The architecture of the OmpC-MlaA complex sheds light on the maintenance of outer membrane lipid asymmetry in Escherichia coli.

A distinctive feature of the Gram-negative bacterial cell envelope is the asymmetric outer membrane (OM), where lipopolysaccharides and phospholipids (PLs) reside in the outer and inner leaflets, respectively. This unique lipid asymmetry renders the OM impermeable to external insults, including antibiotics and bile salts. In Escherichia coli, the complex comprising osmoporin OmpC and the OM lipoprotein MlaA is believed to maintain lipid asymmetry by removing mislocalized PLs from the outer leaflet of the OM. How this complex performs this function is unknown. Here, we defined the molecular architecture of the OmpC-MlaA complex to gain insights into its role in PL transport. Using in vivo photo-cross-linking and molecular dynamics simulations, we established that MlaA interacts extensively with OmpC and is located entirely within the lipid bilayer. In addition, MlaA forms a hydrophilic channel, likely enabling PL translocation across the OM. We further showed that flexibility in a hairpin loop adjacent to the channel is critical in modulating MlaA activity. Finally, we demonstrated that OmpC plays a functional role in maintaining OM lipid asymmetry together with MlaA. Our work offers glimpses into how the OmpC-MlaA complex transports PLs across the OM and has important implications for future antibacterial drug development.

Multiscale modeling of innate immune receptors: Endotoxin recognition and regulation by host defense peptides.

The innate immune system provides a first line of defense against foreign microorganisms, and is typified by the Toll-like receptor (TLR) family. TLR4 is of particular interest, since over-stimulation of its pathway by excess lipopolysaccharide (LPS) molecules from the outer membranes of Gram-negative bacteria can result in sepsis, which causes millions of deaths each year. In this review, we outline our use of molecular simulation approaches to gain a better understanding of the determinants of LPS recognition, towards the search for novel immunotherapeutics. We first describe how atomic-resolution simulations have enabled us to elucidate the regulatory conformational changes in TLR4 associated with different LPS analogues, and hence a means to rationalize experimental structure-activity data. Furthermore, multiscale modelling strategies have provided a detailed description of the thermodynamics and intermediate structures associated with the entire TLR4 relay - which consists of a number of transient receptor/coreceptor complexes - allowing us trace the pathway of LPS transfer from bacterial membranes to the terminal receptor complex at the plasma membrane surface. Finally, we describe our efforts to leverage these computational models, in order to elucidate previously undisclosed anti-inflammatory mechanisms of endogenous host-defense peptides found in wounds. Collectively, this work represents a promising avenue for the development of novel anti-septic treatments, inspired by nature's innate defense strategies.

A polar SxxS motif drives assembly of the transmembrane domains of Toll-like receptor 4.

Like all members of the Toll-like receptor (TLR) family, TLR4 comprises of a large ectodomain (ECD) involved in ligand recognition at the cell-surface, and a cytosolic Toll/interleukin-1 receptor (TIR) signalling domain, linked by a lipid membrane-anchored transmembrane (TM) domain (TMD). Binding of immunostimulatory pathogen-associated molecular patterns (PAMPs) such as bacterial lipopolysaccharide (LPS) to myeloid differentiation factor 2 (MD-2) coreceptor-complexed TLR4 leads to its dimerization, resulting in productive juxtaposition of TIR domains and the initiation of pro-inflammatory innate immune responses. Whilst the process of PAMP recognition is relatively well understood, thanks to numerous high-resolution crystallographic structures of ECDs, the mechanism by which such recognition is translated into TMD dimerization and activating conformational changes is less clear. Based on available biophysical and biochemical experimental data, ab initio modelling, and multiscale molecular dynamics (MD) simulations entailing a total of >13µs and >200µs of atomistic and coarse-grained sampling, respectively, we investigate the structural basis for TLR4 TMD dimerization within a biologically relevant lipid membrane environment. A key polar-xx-polar (637SxxS640) motif is shown to drive association of the TLR4 TMDs, and to maintain a flexible interface, which may be disrupted by selected point mutations. Furthermore, MD simulations of various TMD+ECD constructs have been used to investigate the coupling between domains, revealing that flexible linkers abrogate dimerization via aggregation of ECDs at the membrane surface, explaining previous biochemical observations. These results improve our understanding of the assembly and signalling mechanisms of TLR4, and pave the way for rational structure-based development of membrane-associated immunomodulatory molecules.

South-east Asian Zika virus strain linked to cluster of cases in Singapore, August 2016.

Zika virus (ZIKV) is an ongoing global public health emergency with 70 countries and territories reporting evidence of ZIKV transmission since 2015. On 27 August 2016, Singapore reported its first case of local ZIKV transmission and identified an ongoing cluster. Here, we report the genome sequences of ZIKV strains from two cases and find through phylogenetic analysis that these strains form an earlier branch distinct from the recent large outbreak in the Americas.

Revisiting the interaction between the chaperone Skp and lipopolysaccharide.

The bacterial outer membrane comprises two main classes of components, lipids and membrane proteins. These nonsoluble compounds are conveyed across the aqueous periplasm along specific molecular transport routes: the lipid lipopolysaccharide (LPS) is shuttled by the Lpt system, whereas outer membrane proteins (Omps) are transported by chaperones, including the periplasmic Skp. In this study, we revisit the specificity of the chaperone-lipid interaction of Skp and LPS. High-resolution NMR spectroscopy measurements indicate that LPS interacts with Skp nonspecifically, accompanied by destabilization of the Skp trimer and similar to denaturation by the nonnatural detergent lauryldimethylamine-N-oxide (LDAO). Bioinformatic analysis of amino acid conservation, structural analysis of LPS-binding proteins, and MD simulations further confirm the absence of a specific LPS binding site on Skp, making a biological relevance of the interaction unlikely. Instead, our analysis reveals a highly conserved salt-bridge network, which likely has a role for Skp function.

The membranes of Gram-negative bacteria: progress in molecular modelling and simulation.

Molecular modelling and simulations have been employed to study the membranes of Gram-negative bacteria for over 20 years. Proteins native to these membranes, as well as antimicrobial peptides and drug molecules have been studied using molecular dynamics simulations in simple models of membranes, usually only comprising one lipid species. Thus, traditionally, the simulations have reflected the majority of in vitro membrane experimental setups, enabling observations from the latter to be rationalized at the molecular level. In the last few years, the sophistication and complexity of membrane models have improved considerably, such that the heterogeneity of the lipid and protein composition of the membranes can now be considered both at the atomistic and coarse-grain levels of granularity. Importantly this means relevant biology is now being retained in the models, thereby linking the in silico and in vivo scenarios. We discuss recent progress in simulations of proteins in simple lipid bilayers, more complex membrane models and finally describe some efforts to overcome timescale limitations of atomistic molecular dynamics simulations of bacterial membranes.

The NorM MATE transporter from N. gonorrhoeae: insights into drug and ion binding from atomistic molecular dynamics simulations.

The multidrug and toxic compound extrusion transporters extrude a wide variety of substrates out of both mammalian and bacterial cells via the electrochemical gradient of protons and cations across the membrane. The substrates transported by these proteins include toxic metabolites and antimicrobial drugs. These proteins contribute to multidrug resistance in both mammalian and bacterial cells and are therefore extremely important from a biomedical perspective. Although specific residues of the protein are known to be responsible for the extrusion of solutes, mechanistic details and indeed structures of all the conformational states remain elusive. Here, we report the first, to our knowledge, simulation study of the recently resolved x-ray structure of the multidrug and toxic compound extrusion transporter, NorM from Neisseria gonorrhoeae (NorM_NG). Multiple, atomistic simulations of the unbound and bound forms of NorM in a phospholipid lipid bilayer allow us to identify the nature of the drug-protein/ion-protein interactions, and secondly determine how these interactions contribute to the conformational rearrangements of the protein. In particular, we identify the molecular rearrangements that occur to enable the Na(+) ion to enter the cation-binding cavity even in the presence of a bound drug molecule. These include side chain flipping of a key residue, GLU-261 from pointing toward the central cavity to pointing toward the cation binding side when bound to a Na(+) ion. Our simulations also provide support for cation binding in the drug-bound and apo states of NorM_NG.

Conformational dynamics and membrane interactions of the E. coli outer membrane protein FecA: a molecular dynamics simulation study.

The TonB-dependent transporters mediate high-affinity binding and active transport of a variety of substrates across the outer membrane of Escherichia coli. The substrates transported by these proteins are large, scarce nutrients that are unable to gain entry into the cell by passive diffusion across the complex, asymmetric bilayer that constitutes the outer membrane. Experimental studies have identified loop regions that are essential for the correct functioning of these proteins. A number of these loops have been implicated in ligand binding. We report the first simulations of an E. coli outer membrane protein in an asymmetric model membrane that incorporates lipopolysaccharide (LPS) molecules. Comparative simulations of the apo and holo forms of the TonB-dependent transporter FecA in different membrane models enable us to identify the nature of the LPS-protein interactions and determine how these interactions impact upon the conformational dynamics of this protein. In particular, our simulations provide molecular-level insights into the influence of the environment and ligand on the dynamics of the functionally important loops of FecA. In addition, we provide insights into the nature of the protein-ligand interactions and ligand induced conformational change in FecA.

Stability and membrane interactions of an autotransport protein: MD simulations of the Hia translocator domain in a complex membrane environment.

Hia is a trimeric autotransporter found in the outer membrane of Haemphilus influenzae. The X-ray structure of Hia translocator domain revealed each monomer to consist of an α-helix connected via a loop to a 4-stranded ß-sheet, thus the topology of the trimeric translocator domain is a 12-stranded ß-barrel containing 3 α-helices that protrude from the mouth of the ß-barrel into the extracellular medium. Molecular dynamics simulations of the Hia monomer and trimer have been employed to explore the interactions between the helices, ß-barrel and connecting loops that may contribute to the stability of the trimer. In simulations of the Hia monomer we show that the central α-helix may stabilise the fold of the 4-stranded ß-sheet. In simulations of the Hia trimer, a H-bond network involving residues in the ß-barrel, α-helices and loops has been identified as providing stability for the trimeric arrangement of the monomers. Glutamine residues located in the loops connecting the α-helices to the ß-barrel are orientated in a triangular arrangement such that each forms 2 hydrogen bonds to each of the corresponding glutamines in the other loops. In the absence of the loops, the ß-barrel becomes distorted. Simulations show that while the trimeric translocator domain ß-barrel is inherently flexible, it is unlikely to accommodate the passenger domain in a folded conformation. Simulations of Hia in an asymmetric model of the outer membrane have revealed membrane-protein interactions that anchor the protein within its native membrane environment.

The nanotube express: Delivering a stapled peptide to the cell surface.

HYPOTHESIS: Carbon nanotubes (CNTs) represent a novel platform for cellular delivery of therapeutic peptides. Chemically-functionalized CNTs may enhance peptide uptake by improving their membrane targeting properties. EXPERIMENTS: Using coarse-grained (CG) molecular dynamics (MD) simulations, we investigate membrane interactions of a peptide conjugated to pristine and chemically-modified CNTs. As proof of principle, we focus on their interactions with PM2, an amphipathic stapled peptide that inhibits the E3 ubiquitin ligase HDM2 from negatively regulating the p53 tumor suppressor. CNT interaction with both simple planar lipid bilayers as well as spherical lipid vesicles was studied, the latter as a surrogate for curved cellular membranes. FINDINGS: Membrane permeation was rapid and spontaneous for both pristine and oxidized CNTs when unconjugated. This was slowed upon addition of a noncovalently attached peptide surface "sheath", which may be an effective way to slow CNT entry and avert membrane rupture. The CNT conjugates were observed to "desheath" their peptide layer at the bilayer interface upon insertion, leaving their cargo behind in the outer leaflet. This suggests that a synergy may exist to optimize CNT safety whilst enhancing the delivery efficiency of "hitchhiking" therapeutic molecules.

Stability and membrane orientation of the fukutin transmembrane domain: a combined multiscale molecular dynamics and circular dichroism study.

The N-terminal domain of fukutin-I has been implicated in the localization of the protein in the endoplasmic reticulum and Golgi Apparatus. It has been proposed to mediate this through its interaction with the thinner lipid bilayers found in these compartments. Here we have employed multiscale molecular dynamics simulations and circular dichroism spectroscopy to explore the structure, stability, and orientation of the short 36-residue N-terminus of fukutin-I (FK1TMD) in lipids with differing tail lengths. Our results show that FK1TMD adopts a stable helical conformation in phosphatidylcholine lipids when oriented with its principal axis perpendicular to the bilayer plane. The stability of the helix is largely insensitive to the lipid tail length, preventing hydrophobic mismatch by virtue of its mobility and ability to tilt within the lipid bilayers. This suggests that changes in FK1TMD tilt in response to bilayer properties may be implicated in the regulation of its trafficking. Coarse-grained simulations of the complex Golgi membrane suggest the N-terminal domain may induce the formation of microdomains in the surrounding membrane through its preferential interaction with 1,2-dipalmitoyl-sn-glycero-3-phosphatidylinositol 4,5-bisphosphate lipids.

Multiscale Modeling and Simulation Approaches to Lipid-Protein Interactions.

Lipid membranes play a crucial role in living systems by compartmentalizing biological processes and forming a barrier between these processes and the environment. Naturally, a large apparatus of biomolecules is responsible for construction, maintenance, transport, and degradation of these lipid barriers. Additional classes of biomolecules are tasked with transport of specific substances or transduction of signals from the environment across lipid membranes. In this article, we intend to describe a set of techniques that enable one to build accurate models of lipid systems and their associated proteins, and to simulate their dynamics over a variety of time and length scales. We discuss the methods and challenges that allow us to derive structural, mechanistic, and thermodynamic information from these models. We also show how these models have recently been applied in research to study some of the most complex lipid-protein systems to date, including bacterial and viral envelopes, neuronal membranes, and mammalian signaling systems.

A Funneled Conformational Landscape Governs Flavivirus Fusion Peptide Interaction with Lipid Membranes.

During host cell infection by flaviviruses such as dengue and Zika, acidic pH within the endosome triggers a conformational change in the envelope protein on the outer surface of the virion. This results in exposure of the ∼15 residue fusion peptide (FP) region, freeing it to induce fusion between the viral and endosomal membranes. A better understanding of the conformational dynamics of the FP in the presence of membranes, and the basis for its selectivity for anionic lipid species present within the endosome, would facilitate its therapeutic targeting with antiviral drugs and antibodies. In this work, multiscale modeling, simulations, and free energy calculations (including a total of ∼75 µs of atomic-resolution sampling), combined with imaging total internal reflection fluorescence correlation spectroscopy experiments, were employed to investigate the mechanisms of interaction of FP variants with lipid bilayers. Wild-type FPs (in the presence or absence of a fluorescein isothiocyanate tag) were shown to possess a funneled conformational landscape governing their exit from solvent and penetration into the lipid phase and to exhibit an electrostatically favored >2-fold affinity for membranes containing anionic species over purely zwitterionic ones. Conversely, the landscape was abolished in a nonfunctional point mutant, leading to a 2-fold drop in host membrane affinity. Collectively, our data reveal how the highly conserved flavivirus FP has evolved to funnel its conformational space toward a maximally fusogenic state anchored within the endosomal membrane. Therapeutically targeting the accessible ensemble of FP conformations may represent a new, rational strategy for blocking viral infection.

A Thermodynamic Funnel Drives Bacterial Lipopolysaccharide Transfer in the TLR4 Pathway.

The Gram-negative bacterial outer membrane contains lipopolysaccharide, which potently stimulates the mammalian innate immune response. This involves a relay of specialized complexes culminating in transfer of lipopolysaccharide from CD14 to Toll-like receptor 4 (TLR4) and its co-receptor MD-2 on the cell surface, leading to activation of downstream inflammatory responses. In this study we develop computational models to trace the TLR4 cascade in near-atomic detail. We demonstrate through rigorous thermodynamic calculations that lipopolysaccharide molecules traversing the receptor cascade fall into a thermodynamic funnel. An affinity gradient for lipopolysaccharide is revealed upon extraction from aggregates or realistic bacterial outer membrane models and transfer through CD14 to the terminal TLR4/MD-2 receptor-co-receptor complex. We subsequently assemble viable CD14/TLR4/MD-2 oligomers at the plasma membrane surface, and observe lipopolysaccharide exchange between CD14 and TLR4/MD-2. Collectively, this work helps to unravel the key structural determinants governing endotoxin recognition in the TLR4 innate immune pathway.

Influence of pH on the activity of thrombin-derived antimicrobial peptides.

The wound environment is characterized by physiological pH changes. Proteolysis of thrombin by wound-derived proteases, such as neutrophil elastase, generates antimicrobial thrombin-derived C-terminal peptides (TCPs), such as HVF18 (HVFRLKKWIQKVIDQFGE). Presence of such TCPs in human wound fluids in vivo, as well as the occurrence of an evolutionarily conserved His residue in the primary amino acid sequence of TCPs, prompted us to investigate the pH-dependent antibacterial action of HVF18, as well as of the prototypic GKY25 (GKYGFYTHVFRLKKWIQKVIDQFGE). We show that protonation of this His residue at pH 5.5 increases the antibacterial activity of both TCPs against Gram-negative Escherichia coli by membrane disruption. Physiological salt level (150 mM NaCl) augments antibacterial activity of GKY25 but diminishes for the shorter HVF18. Replacing His with Leu or Ser in GKY25 abolishes the His protonation-dependent increase in antibacterial activity at pH 5.5, whereas substitution with Lys maintains activity at neutral (pH 7.4) and acidic pH. Interestingly, both TCPs display decreased binding affinities to human CD14 with decreasing pH, suggesting a likely switch in mode-of-action, from anti-inflammatory at neutral pH to antibacterial at acidic pH. Together, the results demonstrate that apart from structural prerequisites such as peptide length, charge, and hydrophobicity, the evolutionarily conserved His residue of TCPs influences their antibacterial effects and reveals a previously unknown aspect of TCPs biological action.

Structural basis for endotoxin neutralisation and anti-inflammatory activity of thrombin-derived C-terminal peptides.

Thrombin-derived C-terminal peptides (TCPs) of about 2 kDa are present in wounds, where they exert anti-endotoxic functions. Employing a combination of nuclear magnetic resonance spectroscopy (NMR), biophysical, mass spectrometry and cellular studies combined with in silico multiscale modelling, we here determine the bound conformation of HVF18 (HVFRLKKWIQKVIDQFGE), a TCP generated by neutrophil elastase, in complex with bacterial lipopolysaccharide (LPS) and define a previously undisclosed interaction between TCPs and human CD14. Further, we show that TCPs bind to the LPS-binding hydrophobic pocket of CD14 and identify the peptide region crucial for TCP interaction with LPS and CD14. Taken together, our results demonstrate the role of structural transitions in LPS complex formation and CD14 interaction, providing a molecular explanation for the previously observed therapeutic effects of TCPs in experimental models of bacterial sepsis and endotoxin shock.

Partial Intrinsic Disorder Governs the Dengue Capsid Protein Conformational Ensemble.

The 11 kDa, positively charged dengue capsid protein (C protein) exists stably as a homodimer and colocalizes with the viral genome within mature viral particles. Its core is composed of four alpha helices encompassing a small hydrophobic patch that may interact with lipids, but approximately 20% of the protein at the N-terminus is intrinsically disordered, making it challenging to elucidate its conformational landscape. Here, we combine small-angle X-ray scattering (SAXS), amide hydrogen-deuterium exchange mass spectrometry (HDXMS), and atomic-resolution molecular dynamics (MD) simulations to probe the dynamics of dengue C proteins. We show that the use of MD force fields (FFs) optimized for intrinsically disordered proteins (IDPs) is necessary to capture their conformational landscape and validate the computationally generated ensembles with reference to SAXS and HDXMS data. Representative ensembles of the C protein dimer are characterized by alternating, clamp-like exposure and occlusion of the internal hydrophobic patch, as well as by residual helical structure at the disordered N-terminus previously identified as a potential source of autoinhibition. Such dynamics are likely to determine the multifunctionality of the C protein during the flavivirus life cycle and hence impact the design of novel antiviral compounds.

Multiscale molecular dynamics simulation approaches to the structure and dynamics of viruses.

Viral pathogens are a significant source of human morbidity and mortality, and have a major impact on societies and economies around the world. One of the challenges inherent in targeting these pathogens with drugs is the tight integration of the viral life cycle with the host's cellular machinery. However, the reliance of the virus on the host cell replication machinery is also an opportunity for therapeutic targeting, as successful entry- and exit-inhibitors have demonstrated. An understanding of the extracellular and intracellular structure and dynamics of the virion - as well as of the entry and exit pathways in host and vector cells - is therefore crucial to the advancement of novel antivirals. In recent years, advances in computing architecture and algorithms have begun to allow us to use simulations to study the structure and dynamics of viral ultrastructures at various stages of their life cycle in atomistic or near-atomistic detail. In this review, we outline specific challenges and solutions that have emerged to allow for structurally detailed modelling of viruses in silico. We focus on the history and state of the art of atomistic and coarse-grained approaches to simulate the dynamics of the large, macromolecular structures associated with viral infection, and on their usefulness in explaining and expanding upon experimental data. We discuss the types of interactions that need to be modeled to describe major components of the virus particle and advances in modelling techniques that allow for the treatment of these systems, highlighting recent key simulation studies.

A Spring-Loaded Mechanism Governs the Clamp-like Dynamics of the Skp Chaperone.

The trimeric periplasmic holdase chaperone Skp binds and stabilizes unfolded outer membrane proteins (OMPs) as part of bacterial OMP biogenesis. Skp binds client proteins in its central cavity, thereby reducing its backbone dynamics, but the molecular mechanisms that govern Skp dynamics and adaptation to differently sized clients remains unknown. Here, we employ a combination of microsecond timescale molecular dynamics simulation, small-angle X-ray scattering, and nuclear magnetic resonance spectroscopy to reveal that Skp is remarkably flexible, and features a molecular spring-loaded mechanism in its "tentacle" arms that enables switching between two distinct conformations on sub-millisecond timescales. The conformational switch is executed around a conserved pivot element within the coiled-coil structures of the tentacles, allowing expansion of the cavity and thus accommodation of differently sized clients. The spring-loaded mechanism shows how a chaperone can efficiently modulate its structure and function in an ATP-independent manner.

Pushing the Envelope: Dengue Viral Membrane Coaxed into Shape by Molecular Simulations.

Dengue virus is a flavivirus responsible for millions of infections per year. Its surface contains a phospholipid bilayer, within which are embedded the envelope (E) and membrane (M) proteins, arranged with icosahedral geometry. Exposure to low pH triggers the E proteins to undergo conformational changes, which precede fusion with the host cell membrane and release of the viral genome. The flavivirus membrane exhibits significant local curvature and deformation, as revealed by cryoelectron microscopy (cryo-EM), but its precise structure and interactions with envelope components remain unclear. We now report simulations of the dengue viral particle that refine its envelope structure in unprecedented detail. Our final models are morphologically consistent with cryo-EM data, and reveal the structural basis for membrane curvature. Electrostatic interactions increased envelope complex stability; this coupling has potential functional significance in the context of the viral fusion mechanism and infective states.