characterizations are discussed. We highlight the recent successes and discuss how important questions can be addressed using current MD simulations techniques. We also outline several natural next steps for computational studies of AARS:tRNA complexes.

The Calcium Goes Meow: Effects of Ions and Glycosylation on Fel d 1, the Major Cat Allergen

The major cat allergen, Fel d 1, is a structurally complex protein with two N-glycosylation sites that may be filled by different glycoforms. In addition, the protein contains three putative Ca2+ binding sites. Since the impact of these Fel d 1 structure modifications on the protein dynamics, physiology and pathology are not well established, the present work employed computational biology techniques to tackle these issues. While conformational effects brought upon by glycosylation were identified, potentially involved in cavity volume regulation, our results indicate that only the central Ca2+ ion remains coordinated to Fel d 1 in biological solutions, impairing its proposed role in modulating phospholipase A2 activity. As these results increase our understanding of Fel d 1 structural biology, they may offer new support for understanding its physiological role and impact into cat-promoted allergy.

The Calcium Goes Meow: Effects of Ions and Glycosylation on Fel d 1, the Major Cat Allergen

The major cat allergen, Fel d 1, is a structurally complex protein with two N-glycosylation sites that may be filled by different glycoforms. In addition, the protein contains three putative Ca²⁺ binding sites. Since the impact of these Fel d 1 structure modifications on the protein dynamics, physiology and pathology are not well established, the present work employed computational biology techniques to tackle these issues. While conformational effects brought upon by glycosylation were identified, potentially involved in cavity volume regulation, our results indicate that only the central Ca²⁺ ion remains coordinated to Fel d 1 in biological solutions, impairing its proposed role in modulating phospholipase A₂ activity. As these results increase our understanding of Fel d 1 structural biology, they may offer new support for understanding its physiological role and impact into cat-promoted allergy.

On the modularity of the intrinsic flexibility of the µ opioid receptor: a computational study

The µ opioid receptor (µOR), the principal target to control pain, belongs to the G protein-coupled receptors (GPCRs) family, one of the most highlighted protein families due to their importance as therapeutic targets. The conformational flexibility of GPCRs is one of their essential characteristics as they take part in ligand recognition and subsequent activation or inactivation mechanisms. It is assessed that the intrinsic mechanical properties of the µOR, more specifically its particular flexibility behavior, would facilitate the accomplishment of specific biological functions, at least in their first steps, even in the absence of a ligand or any chemical species usually present in its biological environment. The study of the mechanical properties of the µOR would thus bring some indications regarding the highly efficient ability of the µOR to transduce cellular message. We therefore investigate the intrinsic flexibility of the µOR in its apo-form using all-atom Molecular Dynamics simulations at the sub-microsecond time scale. We particularly consider the µOR embedded in a simplified membrane model without specific ions, particular lipids, such as cholesterol moieties, or any other chemical species that could affect the flexibility of the µOR. Our analyses highlighted an important local effect due to the various bendability of the helices resulting in a diversity of shape and volume sizes adopted by the µOR binding site. Such property explains why the µOR can interact with ligands presenting highly diverse structural geometry. By investigating the topology of the µOR binding site, a conformational global effect is depicted: the correlation between the motional modes of the extra- and intracellular parts of µOR on one hand, along with a clear rigidity of the central µOR domain on the other hand. Our results show how the modularity of the µOR flexibility is related to its pre-ability to activate and to present a basal activity.

Signaling mechanisms of LOV domains: new insights from molecular dynamics studies

Phototropins are one of several classes of photoreceptors used by plants and algae to respond to light. These proteins contain flavin-binding LOV (Light-Oxygen-Voltage) domains that form covalent cysteine-flavin adducts upon exposure to blue light, leading to the enhancement of phototropin kinase activity. Several lines of evidence suggest that adduct formation in the phototropin LOV2 domains leads to the dissociation of an alpha helix (Jα) from these domains as part of the light-induced activation process. However, crystal structures of LOV domains both in the presence and absence of the Jα helix show very few differences between dark and illuminated states, and thus the precise mechanism through which adduct formation triggers helical dissociation remains poorly understood. Using Avena sativa phototropin 1 LOV2 as a model system, we have studied the interactions of the LOV domain core with the Jα helix through a series of equilibrium molecular dynamics simulations. Here we show that conformational transitions of a conserved glutamine residue in the flavin binding pocket are coupled to altered dynamics of the Jα helix both through a shift in dynamics of the main β-sheet of the LOV domain core and through a secondary pathway involving the N-terminal A'α helix.

Synthetic and editing mechanisms of aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases (aaRS) ensure the faithful transmission of genetic information in all living cells. The 24 known aaRS families are divided into 2 structurally distinct classes (class I and class II), each featuring a catalytic domain with a common fold that binds ATP, amino acid, and the 3'-terminus of tRNA. In a common two-step reaction, each aaRS first uses the energy stored in ATP to synthesize an activated aminoacyl adenylate intermediate. In the second step, either the 2'- or 3'-hydroxyl oxygen atom of the 3'-A76 tRNA nucleotide functions as a nucleophile in synthesis of aminoacyl-tRNA. Ten of the 24 aaRS families are unable to distinguish cognate from noncognate amino acids in the synthetic reactions alone. These enzymes possess additional editing activities for hydrolysis of misactivated amino acids and misacylated tRNAs, with clearance of the latter species accomplished in spatially separate post-transfer editing domains. A distinct class of trans-acting proteins that are homologous to class II editing domains also perform hydrolytic editing of some misacylated tRNAs. Here we review essential themes in catalysis with a view toward integrating the kinetic, stereochemical, and structural mechanisms of the enzymes. Although the aaRS have now been the subject of investigation for many decades, it will be seen that a significant number of questions regarding fundamental catalytic functioning still remain unresolved.

Aminoacyl-tRNA Synthetases in the Bacterial World

Aminoacyl-tRNAsynthetases (aaRSs) are modular enzymesglobally conserved in the three kingdoms of life. All catalyze the same two-step reaction, i.e., the attachment of a proteinogenic amino acid on their cognate tRNAs, thereby mediating the correct expression of the genetic code. In addition, some aaRSs acquired other functions beyond this key role in translation.Genomics and X-ray crystallography have revealed great structural diversity in aaRSs (e.g.,in oligomery and modularity, in ranking into two distinct groups each subdivided in 3 subgroups, by additional domains appended on the catalytic modules). AaRSs show hugestructural plasticity related to function andlimited idiosyncrasies that are kingdom or even speciesspecific (e.g.,the presence in many Bacteria of non discriminating aaRSs compensating for the absence of one or two specific aaRSs, notably AsnRS and/or GlnRS).Diversity, as well, occurs in the mechanisms of aaRS gene regulation that are not conserved in evolution, notably betweendistant groups such as Gram-positive and Gram-negative Bacteria.Thereview focuses on bacterial aaRSs (and their paralogs) and covers their structure, function, regulation,and evolution. Structure/function relationships are emphasized, notably the enzymology of tRNA aminoacylation and the editing mechanisms for correction of activation and charging errors. The huge amount of genomic and structural data that accumulatedin last two decades is reviewed,showing how thefield moved from essentially reductionist biologytowards more global and integrated approaches. Likewise, the alternative functions of aaRSs and those of aaRSparalogs (e.g., during cellwall biogenesis and other metabolic processes in or outside protein synthesis) are reviewed. Since aaRS phylogenies present promiscuous bacterial, archaeal, and eukaryal features, similarities and differences in the properties of aaRSs from the three kingdoms of life are pinpointedthroughout the reviewand distinctive characteristics of bacterium-like synthetases from organelles are outlined.

NetworkView: 3D display and analysis of protein·RNA interaction networks

SUMMARY

NetworkView is an application for the display and analysis of protein·RNA interaction networks derived from structure and/or dynamics. These networks typically model individual protein residues and nucleic acid monomers as nodes and their pairwise contacts as edges with associated weights. NetworkView projects the network onto the underlying 3D molecular structure so that visualization and analysis of the network can be coupled to physical and biological properties. NetworkView is implemented as a plugin to the molecular visualization software VMD.

AVAILABILITY AND IMPLEMENTATION

NetworkView is included with VMD, which is available at http://www.ks.uiuc.edu/Research/vmd/. Documentation, tutorials and supporting programs are available at http://www.scs.illinois.edu/schulten/software/.

CONTACT

networkview@scs.illinois.edu