Biology subjects

Tajkhorshid, E.

Publications and source records attributed to Tajkhorshid, E..

At least 37 records · Page 2Linked to original sources

bioRxiv2022

Structures and membrane interactions of native serotonin transporter in complexes with psychostimulants

The serotonin transporter (SERT) is a member of the SLC6 neurotransmitter transporter family that mediates serotonin reuptake at presynaptic nerve terminals. SERT is the target of both therapeutic antidepressant drugs and illicit psychostimulant substances such as cocaine and methamphetamines, which are small molecules that perturb normal serotonergic transmission by interfering with serotonin transport. Despite decades of studies, important functional aspects of SERT such as the oligomerization state of native SERT and its interactions with potential proteins remain unresolved. Here we develop methods to isolate SERT from porcine brain (pSERT) using a mild, non-ionic detergent, utilize fluorescence- detection size-exclusion chromatography to investigate its oligomerization state and interactions with other proteins, and employ single-particle cryo-electron microscopy to elucidate the structures of pSERT in complexes with methamphetamine or cocaine, providing structural insights into psychostimulant recognition and accompanying pSERT conformations. Methamphetamine and cocaine both bind to SERT central site, stabilizing the transporter in an outward open conformation. We also identify densities attributable to multiple cholesterol or cholesteryl hemisuccinate (CHS) molecules, as well as to a detergent molecule bound to SERT allosteric site. Under our conditions of isolation, we find that pSERT is best described as a monomeric entity, isolated without interacting proteins, and is ensconced by multiple cholesterol or CHS molecules. SignificanceThe serotonin transporter (SERT) is the target of antidepressants and illicit psychostimulants. Despite its importance in the nervous, cardiovascular and gastrointestinal systems, there is no direct knowledge of SERTs oligomerization state(s) and interactions with other proteins. Here, we develop methods to isolate porcine SERT (pSERT) from native brain tissue in the presence of a mild, non-ionic detergent, and investigated its properties by biochemical, structural and computational methods. We show how cocaine and methamphetamine exert their pharmacological effect on SERT, binding to a site halfway across the membrane-spanning region of the transporter, stabilizing pSERT in an outward open conformation. pSERT is best described as a monomeric entity, requiring neither oligomerization or additional proteins for its structure or function.

biophysics↗

bioRxiv2022

Backbone amides are key determinants of Cl- selectivity in CLC ion channels

Chloride homeostasis is tightly regulated in cellular compartments by dedicated channels and transporters. Whereas CLC-type channels select for Cl- over other anions, all other chloride channels are indiscriminate in their anionic permeability. Pore-lining side chains are thought to determine Cl- selectivity in CLC channels. However, orientation and functional roles of these side chains is not conserved among CLCs. All CLC pores are lined by backbone amides in a conserved structural arrangement, suggesting a role of mainchain groups in selectivity. We replaced pore-lining residues in the CLC-0 and bCLC-k channels with their respective -hydroxy acid counterparts using nonsense suppression method. This exchanges peptide-bond amides with ester-bond oxygens, incapable of hydrogen-bonding with permeating anions. Backbone substitutions functionally degrade inter-anion discrimination in a site-specific manner. These effects depend on the presence of a glutamate side chain that competes with ions permeating through the pore. Molecular dynamics simulations show that ion energetics within the bCLC-k pore are primarily determined by interactions with backbone amides. Insertion of an -hydroxy acid significantly alters ion selectivity and global pore hydration. We propose that backbone amides are conserved determinants of Cl- specificity in CLC channels in a mechanism reminiscent of that described for K+ channels.

biophysics↗

bioRxiv2022

Lipid-mediated organization of prestin in the outer hair cell membrane and its implications in sound amplification

Prestin is a high-density motor protein in the outer hair cells (OHCs), whose conformational response to acoustic signals alters the shape of the cell, thereby playing a major role in sound amplification by the cochlea. Despite recent structural determination in active and inhibited states, the details of prestins intimate interactions with the membrane, which are central to its function remained unresolved. Here, employing a large set (collectively, more than 0.5 ms) of coarse-grained molecule dynamics simulations, we characterize the nature of prestins lipid-protein interactions, demonstrating their impact on the organization of prestin at densities relevant to the OHCs and its effectiveness in reshaping OHCs. Beyond local enrichment/depletion of various lipid types, prestin causes drastic anisotropic membrane deformation, which in turn mediates a preferential membrane organization of prestin in which deformation patterns by neighboring prestin copies are aligned constructively. The reduced membrane rigidity accompanying this arrangement is hypothesized to maximize the mechanical impact of prestin on OHC reshaping during cochlear sound amplification. Prestins preferential arrangement is further verified by extended simulations demonstrating strong correlation between prestin neighbors in their orientations. These results demonstrate a strong case of protein-protein cooperative communication in membrane, purely mediated by their interactions with lipids.

biophysics↗

bioRxiv2022

Structure of C. elegans TMC-1 complex illuminates auditory mechanosensory transduction

The initial step in the sensory transduction pathway underpinning hearing and balance in mammals involves the conversion of force into the gating of a mechanosensory transduction (MT) channel. Despite the profound socioeconomic impacts of hearing disorders and the fundamental biological significance of understanding MT, the composition, structure and mechanism of the MT complex has remained elusive. Here we report the single particle cryo-EM structure of the native MT TMC-1 complex isolated from C. elegans. The 2-fold symmetric complex is composed of 2 copies each of the pore-forming TMC-1 subunit, the calcium-binding protein CALM-1 and the transmembrane inner ear protein TMIE. CALM-1 makes extensive contacts with the cytoplasmic face of the TMC-1 subunits while the single-pass TMIE subunits reside on the periphery of the complex, poised like the handles of an accordion. A subset of particles in addition harbors a single arrestin-like protein, ARRD-6, bound to a CALM-1 domain. Single- particle reconstructions and molecular dynamics simulations show how the MT complex deforms the membrane bilayer and suggest crucial roles for lipid-protein interactions in the mechanism by which mechanical force is transduced to ion channel gating.

biochemistry↗

bioRxiv2022

Elevator mechanism dynamics in a sodium-coupled dicarboxylate transporter

VcINDY, the sodium-dependent dicarboxylate transporter from Vibrio cholerae, is responsible for C4- and C5-carboxylate uptake into cells. The molecular mechanism of how VcINDY physically moves substrates across the membrane, and does so in an energetically efficient manner, is unclear. Here, we use single-molecule fluorescence resonance energy transfer experiments to directly observe the individual mechanistic steps that VcINDY takes to translocate substrates across a lipid bilayer, and then test key predictions of transport cycle mechanistic models. Our data provide the first direct evidence that VcINDY undergoes stochastic, elevator-type conformational motions that enable substrate translocation. Kinetic analysis suggests that the two protomers of the VcINDY homodimer undergo those motions in a non-cooperative manner, and thus catalyze two independent transport reactions. The relative substrate independence of those motions supports the notion that the VcINDY transport cycle maintains strict co-substrate coupling using a cooperative binding mechanism. Finally, thermodynamic modeling provides insight into how such a cooperative binding mechanism provides a generalized approach to optimizing transport for many secondary active transporters. Significance StatementTransporter proteins use energy to move molecular materials into and out of cells. To be efficient, the transporter motions responsible for moving the molecules must be tightly choreographed to avoid wasting energy without transporting anything. By measuring the motions and kinetics of a prototypical transporter (VcINDY) at the single-molecule level, this study finds the first evidence that transporters like VcINDY achieve efficient transport by coordinating constantly dynamic, "elevator-type" motions while sitting in the cellular membrane. The efficiency of these surprisingly dynamic transporters is then revealed by thermodynamic modeling, which explains the molecular basis behind how highly cooperative, substrate binding reactions may have evolved as the optimal strategy for maximizing transporter efficiency.

biophysics↗

bioRxiv2022

Tuning aromatic contributions by site-specific encoding of fluorinated phenylalanine residues in bacterial and mammalian cells

The aromatic side-chains of phenylalanine, tyrosine, and tryptophan interact with their environments via both hydrophobic and electrostatic interactions. Determining the extent to which these contribute to protein function and stability is not possible with conventional mutagenesis. Serial fluorination of a given aromatic is a validated method in vitro and in silico to specifically alter electrostatic characteristics, but this approach is restricted to a select few experimental systems. Here, we report a new group of pyrrolysine-based aminoacyl-tRNA synthetase/tRNA pairs that enable the site-specific encoding of a varied spectrum of fluorinated phenylalanine amino acids in E. coli and mammalian (HEK 293T) cells. By allowing the cross-kingdom expression of proteins bearing these unnatural amino acids at biochemical scale, these tools will enable deconstruction of biological mechanisms which utilize aromatic-pi interactions in structural and cellular contexts. Statement of SignificanceThe aromatic side-chains of phenylalanine, tyrosine, and tryptophan are crucial for protein function and pharmacology due to their hydrophobic and electrostatic contributions to catalytic centers and ligand-binding pockets. However, few experimental approaches can chemically assess the functional roles of aromatics in cellular environments. The accepted computational method for aromatic interrogation is via serial fluorination, which lacks an experimental correlate in bacterial or mammalian cell systems. We have identified a family of synthetases to encode multiple different types of fluorinated phenylalanine residues in E. coli and HEK cells via nonsense suppression. The efficiency of these synthetases is sufficient to support biochemical characterization and structural determination of proteins with site-specific incorporation of unnatural phenylalanine analogs.

biochemistry↗

bioRxiv2022

Protonation-dependent alternating access of a spinster transporter, an emerging family of broad-specificity efflux pumps

Spinster (Spns) lipid transporters are critical for transporting sphingosine-1-phosphate (S1P) across cellular membranes. In humans, Spns2 functions as the main S1P transporter in endothelial cells, making it a potential drug target for modulating S1P signaling. Here, we employed an integrated approach in lipid membranes to identify unknown conformational states of a bacterial Spns from Hyphomonas neptunium (HnSpns) and to define its proton- and substrate-coupled conformational dynamics. Our systematic study reveals conserved residues critical for protonation steps and their regulation, and how sequential protonation of these proton switches coordinates the conformational transitions in the context of a noncanonical ligand-dependent alternating access. A conserved periplasmic salt bridge (Asp60TM2:Arg289TM7) keeps the transporter in a closed conformation, while proton-dependent conformational dynamics are significantly enhanced on the periplasmic side, providing a pathway for ligand exchange. Furthermore, our resistance assays reveal substrate polyspecificity and HnSpns multidrug resistance (MDR) activity that underscore the previously unknown role of Spns proteins in MDR, beyond their activity in sphingolipid transport and signaling.

biophysics↗

bioRxiv2021

Post-Translational Modifications Optimize the Ability of SARS-CoV-2 Spike for Effective Interaction with Host Cell Receptors

SARS-CoV2 spike glycoprotein is prime target for vaccines and for diagnostics and therapeutic antibodies against the virus. While anchored in the viral envelope, for effective virulance, the spike needs to maintain structural flexibility to recognize the host cell surface receptors and bind to them, a property that can heavily hinge upon the dynamics of the unresolved domains, most prominently the stalk. Construction of the complete, membrane-bound spike model and the description of its dynamics remain critical steps in understanding the inner working of this key element in viral infection. Using a hybrid approach, combining homology modeling, protein-protein docking and MD simulations, guided by biochemical and glycomics data, we have developed a full-length, membrane-bound, palmitoylated and fully-glycosylated spike structure in a native membrane. Multi-microsecond MD simulations of this model, the longest known trajectory of the full-spike, reveals conformational dynamics employed by the protein to explore the crowded surface of the host cell. In agreement with cryoEM, three flexiblele hinges in stalk allow for global conformational heterogeneity of spike in the fully-glycosyslated system mediated by glycan-glycan and glycan-lipid interactions. Dynamical range of spike is considerably reduced in its non-glycosylated form, confining the area explored by the spike on the host cell surface. Furthermore, palmitoylation of the membrane domain amplify the local curvature that may prime the fusion. We show that the identified hinge regions are highly conserved in SARS coronaviruses, highlighting their functional importance in enhancing viral infection, and thereby provide novel points for discovery of alternative therapeutics against the virus. SignificanceSARS-CoV2 Spike protein, which forms the basis for high pathogenicity and transmissibility of the virus, is also prime target for the development of both diagnostics and vaccines for the debilitating disease caused by the virus. We present a full model of spike methodically crafted and used to study its atomic-level dynamics by multiple-{micro}s simulations. The results shed new light on the impact of posttranslational modifications in the pathogenicity of the virus. We show how glycan-glycan and glycan-lipid interactions broaden the proteins dynamical range, and thereby, its effective interaction with the surface receptors on the host cell. Palmitoylation of spike membrane domain, on the other hand, results in a unique deformation pattern that might prime the membrane for fusion.

biophysics↗

bioRxiv2021

Microscopic Characterization of the Chloride Permeation Pathway in the Human Excitatory Amino Acid Transporter 1 (EAAT1)

Excitatory amino acid transporters (EAATs) are glutamate transporters that belong to the solute carrier 1A (SLC1A) family. They couple glutamate transport to the co-transport of three sodium (Na+) ions and one proton (H+) and the counter-transport of one potassium (K+) ion. In addition to this coupled transport, binding of substrate and Na+ ions to EAATs activates a thermodynamically uncoupled chloride (Cl-) conductance. Structures of SLC1A family members have revealed that these transporters use a twisting elevator mechanism of transport, where a mobile transport domain carries substrate and coupled ions across the membrane, while a static scaffold domain anchors the transporter in the membrane. We have recently demonstrated that the uncoupled Cl- conductance is activated by the formation of an aqueous pore at the domain interface during the transport cycle in archaeal GltPh. However, a pathway for the uncoupled Cl- conductance has not been reported for the EAATs and it is unclear if such a pathway is conserved. Here, we employ all-atom molecular dynamics (MD) simulations combined with enhanced sampling, free-energy calculations, and experimental mutagenesis to approximate large-scale conformational changes during the transport process and identified a Cl- conducting conformation in human EAAT1. We were able to extensively sample the large-scale structural transitions, allowing us to capture an intermediate conformation formed during the transport cycle with a continuous aqueous pore at the domain interface. The free-energy calculations performed for the conduction of Cl- and Na+ ions through the captured conformation, highlight the presence of two hydrophobic gates which control the selective movement of Cl- through the aqueous pathway. Overall, our findings provide insights into the mechanism by which a human glutamate transporter can support the dual functions of active transport and passive Cl- permeation and confirming the commonality of this mechanism in different members of the SLC1A family.

biophysics↗

bioRxiv2021

Intelligent Resolution: Integrating Cryo-EM with AI-driven Multi-resolution Simulations to Observe the SARS-CoV-2 Replication-Transcription Machinery in Action

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) replication transcription complex (RTC) is a multi-domain protein responsible for replicating and transcribing the viral mRNA inside a human cell. Attacking RTC function with pharmaceutical compounds is a pathway to treating COVID-19. Conventional tools, e.g., cryo-electron microscopy and all-atom molecular dynamics (AAMD), do not provide sufficiently high resolution or timescale to capture important dynamics of this molecular machine. Consequently, we develop an innovative workflow that bridges the gap between these resolutions, using mesoscale fluctuating finite element analysis (FFEA) continuum simulations and a hierarchy of AI-methods that continually learn and infer features for maintaining consistency between AAMD and FFEA simulations. We leverage a multi-site distributed workflow manager to orchestrate AI, FFEA, and AAMD jobs, providing optimal resource utilization across HPC centers. Our study provides unprecedented access to study the SARS-CoV-2 RTC machinery, while providing general capability for AI-enabled multi-resolution simulations at scale.

biophysics↗

bioRxiv2021

Ataxia-linked SLC1A3 mutations alter EAAT1 chloride channel activity and glial regulation of CNS function

Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system (CNS). Excitatory Amino Acid Transporters (EAATs) regulate extracellular glutamate by transporting it into cells, mostly glia, to terminate neurotransmission and to avoid neurotoxicity. EAATs are also chloride (Cl-) channels, but the physiological role of Cl- conductance through EAATs is poorly understood. Mutations of human EAAT1 (hEAAT1) have been identified in patients with episodic ataxia type 6 (EA6). One mutation showed increased Cl- channel activity and decreased glutamate transport, but the relative contributions of each function of hEAAT1 to mechanisms underlying the pathology of EA6 remain unclear. Here we investigated the effects of five additional EA6-related mutations on hEAAT1 function in Xenopus laevis oocytes, and on CNS function in a Drosophila melanogaster model of locomotor behavior. Our results indicate that mutations with decreased hEAAT1 Cl- channel activity and functional glutamate transport can also contribute to the pathology of EA6, highlighting the importance of Cl- homeostasis in glial cells for proper CNS function. We also identified a novel mechanism involving an ectopic sodium (Na+) leak conductance in glial cells. Together, these results strongly support the idea that EA6 is primarily an ion channelopathy of CNS glia.

neuroscience↗

bioRxiv2021

Structural transitions permitting ligand entry and exit in bacterial fatty acid binding proteins.

Fatty acid (FA) transfer proteins extract FA from membranes and sequester their ligand to facilitate its movement through the cytosol. While detailed views of soluble protein-FA complexes are available, how FA exchange occurs at the membrane has remained unknown. Staphylococcus aureus FakB1 is a prototypical bacterial FA transfer protein that binds palmitate within a narrow, buried tunnel. Here, we determine the conformational change from this closed state to an open state that engages the phospholipid bilayer. Upon membrane binding, a dynamic loop in FakB1 that covers the FA binding site disengages and folds into an amphipathic helix. This helix inserts below the phosphate plane of the bilayer to create a diffusion channel for the FA to exchange between the protein and the membrane. The structure of the bilayer-associated conformation of FakB1 has local similarities with mammalian FA binding proteins and provides a general conceptual framework for how these proteins interact with the membrane to promote lipid transfer.

biophysics↗

bioRxiv2021

Investigation of gating in outer membrane porins provides new perspectives on antibiotic resistance mechanisms

Gram-negative bacteria pose a serious public health concern, primarily due to a higher frequency of antibiotic resistance conferred to them as a result of low permeability of their outer membrane (OM). Antibiotics capable of traversing the OM typically permeate through OM porins; thus, understanding the permeation properties of these porins is instrumental to the development of new antibiotics. A common macroscopic feature of many OM porins is their ability to transition between functionally distinct open and closed states that regulate transport properties and rate. To obtain a molecular basis for these processes, we performed tens of microseconds of molecular dynamics simulations of E. coli OM porin, OmpF. We observed that large-scale motion of the internal loop, L3, leads to widening and narrowing of the pore, suggesting its potential role in gating. Furthermore, Markov state analysis revealed multiple energetically stable conformations of L3 corresponding to open and closed states of the porin. Dynamics between these functional states occurs on the time scale of tens of microseconds and are mediated by the movement of highly conserved acidic residues of L3 to form H-bonds with opposing sides of the barrel wall of the pore. To validate our mechanism, we mutated key residues involved in the gating process that alter the H-bond pattern in the open/closed states and performed additional simulations. These mutations shifted the dynamic equilibrium of the pore towards open or closed states. Complementarily, the mutations favoring the open/closed states lead to increased/decreased accumulation of multiple antibiotics in our whole-cell accumulation assays. Notably, porins containing one of the mutations favoring the closed state has previously been found in antibiotic resistant bacterial strains. Overall, our 180 {micro}s of simulation data (wild type and mutants) with concerted experiments suggests that regulation of the dynamic equilibrium between open and closed states of OM porins could be a mechanism by which Gram-negative bacteria acquire antibiotic resistance.

biophysics↗

bioRxiv2021

Rationalizing generation of broad spectrum antibiotics with the addition of a primary amine

Antibiotic resistance of Gram-negative bacteria is largely attributed to the low permeability of their outer membrane (OM). Recently, we disclosed the eNTRy rules, a key lesson of which is that the introduction of a primary amine enhances OM permeation in certain contexts. To understand the molecular basis for this finding, we perform an extensive set of molecular dynamics (MD) simulations and free energy calculations comparing the permeation of aminated and amine-free antibiotic derivatives through the most abundant OM porin of E. coli, OmpF. To improve sampling of conformationally flexible drugs in MD simulations, we developed a novel, Monte Carlo and graph theory based algorithm to probe more efficiently the rotational and translational degrees of freedom visited during the permeation of the antibiotic molecule through OmpF. The resulting pathways were then used for free-energy calculations, revealing a lower barrier against the permeation of the aminated compound, substantiating its greater OM permeability. Further analysis revealed that the amine facilitates permeation by enabling the antibiotic to align its dipole to the luminal electric field of the porin and while forming favorable electrostatic interactions with specific, highly-conserved charged residues. The importance of these interactions in permeation was further validated with experimental mutagenesis and whole cell accumulation assays. Overall, this study provides insights on the importance of the primary amine for antibiotic permeation into Gram-negative pathogens that could help the design of future antibiotics. We also offer a new computational approach for calculating free-energy of processes where relevant molecular conformations cannot be efficiently captured.

biophysics↗

bioRxiv2021

PIP2-dependent coupling of voltage sensor and pore domains in Kv7.2

Phosphatidylinositol-4,5-bisphosphate (PIP2) is a signaling lipid which regulates voltage-gated Kv7/KCNQ potassium channels. Altered PIP2 sensitivity of neuronal Kv7.2 channel is involved in KCNQ2 epileptic encephalopathy. However, the molecular action of PIP2 on Kv7.2 gating remains largely elusive. Here, we use molecular dynamics simulations and electrophysiology to characterize PIP2 binding sites in a human Kv7.2 channel. In the closed state, PIP2 localizes to the periphery of the voltage-sensing domain (VSD). In the open state, PIP2 binds to 4 distinct interfaces formed by the cytoplasmic ends of the VSD, the gate, intracellular helices A and B and their linkers. PIP2 binding induces bilayer-interacting conformation of helices A and B and the correlated motion of the VSD and the pore domain, whereas charge-neutralizing mutations block this coupling and reduce PIP2 sensitivity of Kv7.2 channels by disrupting PIP2 binding. These findings reveal the allosteric role of PIP2 in Kv7.2 channel activation.

biophysics↗

bioRxiv2021

Extended-Ensemble Docking to Probe Evolution of Ligand Binding Sites in Proteins Undergoing Large-Scale Structural Changes for Their Function

Proteins can sample a broad landscape as they undergo conformational transition between different functional states. As key players in almost all cellular processes, proteins are important drug targets. Considering the different conformational states of a protein is therefore central for a successful drug-design strategy. Here we introduce a novel docking protocol, termed as extended-ensemble docking, pertaining to proteins that undergo large-scale (global) conformational changes during their function. In its application to multidrug ABC-transporter P-glycoprotein (Pgp), extensive non-equilibrium molecular dynamics simulations employing system-specific collective variables capturing the alternate access mechanism of Pgp, are first used to construct the transition cycle of the transporter. An extended set of conformational states representing the full transition between the inward- and the outward-facing states of Pgp, is then used to seed high-throughput docking calculations of a set of known substrates, non-substrates, and modulators of the transporter. Large differences are observed in the predicted binding affinities to the conformational ensemble, with compounds showing stronger binding affinities to intermediate conformations compared to the starting crystal structure. Hierarchical clustering of the individual binding modes of the different compounds shows all ligands preferably bind to the large central cavity of the protein, formed at the apex of the transmembrane domain (TMD), whereas only small binding populations are observed in the previously described R and H sites present within the individual TMD leaflets. Based on the results, the central cavity is further divided into two major subsites: first subsite preferably binds smaller substrates and high-affinity inhibitors, whereas the second one shows preference for larger substrates and low-affinity modulators. These central sites along with the low-affinity interaction sites present within the individual TMD leaflets may respectively correspond to the proposed high- and low-affinity binding sites in Pgp. We propose further optimization strategy for developing more potent inhibitor of Pgp, based on increasing its specificity to the extended ensemble of the protein instead of using a single protein structure, as well as its selectivity for the high-affinity binding site. In contrast to earlier in-silico studies using single static structures of Pgp, our results show better agreement with experimental studies, pointing to the importance of incorporating the global conformational flexibility of proteins in future drug-discovery endeavors.

biophysics↗

bioRxiv2021

Molecular Mechanism of Capsid Disassembly in Hepatitis B Virus

The disassembly of a viral capsid leading to the release of its genetic material into the host cell is a fundamental step in viral infection. Hepatitis B virus (HBV) in particular consists of identical capsid protein monomers that dimerize and also arrange themselves into pentamers or hexamers on the capsid surface. By applying atomistic molecular dynamics simulation to an entire solvated HBV capsid subject to a uniform mechanical stress protocol, we monitor the disassembly process down to the level of individual amino acids. The strain of an external force combined with thermal fluctuations causes structurally heterogeneous cracks to appear in the HBV capsid. Unlike the expectation from purely mechanical considerations, the cracks mainly occur within and between hexameric sites, whereas pentameric sites remain largely intact. Only a small subset of the capsid protein monomers governs disassembly. These monomers are distributed across the whole capsid, but belong to regions with a high degree of collective motion that we label communities. Cross-talk within these communities is a mechanism of crack propagation leading to destabilization of the entire capsid, and eventually its disassembly. We identify specific residues whose interactions are most readily lost during disassembly: R127, I139, Y132, N136, A137, and V149 are among the hotspots at the interfaces between dimers that lie within or span hexamers, leading to dissociation. The majority of these hotspots are evolutionary conserved, indicating that they are important for disassembly by avoiding over-stabilization of capsids. SignificanceHepatitis B virus (HBV) is a DNA virus that is 100 times more infectious than HIV. Despite the availability of a vaccine, the chronic infection rate of this virus is still about 300 million people globally. HBV chronic infection, for which no cure is currently available, can lead to liver cancer. Therefore, there is an unmet need to investigate the infection cycle of the virus. One of the most crucial steps in virus replication cycle is the release of its genetic material to the nucleus. During this step, the viral capsid enclosing the genetic material disassembles. However, its mechanism is unknown. Here, we utilize molecular simulations to shed light on the events leading to the capsid disassembly with atomistic detail.

biophysics↗

bioRxiv2020

Structural Basis of Complex Formation Between Mitochondrial Anion Channel VDAC1 and Hexokinase-II

Complex formation between hexokinase-II (HKII) and the mitochondrial channel VDAC1 plays a crucial role in regulating cell growth and survival; however, structural details of this complex remain elusive. We hypothesize that a conserved, hydrophobic helix (H-anchor) of HKII first inserts into the outer membrane of mitochondria (OMM) and then interacts with VDAC1 on the cytosolic leaflet of OMM to form a binary complex. To systematically investigate this process, we adopted a hybrid approach: 1) the membrane binding of HKII was first described with molecular dynamics (MD) simulations employing a membrane mimetic model with enhanced lipid diffusion, then 2) the resulting membrane-bound HKII was used to form complex with VDAC1 in millisecond-scale Brownian dynamics (BD) simulations. We show that H-anchor inserts its first 10 residues into the membrane, substantiating previous experimental findings. The insertion depth of the H-anchor was used to derive positional restraints in subsequent BD simulations to preserve the membrane-bound pose of HKII during the formation of the HKII/VDAC1 binary complex. Multiple BD-derived structural models were further refined with MD simulations, resulting in one stable complex. A major feature in the complex is the partial (not complete) blockade of VDAC1s permeation pathway by HKII, a result supported by our comparative electrophysiological measurements of the channel in the presence and absence of HKII. Additionally, we showed how VDAC1 phosphorylation disrupts HKII binding, a feature that is verified by our electrophysiology recordings and have implications in mitochondria-mediated cell death.

biophysics↗