Prefusion spike protein conformational changes are slower in SARS-CoV-2 than in SARS-CoV-1.

Within the last 2 decades, severe acute respiratory syndrome coronaviruses 1 and 2 (SARS-CoV-1 and SARS-CoV-2) have caused two major outbreaks; yet, for reasons not fully understood, the coronavirus disease 2019 pandemic caused by SARS-CoV-2 has been significantly more widespread than the 2003 SARS epidemic caused by SARS-CoV-1, despite striking similarities between these two viruses. The SARS-CoV-1 and SARS-CoV-2 spike proteins, both of which bind to host cell angiotensin-converting enzyme 2, have been implied to be a potential source of their differential transmissibility. However, the mechanistic details of prefusion spike protein binding to angiotensin-converting enzyme 2 remain elusive at the molecular level. Here, we performed an extensive set of equilibrium and nonequilibrium microsecond-level all-atom molecular dynamics simulations of SARS-CoV-1 and SARS-CoV-2 prefusion spike proteins to determine their differential dynamic behavior. Our results indicate that the active form of the SARS-CoV-2 spike protein is more stable than that of SARS-CoV-1 and the energy barrier associated with the activation is higher in SARS-CoV-2. These results suggest that not only the receptor-binding domain but also other domains such as the N-terminal domain could play a crucial role in the differential binding behavior of SARS-CoV-1 and SARS-CoV-2 spike proteins.

cpSRP43 Is Both Highly Flexible and Stable: Structural Insights Using a Combined Experimental and Computational Approach.

The novel multidomain protein, cpSRP43, is a unique subunit of the post-translational chloroplast signal recognition particle (cpSRP) targeting pathway in higher plants. The cpSRP pathway is responsible for targeting and insertion of light-harvesting chlorophyll a/b binding proteins (LHCPs) to the thylakoid membrane. Upon emergence into the stroma, LHCPs form a soluble transit complex with the cpSRP heterodimer, which is composed of cpSRP43 and cpSRP54. cpSRP43 is irreplaceable as a chaperone to LHCPs in their translocation to the thylakoid membrane and remarkable in its ability to dissolve aggregates of LHCPs without the need for external energy input. In previous studies, cpSRP43 has demonstrated significant flexibility and interdomain dynamics. In this study, we explore the structural stability and flexibility of cpSRP43 using a combination of computational and experimental techniques and find that this protein is concurrently highly stable and flexible. In addition to microsecond-level unbiased molecular dynamics (MD), biased MD simulations based on system-specific collective variables are used along with biophysical experimentation to explain the basis of the flexibility and stability of cpSRP43, showing that the free and cpSRP54-bound cpSRP43 has substantially different conformations and conformational dynamics.

Addressing the Embeddability Problem in Transition Rate Estimation.

Markov State Models (MSM) and related techniques have gained significant traction as a tool for analyzing and guiding molecular dynamics (MD) simulations due to their ability to extract structural, thermodynamic, and kinetic information on proteins using computationally feasible MD simulations. The MSM analysis often relies on spectral decomposition of empirically generated transition matrices. This work discusses an alternative approach for extracting the thermodynamic and kinetic information from the so-called rate/generator matrix rather than the transition matrix. Although the rate matrix itself is built from the empirical transition matrix, it provides an alternative approach for estimating both thermodynamic and kinetic quantities, particularly in diffusive processes. A fundamental issue with this approach is known as the embeddability problem. The key contribution of this work is the introduction of a novel method to address the embeddability problem as well as the collection and utilization of existing algorithms previously used in the literature. The algorithms are tested on data from a one-dimensional toy model to show the workings of these methods and discuss the robustness of each method in dependence of lag time and trajectory length.

Transient local secondary structure in the intrinsically disordered C-term of the Albino3 insertase.

Albino3 (Alb3) is an integral membrane protein fundamental to the targeting and insertion of light-harvesting complex (LHC) proteins into the thylakoid membrane. Alb3 contains a stroma-exposed C-terminus (Alb3-Cterm) that is responsible for binding the LHC-loaded transit complex before LHC membrane insertion. Alb3-Cterm has been reported to be intrinsically disordered, but precise mechanistic details underlying how it recognizes and binds to the transit complex are lacking, and the functional roles of its four different motifs have been debated. Using a novel combination of experimental and computational techniques such as single-molecule fluorescence resonance energy transfer, circular dichroism with deconvolution analysis, site-directed mutagenesis, trypsin digestion assays, and all-atom molecular dynamics simulations in conjunction with enhanced sampling techniques, we show that Alb3-Cterm contains transient secondary structure in motifs I and II. The excellent agreement between the experimental and computational data provides a quantitatively consistent picture and allows us to identify a heterogeneous structural ensemble that highlights the local and transient nature of the secondary structure. This structural ensemble was used to predict both the inter-residue distance distributions of single molecules and the apparent unfolding free energy of the transient secondary structure, which were both in excellent agreement with those determined experimentally. We hypothesize that this transient local secondary structure may play an important role in the recognition of Alb3-Cterm for the LHC-loaded transit complex, and these results should provide a framework to better understand protein targeting by the Alb3-Oxa1-YidC family of insertases.

Activities and polymorphisms of MMP-2 and MMP-9, smoking, diabetes and risk of prostate cancer.

Matrix metalloproteinases (MMPs) are a group of zinc dependent enzymes that are involved in tumor cell invasion and metastasis. The role of MMP-2 and -9 genetic polymorphism in different malignancies has been the subject of numerous studies. The present research has attempted to discover any positive correlation between MMP-2 and MMP-9 SNPs and prostate cancer (PCa) in patients with a history of either diabetes or smoking habits. 112 PCa-patients and 150 unrelated healthy-controls that matched for age and sex were selected for present case-control study. MMP-2 -1575G/A and MMP-9 -1562 C/T polymorphisms detected by PCR-RFLP, serum tissue inhibitors of metalloproteinases (TIMP-1 and TIMP-2), testosterone, prostate-specific antigen (PSA), free-prostate-specific-antigen (fPSA), and fPSA/PSA levels were detected by ELISA and enzyme assay, respectively. MMP-2 and MMP-9 activities were measured by gelatin-zymography. Covariates were considered as age, status of cigarette smoking, and a possible history of diabetes mellitus (DM). The frequency of -1575 MMP-2 A/A + A/G and -1562 MMP-9 C/T + T/T genotypes were higher in PCa-patients with DM (74.3%,p = 0.003) and with smoking habits (72.5%,p = 0.005). These genotypes were associated with the increased risk of prostate cancer in smokers (3.52-folds) and in individuals with history of DM (4.34-folds). A significant positive association was found between level of TIMPs (TIMP -1 and TIMP-2) and BMI in PCa-patients and also between testosterone levels and MMP-9 activity in healthy control subjects. For the first time, this study demonstrated that activities of MMP-2 -1575G/A and MMP-9 -1562C/T variants in association with smoking and diabetes are considered significant risk factors for PCa.

Chemomechanical Coupling in Hexameric Protein-Protein Interfaces Harnesses Energy within V-Type ATPases.

ATP synthase is the most prominent bioenergetic macromolecular motor in all life forms, utilizing the proton gradient across the cell membrane to fuel the synthesis of ATP. Notwithstanding the wealth of available biochemical and structural information inferred from years of experiments, the precise molecular mechanism whereby vacuolar (V-type) ATP synthase fulfills its biological function remains largely fragmentary. Recently, crystallographers provided the first high-resolution view of ATP activity in Enterococcus hirae V1-ATPase. Employing a combination of transition-path sampling and high-performance free-energy methods, the sequence of conformational transitions involved in a functional cycle accompanying ATP hydrolysis has been investigated in unprecedented detail over an aggregate simulation time of 65 µs. Our simulated pathways reveal that the chemical energy produced by ATP hydrolysis is harnessed via the concerted motion of the protein-protein interfaces in the V1-ring, and is nearly entirely consumed in the rotation of the central stalk. Surprisingly, in an ATPase devoid of a central stalk, the interfaces of this ring are perfectly designed for inducing ATP hydrolysis. However, in a complete V1-ATPase, the mechanical property of the central stalk is a key determinant of the rate of ATP turnover. The simulations further unveil a sequence of events, whereby unbinding of the hydrolysis product (ADP + Pi) is followed by ATP uptake, which, in turn, leads to the torque generation step and rotation of the center stalk. Molecular trajectories also bring to light multiple intermediates, two of which have been isolated in independent crystallography experiments.

Mechanistic picture for conformational transition of a membrane transporter at atomic resolution.

During their transport cycle, ATP-binding cassette (ABC) transporters undergo large-scale conformational changes between inward- and outward-facing states. Using an approach based on designing system-specific reaction coordinates and using nonequilibrium work relations, we have performed extensive all-atom molecular dynamics simulations in the presence of explicit membrane/solvent to sample a large number of mechanistically distinct pathways for the conformational transition of MsbA, a bacterial ABC exporter whose structure has been solved in multiple functional states. The computational approach developed here is based on (i) extensive exploration of system-specific biasing protocols (e.g., using collective variables designed based on available low-resolution crystal structures) and (ii) using nonequilibrium work relations for comparing the relevance of the transition pathways. The most relevant transition pathway identified using this approach involves several distinct stages reflecting the complex nature of the structural changes associated with the function of the protein. The opening of the cytoplasmic gate during the outward- to inward-facing transition of apo MsbA is found to be disfavored when the periplasmic gate is open and facilitated by a twisting motion of the nucleotide-binding domains that involves a dramatic change in their relative orientation. These results highlight the cooperativity between the transmembrane and the nucleotide-binding domains in the conformational transition of ABC exporters. The approach introduced here provides a framework to study large-scale conformational changes of other membrane transporters whose computational investigation at an atomic resolution may not be currently feasible using conventional methods.

Reaction path ensemble of the B-Z-DNA transition: a comprehensive atomistic study.

Since its discovery in 1979, left-handed Z-DNA has evolved from an in vitro curiosity to a challenging DNA structure with crucial roles in gene expression, regulation and recombination. A fundamental question that has puzzled researchers for decades is how the transition from B-DNA, the prevalent right-handed form of DNA, to Z-DNA is accomplished. Due to the complexity of the B-Z-DNA transition, experimental and computational studies have resulted in several different, apparently contradictory models. Here, we use molecular dynamics simulations coupled with state-of-the-art enhanced sampling techniques operating through non-conventional reaction coordinates, to investigate the B-Z-DNA transition at the atomic level. Our results show a complex free energy landscape, where several phenomena such as over-stretching, unpeeling, base pair extrusion and base pair flipping are observed resulting in interconversions between different DNA conformations such as B-DNA, Z-DNA and S-DNA. In particular, different minimum free energy paths allow for the coexistence of different mechanisms (such as zipper and stretch-collapse mechanisms) that previously had been proposed as independent, disconnected models. We find that the B-Z-DNA transition--in absence of other molecular partners--can encompass more than one mechanism of comparable free energy, and is therefore better described in terms of a reaction path ensemble.

Conformational dynamics at the inner gate of KcsA during activation.

The potassium channel KcsA offers a unique opportunity to explicitly study the dynamics of the moving parts of ion channels, yet our understanding of the extent and dynamic behavior of the physiologically relevant structural changes at the inner gate in KcsA remains incomplete. Here, we use electron paramagnetic resonance, nuclear magnetic resonance, and molecular dynamics simulations to characterize the extent of pH-dependent conformational changes of the inner gate in lipid bilayers or detergent micelles. Our results show that under physiological conditions the inner gate experiences a maximal diagonal opening of ∼24 Šwith the largest degree of dynamics near the pKa of activation (pH ∼3.9). These results extend the observation that the C-terminus is necessary to limit the extent of opening and imply that the inner gate regulates the extent of conformational change at the zone of allosteric coupling and at the selectivity filter.

Investigating rare events with nonequilibrium work measurements. II. Transition and reaction rates.

We present a formalism for investigating transition pathways and transition probabilities for rare events in biomolecular systems. The formalism is based on combining Transition Path Theory with the results of nonequilibrium work relations, and shows that the equilibrium and nonequilibrium transition rates are in fact related. Aside from its fundamental importance, this allows for the calculation of relative equilibrium reaction rates with driven nonequilibrium simulations such as Steered Molecular Dynamics. The workings of the formalism are illustrated with a few typical numerical examples.

Investigating rare events with nonequilibrium work measurements. I. Nonequilibrium transition path probabilities.

We have developed a formalism for investigating transition pathways and transition probabilities for rare events in biomolecular systems. In this paper, we set the theoretical framework for employing nonequilibrium work relations to estimate the relative reaction rates associated with different classes of transition pathways. Particularly, we derive an extension of Crook's transient fluctuation theorem, which relates the relative transition rates of driven systems in the forward and reverse directions, and allows for the calculation of these relative rates using work measurements (e.g., in Steered Molecular Dynamics). The formalism presented here can be combined with Transition Path Theory to relate the equilibrium and driven transition rates. The usefulness of this framework is illustrated by means of a Gaussian model and a driven proline dimer.

Lipid-dependent conformational dynamics of bacterial ATP-binding cassette transporter Sav1866.

Sav1866, a bacterial ATP-binding cassette (ABC) exporter, plays a crucial role in cellular processes by facilitating the efflux of a diverse range of substrates, including drugs, chemotherapeutic agents, peptides, and lipids. This efflux activity significantly impacts the effectiveness of various therapies against bacterial infections. In our recent investigation, we focused on understanding the conformational dynamics of Sav1866 within different lipid environments. Specifically, we explored its behavior in environments composed of DMPC and POPE lipids, which exhibit crucial distinctions not only in their headgroup polarity but also in the length and saturation of their hydrophobic tails. Our extensive set of equilibrium microsecond-level all-atom molecular dynamics (MD) simulations revealed significant distinctions in transporter behavior influenced by these lipid compositions. We observed a rapid transition to an occluded-inward-facing (IF-occ) conformation in POPE environments, contrasting with a channel-like behavior in DMPC environments, deviating from the expected alternating access mechanism (AAM). These findings underscore the significant impact of lipid compositions on ABC transporter function, offering new perspectives on membrane transport mechanisms.

Differential Behavior of Conformational Dynamics in Active and Inactive States of Cannabinoid Receptor 1.

The cannabinoid receptor CB1 is a G protein-coupled receptor that regulates critical physiological processes including pain, appetite, and cognition. Understanding the conformational dynamics of CB1 associated with transitions between inactive and active signaling states is imperative for developing targeted modulators. Using microsecond-level all-atom molecular dynamics simulations, we identified marked differences in the conformational ensembles of inactive and active CB1 states in apo conditions. The inactive state exhibited substantially increased structural heterogeneity and plasticity compared to the more rigidified active state in the absence of stabilizing ligands. Transmembrane helices TM3 and TM7 were identified as distinguishing factors modulating the state-dependent dynamics. TM7 displayed amplified fluctuations selectively in the inactive state simulations attributed to disruption of conserved electrostatic contacts anchoring it to surrounding helices in the active state. Additionally, we identified significant reorganization of key salt bridge and hydrogen bond networks known to control CB1 activation between states. For instance, a conserved D213-Y224 hydrogen bond and D184-K192 salt bridge interactions showed marked rearrangements between the states. Collectively, these findings reveal the specialized role of TM7 in directing state-dependent CB1 dynamics through electrostatic switch mechanisms. By elucidating the intrinsic enhanced flexibility of inactive CB1, this study provides valuable insights into the conformational landscape enabling functional transitions. Our perspective advances understanding of CB1 activation mechanisms and offers opportunities for structure-based drug discovery targeting the state-specific conformational dynamics of this receptor.

Cholesterol Dependence of the Conformational Changes in Metabotropic Glutamate Receptor 1.

Metabotropic glutamate receptors (mGluRs) are class C G protein-coupled receptors that function as obligate dimers in regulating neurotransmission and synaptic plasticity in the central nervous system. The mGluR1 subtype has been shown to be modulated by the membrane lipid environment, particularly cholesterol, though the molecular mechanisms remain elusive. In this study, we employed all-atom molecular dynamics simulations to investigate the effects of cholesterol on the conformational dynamics of the mGluR1 seven-transmembrane (7TM) domain in an inactive state model. Simulations were performed with three different cholesterol concentrations (0%, 10%, and 25%) in a palmitoyl-oleoyl phosphatidylcholine (POPC) lipid bilayer system. Our results demonstrate that cholesterol induces conformational changes in the mGluR1 dimer more significantly than in the individual protomers. Notably, cholesterol modulates the dynamics and conformations of the TM1 and TM2 helices at the dimer interface. Interestingly, an intermediate cholesterol concentration of 10% elicits more pronounced conformational changes compared to both cholesterol-depleted (0%) and cholesterol-enriched (25%) systems. Specific electrostatic interaction unique to the 10% cholesterol system further corroborate these conformational differences. Given the high sequence conservation of the 7TM domains across mGluR subtypes, the cholesterol-dependent effects observed in mGluR1 are likely applicable to other members of this receptor family. Our findings provide atomistic insights into how cholesterol modulates the conformational landscape of mGluRs, which could impact their function and signaling mechanisms.

Bioorthogonal click labeling of an amber-free HIV-1 provirus for in-virus single molecule imaging.

Structural dynamics of human immunodeficiency virus 1 (HIV-1) envelope (Env) glycoprotein mediate cell entry and facilitate immune evasion. Single-molecule FRET using peptides for Env labeling revealed structural dynamics of Env, but peptide use risks potential effects on structural integrity/dynamics. While incorporating noncanonical amino acids (ncAAs) into Env by amber stop-codon suppression, followed by click chemistry, offers a minimally invasive approach, this has proved to be technically challenging for HIV-1. Here, we develope an intact amber-free HIV-1 system that overcomes hurdles of preexisting viral amber codons. We achieved dual-ncAA incorporation into Env on amber-free virions, enabling single-molecule Förster resonance energy transfer (smFRET) studies of click-labeled Env that validated the previous peptide-based labeling approaches by confirming the intrinsic propensity of Env to dynamically sample multiple conformational states. Amber-free click-labeled Env also enabled real-time tracking of single virion internalization and trafficking in cells. Our system thus permits in-virus bioorthogonal labeling of proteins, compatible with studies of virus entry, trafficking, and egress from cells.

Characterizing a histidine switch controlling pH-dependent conformational changes of the influenza virus hemagglutinin.

During the fusion of the influenza virus to the host cell, bending of the HA2 chain of hemagglutinin into a hairpin-shaped structure in a pH-dependent manner facilitates the fusion of the viral envelope and the endosomal membrane. To characterize the structural and dynamical responses of the hinge region of HA2 to pH changes and examine the role of a conserved histidine in this region (the hinge histidine), we have performed an extensive set of molecular dynamics (MD) simulations of 26-residue peptides encompassing the hinge regions of several hemagglutinin subtypes under both neutral and low pH conditions, modeled by the change of the protonation state of the hinge histidine. More than 70 sets of MD simulations (collectively amounting to 25.1 µs) were performed in both implicit and explicit solvents to study the effect of histidine protonation on structural dynamics of the hinge region. In both explicit and implicit solvent simulations, hinge bending was consistently observed upon the protonation of the histidine in all the simulations starting with an initial straight helical conformation, whereas the systems with a neutral histidine retained their primarily straight conformation throughout the simulations. Conversely, the MD simulations starting from an initially bent conformation resulted in the formation of a straight helical structure upon the neutralization of the hinge histidine, whereas the bent structure was maintained when the hinge histidine remained protonated. Finally, mutation of the hinge histidine to alanine abolishes the bending response of the peptide altogether. A molecular mechanism based on the interaction of the hinge histidine with neighboring acidic residues is proposed to be responsible for its role in controlling the conformation of the hinge. We propose that this might present a common mechanism for pH-controlled structural changes in helical structures when histidines act as the pH sensor.

Are long-range structural correlations behind the aggregration phenomena of polyglutamine diseases?

We have characterized the conformational ensembles of polyglutamine Qn peptides of various lengths n (ranging from 6 to 40), both with and without the presence of a C-terminal polyproline hexapeptide. For this, we used state-of-the-art molecular dynamics simulations combined with a novel statistical analysis to characterize the various properties of the backbone dihedral angles and secondary structural motifs of the glutamine residues. For Q40 (i.e., just above the pathological length ≃36 for Huntington's disease), the equilibrium conformations of the monomer consist primarily of disordered, compact structures with non-negligible α-helical and turn content. We also observed a relatively small population of extended structures suitable for forming aggregates including ß- and α-strands, and ß- and α-hairpins. Most importantly, for Q40 we find that there exists a long-range correlation (ranging for at least 20 residues) among the backbone dihedral angles of the Q residues. For polyglutamine peptides below the pathological length, the population of the extended strands and hairpins is considerably smaller, and the correlations are short-range (at most 5 residues apart). Adding a C-terminal hexaproline to Q40 suppresses both the population of these rare motifs and the long-range correlation of the dihedral angles. We argue that the long-range correlation of the polyglutamine homopeptide, along with the presence of these rare motifs, could be responsible for its aggregation phenomena.

Ins and Outs of Rocker Switch Mechanism in Major Facilitator Superfamily of Transporters.

The major facilitator superfamily (MFS) of transporters consists of three classes of membrane transporters: symporters, uniporters, and antiporters. Despite such diverse functions, MFS transporters are believed to undergo similar conformational changes within their distinct transport cycles, known as the rocker-switch mechanism. While the similarities between conformational changes are noteworthy, the differences are also important since they could potentially explain the distinct functions of symporters, uniporters, and antiporters of the MFS superfamily. We reviewed a variety of experimental and computational structural data on a select number of antiporters, symporters, and uniporters from the MFS family to compare the similarities and differences of the conformational dynamics of three different classes of transporters.

The Alternating Access Mechanism in Mammalian Multidrug Resistance Transporters and Their Bacterial Homologs.

Multidrug resistance (MDR) proteins belonging to the ATP-Binding Cassette (ABC) transporter group play a crucial role in the export of cytotoxic drugs across cell membranes. These proteins are particularly fascinating due to their ability to confer drug resistance, which subsequently leads to the failure of therapeutic interventions and hinders successful treatments. One key mechanism by which multidrug resistance (MDR) proteins carry out their transport function is through alternating access. This mechanism involves intricate conformational changes that enable the binding and transport of substrates across cellular membranes. In this extensive review, we provide an overview of ABC transporters, including their classifications and structural similarities. We focus specifically on well-known mammalian multidrug resistance proteins such as MRP1 and Pgp (MDR1), as well as bacterial counterparts such as Sav1866 and lipid flippase MsbA. By exploring the structural and functional features of these MDR proteins, we shed light on the roles of their nucleotide-binding domains (NBDs) and transmembrane domains (TMDs) in the transport process. Notably, while the structures of NBDs in prokaryotic ABC proteins, such as Sav1866, MsbA, and mammalian Pgp, are identical, MRP1 exhibits distinct characteristics in its NBDs. Our review also emphasizes the importance of two ATP molecules for the formation of an interface between the two binding sites of NBD domains across all these transporters. ATP hydrolysis occurs following substrate transport and is vital for recycling the transporters in subsequent cycles of substrate transportation. Specifically, among the studied transporters, only NBD2 in MRP1 possesses the ability to hydrolyze ATP, while both NBDs of Pgp, Sav1866, and MsbA are capable of carrying out this reaction. Furthermore, we highlight recent advancements in the study of MDR proteins and the alternating access mechanism. We discuss the experimental and computational approaches utilized to investigate the structure and dynamics of MDR proteins, providing valuable insights into their conformational changes and substrate transport. This review not only contributes to an enhanced understanding of multidrug resistance proteins but also holds immense potential for guiding future research and facilitating the development of effective strategies to overcome multidrug resistance, thus improving therapeutic interventions.

Cholesterol in Class C GPCRs: Role, Relevance, and Localization.

G-protein coupled receptors (GPCRs), one of the largest superfamilies of cell-surface receptors, are heptahelical integral membrane proteins that play critical roles in virtually every organ system. G-protein-coupled receptors operate in membranes rich in cholesterol, with an imbalance in cholesterol level within the vicinity of GPCR transmembrane domains affecting the structure and/or function of many GPCRs, a phenomenon that has been linked to several diseases. These effects of cholesterol could result in indirect changes by altering the mechanical properties of the lipid environment or direct changes by binding to specific sites on the protein. There are a number of studies and reviews on how cholesterol modulates class A GPCRs; however, this area of study is yet to be explored for class C GPCRs, which are characterized by a large extracellular region and often form constitutive dimers. This review highlights specific sites of interaction, functions, and structural dynamics involved in the cholesterol recognition of the class C GPCRs. We summarize recent data from some typical family members to explain the effects of membrane cholesterol on the structural features and functions of class C GPCRs and speculate on their corresponding therapeutic potential.