Binding dynamics of hepatitis C virus' NS5A amphipathic peptide to cell and model membranes

Lipid Membrane Interface Viewpoint: From Viral Entry to Antiviral and Vaccine Development

Membrane-enveloped viruses are responsible for most viral pandemics in history, and more effort is needed to advance broadly applicable countermeasures to mitigate the impact of future outbreaks. In this Perspective, we discuss how biosensing techniques associated with lipid model membrane platforms are contributing to improving our mechanistic knowledge of membrane fusion and destabilization that is closely linked to viral entry as well as vaccine and antiviral drug development. A key benefit of these platforms is the simplicity of interpreting the results which can be complemented by other techniques to decipher more complicated biological observations and evaluate the biophysical functionalities that can be correlated to biological activities. Then, we introduce exciting application examples of membrane-targeting antivirals that have been refined over time and will continue to improve based on biophysical insights. Two ways to abrogate the function of viral membranes are introduced here: (1) selective disruption of the viral membrane structure and (2) alteration of the membrane component. While both methods are suitable for broadly useful antivirals, the latter also has the potential to produce an inactivated vaccine. Collectively, we emphasize how biosensing tools based on membrane interfacial science can provide valuable information that could be translated into biomedicines and improve their selectivity and performance.

Modelling the adsorption of phospholipid vesicles to a silicon dioxide surface using Langmuir kinetics

Supported Lipid Bilayers (SLBs) are model biological membranes that have been developed to study the interactions between biomolecules in a cell membrane. Though forming SLBs is relatively easy, their formation mechanism remains a topic of debate. When buffered solutions containing phosphatidylcholine vesicles are flowed over a silicon dioxide (SiO₂) surface they adsorb intact to the surface to form a Supported Vesicle Layer (SVL) if the pH of the buffer is above 9. We have run experiments with buffers with a pH at or above 9 to study the kinetics of the adsorption of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicles to an SiO₂ surface, which is the first step in the formation of an SLB. We used a quartz crystal microbalance (QCM) to monitor the real-time changes in the mass of the SVL as it formed from solutions with different lipid concentrations. Increases in the maximum frequency change with increasing lipid concentration indicated that both adsorption and desorption of DOPC vesicles were occurring, and that an equilibrium was established between the DOPC vesicles in the SVL and in the bulk solution. From the data acquired we were able to determine that the equilibrium constant for the adsorption and desorption of DOPC vesicles was 18 ± 1. The data was fitted to a Langmuir adsorption model from which the rate constants for the adsorption and desorption of DOPC vesicles were determined to be k_a = (0.0107 ± 0.0004) mL mg^-1 s^-1 and k_d = (5.8 ± 0.3) × 10^-4 s^-1. The best fit to the experimental data was achieved if a parameter (α = (0.035 ± 0.003) s^-1) was used to account for the time taken for the lipid concentration to reach its steady state value in the flow cell used in the experiments.

Quantitative description of the response of finite size adsorbates on a quartz crystal microbalance in liquids using analytical hydrodynamics

Despite being a fundamental tool in soft matter research and biosensing, quartz crystal microbalance (QCM) analyses of discrete macromolecules in liquids so far lack a firm theoretical basis. Quite often, acoustic signals of discrete particles are qualitatively interpreted using ad hoc frameworks based on effective electrical circuits, effective springs and trapped-solvent models with many fitting parameters. Nevertheless, due to its extreme sensitivity, the QCM technique pledges to become an accurate predictive tool. Using unsteady low Reynolds hydrodynamics we derive analytical expressions for the acoustic impedance of adsorbed discrete spheres. The present approach is successfully validated against 3D simulations and a plethora of experimental results covering more than a decade of research on proteins, viruses, liposomes, and massive nanoparticles, with sizes ranging from a few to hundreds of nanometers. The agreement without fitting parameters indicates that the acoustic response is dominated by the hydrodynamic propagation of the particle surface stress over the resonator. Understanding this leading contribution is a prerequisite for deciphering the secondary contributions arising from the relevant specific molecular and physico-chemical forces.

Soluble resistance-related calcium-binding protein participates in multiple diseases via protein-protein interactions

Soluble resistance-related calcium-binding protein (sorcin), a 22 kDa penta-EF-hand protein, has been intensively studied in cancers and multidrug resistance over a prolonged period. Sorcin is widely distributed in tissues and participates in the regulation of Ca²⁺ homeostasis and Ca²⁺-dependent signaling. Protein-protein interactions (PPIs) are essential for regulating protein functions in almost all biological processes. Sorcin interaction partners tend to vary in type, including Ca²⁺ receptors, Ca²⁺ transporters, endoplasmic reticulum stress markers, transcriptional regulatory elements, immunomodulation-related factors, and viral proteins. Recent studies have shown that sorcin is involved in a broad range of pathological conditions, such as cardiomyopathy, type 2 diabetes mellitus, neurodegenerative diseases, liver diseases, and viral infections. As a multifunctional cellular protein, in these diseases, sorcin has a role by interacting with or regulating the expression of other proteins, such as sarcoplasmic reticulum/endoplasmic reticulum Ca²⁺ ATPase, ryanodine receptors, presenilin 2, L-type Ca²⁺ channels, carbohydrate-responsive element-binding protein, tau, α-synuclein, signal transducer and activator of transcription 3, HCV nonstructural 5A protein, and viral capsid protein 1. This review summarizes the roles that sorcin plays in various diseases, mainly via different PPIs, and focuses principally on non-neoplastic diseases to help acquire a more comprehensive understanding of sorcin's multifunctional characteristics.

Comparing the Membrane-Interaction Profiles of Two Antiviral Peptides: Insights into Structure-Function Relationship

In recent years, certain amphipathic, α-helical peptides have been discovered that inhibit medically important enveloped viruses by disrupting the lipid membrane surrounding individual virus particles. Interestingly, only a small subset of amphipathic, α-helical peptides demonstrate inhibitory activity, and there is broad interest in understanding how the structures of these peptides contribute to functional activity against lipid membranes. To address this question, herein, we employed multiple surface-sensitive measurement techniques along with computational simulations in order to investigate how AH and C5A peptides, two of the most biologically active peptides in this class, interact with model lipid membranes while gaining insight into membrane-induced peptide conformational changes. Circular dichroism spectroscopy experiments revealed that both AH and C5A peptides undergo pronounced coil-to-helix transitions in the presence of lipid membrane environments, and the C5A conformational change was the largest. Time-lapsed fluorescence microscopy measurements were conducted to monitor the interaction of peptides with arrays of tethered, individual lipid vesicles and showed that C5A potently lyses lipid vesicles indiscriminate of vesicle size at peptide concentrations as low as 10 nM whereas AH peptide preferentially lyses lipid vesicles with high membrane curvature and is less potent than C5A. These findings were complemented by electrochemical impedance spectroscopy measurements on a tethered lipid bilayer membrane platform, which indicated that C5A solubilizes lipid membranes in a manner that is distinct from how AH disrupts lipid membranes via pore formation. Computational simulations supported that the distinct membrane-interaction profiles arise from different helical folding patterns, whereby AH monomers predominantly exist as two shorter helices with a hinge in-between and C5A monomers form a single helix. Taken together, our findings demonstrate that membrane-active antiviral peptides can exhibit distinct membrane-interaction profiles that confer different degrees of targeting selectivity, and the corresponding structural insights will be useful for peptide engineering applications.

Molecular details of spontaneous insertion and interaction of HCV non-structure 3 protease protein domain with PIP2-containing membrane

Hepatitis C virus (HCV), known as the leading cause of liver cirrhosis, viral hepatitis, and hepatocellular carcinoma, has been affecting more than 150 million people globally. The HCV non-structure 3 (NS3) protease protein domain plays a key role in HCV replication and pathogenesis; and is currently a primary target for HCV antiviral therapy. Through unbiased molecular dynamics simulations which take advantage of the novel highly mobile membrane mimetic model, we constructed the membrane-bound state of the protein domain at the atomic level. Our results indicated that protease domain of HCV NS3 protein can spontaneously bind and penetrate to an endoplasmic reticulum complex membrane containing phosphatidylinositol 4,5-bisphosphate (PIP2). An amphipathic helix α₀ and loop S1 show their anchoring role to keep the protein on the membrane surface. Proper orientation of the protein domain at membrane surface was identified through measuring tilt angles of two specific vectors, wherein residue R161 plays a crucial role in its final orientation. Remarkably, PIP2 molecules were observed to bind to three main sites of the protease domain via specific electrostatic contacts and hydrogen bonds. PIP2-interaction determines the protein orientation at the membrane while both hydrophobic interplay and PIP2-interaction can stabilize the NS3 - membrane complex. Simulated results provide us with a detailed characterization of insertion, orientation and PIP2-interaction of NS3 protease domain at membrane environment, thus enhancing our understanding of structural functions and mechanism for the association of HCV non-structure 3 protein with respect to ER membranes.

Quantitative Evaluation of Viral Protein Binding to Phosphoinositide Receptors and Pharmacological Inhibition

There is significant interest in developing analytical methods to characterize molecular recognition events between proteins and phosphoinositides, which are a medically important class of carbohydrate-functionalized lipids. Within this scope, one area of high priority involves quantitatively evaluating drug candidates that pharmacologically inhibit protein-phosphoinositide interactions. As full-length proteins are often difficult to produce, establishing methods to study these interactions with shorter, bioactive peptides would be advantageous. Herein, we report an atomic force microscopy (AFM)-based force spectroscopic approach to detect the specific interaction between an amphipathic, α-helical (AH) peptide derived from the hepatitis C virus NS5A protein and its biological target, the phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P₂] phosphoinositide receptor. After optimization of the peptide tethering strategy and measurement parameters, the binding specificity of AH peptide for PI(4,5)P₂ receptors was comparatively evaluated across a panel of phosphoinositides and the influence of ionic strength on AH-PI(4,5)P₂ binding strength was tested. Importantly, these capabilities were translated into the development of a novel experimental methodology to determine the inhibitory activity of a small-molecule drug candidate acting against the AH-PI(4,5)P₂ interaction, and extracted kinetic parameters agree well with literature values obtained by conventional biochemical methods. Taken together, our findings provide a nanomechanical basis for explaining the high binding specificity of the NS5A AH to PI(4,5)P₂ receptors, in turn establishing an analytical framework to study phosphoinositide-binding viral peptides and proteins as well as a broadly applicable approach to evaluate candidate inhibitors of protein-phosphoinositide interactions.

Cholesterol-Enriched Domain Formation Induced by Viral-Encoded, Membrane-Active Amphipathic Peptide

The α-helical (AH) domain of the hepatitis C virus nonstructural protein NS5A, anchored at the cytoplasmic leaflet of the endoplasmic reticulum, plays a role in viral replication. However, the peptides derived from this domain also exhibit remarkably broad-spectrum virocidal activity, raising questions about their modes of membrane association. Here, using giant lipid vesicles, we show that the AH peptide discriminates between membrane compositions. In cholesterol-containing membranes, peptide binding induces microdomain formation. By contrast, cholesterol-depleted membranes undergo global softening at elevated peptide concentrations. Furthermore, in mixed populations, the presence of ∼100 nm vesicles of viral dimensions suppresses these peptide-induced perturbations in giant unilamellar vesicles, suggesting size-dependent membrane association. These synergistic composition- and size-dependent interactions explain, in part, how the AH domain might on the one hand segregate molecules needed for viral assembly and on the other hand furnish peptides that exhibit broad-spectrum virocidal activity.

Reconstitution and Functional Analysis of a Full-Length Hepatitis C Virus NS5B Polymerase on a Supported Lipid Bilayer

Therapeutic targeting of membrane-associated viral proteins is complicated by the challenge of investigating their enzymatic activities in the native membrane-bound state. To permit functional characterization of these proteins, we hypothesized that the supported lipid bilayer (SLB) can support in situ reconstitution of membrane-associated viral protein complexes. As proof-of-principle, we selected the hepatitis C virus (HCV) NS5B polymerase which is essential for HCV genome replication, and determined that the SLB platform enables functional reconstitution of membrane protein activity. Quartz crystal microbalance with dissipation (QCM-D) monitoring enabled label-free detection of full-length NS5B membrane association, its interaction with replicase subunits NS3, NS5A, and template RNA, and most importantly its RNA synthesis activity. This latter activity could be inhibited by the addition of candidate small molecule drugs. Collectively, our results demonstrate that the SLB platform can support functional studies of membrane-associated viral proteins engaged in critical biological activities.

Phosphatidylinositol 4,5-bisphosphate is an HCV NS5A ligand and mediates replication of the viral genome

BACKGROUND & AIMS

Phosphoinositides (PIs) bind and regulate localization of proteins via a variety of structural motifs. PI 4,5-bisphosphate (PI[4,5]P2) interacts with and modulates the function of several proteins involved in intracellular vesicular membrane trafficking. We investigated interactions between PI(4,5)P2 and hepatitis C virus (HCV) nonstructural protein 5A (NS5A) and effects on the viral life cycle.

METHODS

We used a combination of quartz crystal microbalance, circular dichroism, molecular genetics, and immunofluorescence to study specific binding of PI(4,5)P2 by the HCV NS5A protein. We evaluated the effects of PI(4,5)P2 on the function of NS5A by expressing wild-type or mutant forms of Bart79I or FL-J6/JFH-5'C19Rluc2AUbi21 RNA in Huh7 cells. We also studied the effects of strategies designed to inhibit PI(4,5)P2 on HCV replication in these cells.

RESULTS

The N-terminal amphipathic helix of NS5A bound specifically to PI(4,5)P2, inducing a conformational change that stabilized the interaction between NS5A and TBC1D20, which is required for HCV replication. A pair of positively charged residues within the amphipathic helix (the basic amino acid PI(4,5)P2 pincer domain) was required for PI(4,5)P2 binding and replication of the HCV-RNA genome. A similar motif was found to be conserved across all HCV isolates, as well as amphipathic helices of many pathogens and apolipoproteins.

CONCLUSIONS

PI(4,5)P2 binds to HCV NS5A to promote replication of the viral RNA genome in hepatocytes. Strategies to disrupt this interaction might be developed to inhibit replication of HCV and other viruses.

Controlling lipid membrane architecture for tunable nanoplasmonic biosensing

Tunable nanoplasmonic biosensing for lipid and protein applications is reported based on controlling lipid membrane architecture on surfaces. The interaction of a peptide with lipid membranes is highly sensitive to the membrane architecture on top of plasmonic nanodisks, and the measurement response varies in a manner which is consistent with the surrounding lipid environment.

A quantitative high-resolution genetic profile rapidly identifies sequence determinants of hepatitis C viral fitness and drug sensitivity

Widely used chemical genetic screens have greatly facilitated the identification of many antiviral agents. However, the regions of interaction and inhibitory mechanisms of many therapeutic candidates have yet to be elucidated. Previous chemical screens identified Daclatasvir (BMS-790052) as a potent nonstructural protein 5A (NS5A) inhibitor for Hepatitis C virus (HCV) infection with an unclear inhibitory mechanism. Here we have developed a quantitative high-resolution genetic (qHRG) approach to systematically map the drug-protein interactions between Daclatasvir and NS5A and profile genetic barriers to Daclatasvir resistance. We implemented saturation mutagenesis in combination with next-generation sequencing technology to systematically quantify the effect of every possible amino acid substitution in the drug-targeted region (domain IA of NS5A) on replication fitness and sensitivity to Daclatasvir. This enabled determination of the residues governing drug-protein interactions. The relative fitness and drug sensitivity profiles also provide a comprehensive reference of the genetic barriers for all possible single amino acid changes during viral evolution, which we utilized to predict clinical outcomes using mathematical models. We envision that this high-resolution profiling methodology will be useful for next-generation drug development to select drugs with higher fitness costs to resistance, and also for informing the rational use of drugs based on viral variant spectra from patients.

The emergence of drug resistance during antiviral treatment limits treatment options and poses challenges to pharmaceutical development. Meanwhile, the search for novel antiviral compounds with chemical genetic screens has led to the identification of antiviral agents with undefined drug mechanisms. Daclatasvir, an effective NS5A inhibitor, is one such example. In traditional methods to identify critical residues governing drug-protein interactions, wild type virus is passaged under drug treatment pressure, enabling the identification of resistant mutations evolved after multiple viral passages. However, this method only characterizes a fraction of the positively selected variants. Here we have simultaneously quantified the relative change in replication fitness as well as the relative sensitivity to Daclatasvir for all possible single amino acid mutations in the NS5A domain IA, thereby identifying the entire panel of positions that interact with the drug. Using mathematical models, we predicted which mutations pose the greatest risk of causing emergence of resistance under different scenarios of treatment compliance. The mutant fitness and drug-sensitivity profiles obtained can also inform the patient-specific use of Daclatasvir and may facilitate the development of second-generation drugs with a higher genetic barrier to resistance.

Model cell membranes: Techniques to form complex biomimetic supported lipid bilayers via vesicle fusion

Vesicle fusion has long provided an easy and reliable method to form supported lipid bilayers (SLBs) from simple, zwitterionic vesicles on siliceous substrates. However, for complex compositions, such as vesicles with high cholesterol content and multiple lipid types, the energy barrier for the vesicle-to-bilayer transition is increased or the required vesicle-vesicle and vesicle-substrate interactions are insufficient for vesicle fusion. Thus, for vesicle compositions that more accurately mimic native membranes, vesicle fusion often fails to form SLBs. In this paper, we review three approaches to overcome these barriers to form complex, biomimetic SLBs via vesicle fusion: (i) optimization of experimental conditions (e.g., temperature, buffer ionic strength, osmotic stress, cation valency, and buffer pH), (ii) α-helical (AH) peptide-induced vesicle fusion, and (iii) bilayer edge-induced vesicle fusion. AH peptide-induced vesicle fusion can form complex SLBs on multiple substrate types without the use of additional equipment. Bilayer edge-induced vesicle fusion uses microfluidics to form SLBs from vesicles with complex composition, including vesicles derived from native cell membranes. Collectively, this review introduces vesicle fusion techniques that can be generalized for many biomimetic vesicle compositions and many substrate types, and thus will aid efforts to reliably create complex SLB platforms on a range of substrates.

Model membrane platforms for biomedicine: case study on antiviral drug development

As one of the most important interfaces in cellular systems, biological membranes have essential functions in many activities such as cellular protection and signaling. Beyond their direct functions, they also serve as scaffolds to support the association of proteins involved in structural support, adhesion, and transport. Unfortunately, biological processes sometimes malfunction and require therapeutic intervention. For those processes which occur within or upon membranes, it is oftentimes difficult to study the mechanism in a biologically relevant, membranous environment. Therefore, the identification of direct therapeutic targets is challenging. In order to overcome this barrier, engineering strategies offer a new approach to interrogate biological activities at membrane interfaces by analyzing them through the principles of the interfacial sciences. Since membranes are complex biological interfaces, the development of simplified model systems which mimic important properties of membranes can enable fundamental characterization of interaction parameters for such processes. We have selected the hepatitis C virus (HCV) as a model viral pathogen to demonstrate how model membrane platforms can aid antiviral drug discovery and development. Responsible for generating the genomic diversity that makes treating HCV infection so difficult, viral replication represents an ideal step in the virus life cycle for therapeutic intervention. To target HCV genome replication, the interaction of viral proteins with model membrane platforms has served as a useful strategy for target identification and characterization. In this review article, we demonstrate how engineering approaches have led to the discovery of a new functional activity encoded within the HCV nonstructural 5A protein. Specifically, its N-terminal amphipathic, α-helix (AH) can rupture lipid vesicles in a size-dependent manner. While this activity has a number of exciting biotechnology and biomedical applications, arguably the most promising one is in antiviral medicine. Based on the similarities between lipid vesicles and the lipid envelopes of virus particles, experimental findings from model membrane platforms led to the prediction that a range of medically important viruses might be susceptible to rupturing treatment with synthetic AH peptide. This hypothesis was tested and validated by molecular virology studies. Broad-spectrum antiviral activity of the AH peptide has been identified against HCV, HIV, herpes simplex virus, and dengue virus, and many more deadly pathogens. As a result, the AH peptide is the first in class of broad-spectrum, lipid envelope-rupturing antiviral agents, and has entered the drug pipeline. In summary, engineering strategies break down complex biological systems into simplified biomimetic models that recapitulate the most important parameters. This approach is particularly advantageous for membrane-associated biological processes because model membrane platforms provide more direct characterization of target interactions than is possible with other methods. Consequently, model membrane platforms hold great promise for solving important biomedical problems and speeding up the translation of biological knowledge into clinical applications.

Single vesicle analysis reveals nanoscale membrane curvature selective pore formation in lipid membranes by an antiviral α-helical peptide

Using tethered sub-100 nm lipid vesicles that mimic enveloped viruses with nanoscale membrane curvature, we have in this work designed a total internal reflection fluorescence microscopy-based single vesicle assay to investigate how an antiviral amphipathic α-helical (AH) peptide interacts with lipid membranes to induce membrane curvature-dependent pore formation and membrane destabilization. Based on a combination of statistics from single vesicle imaging, binding kinetics data, and theoretical analysis, we propose a mechanistic model that is consistent with the experimentally observed peptide association and pore formation kinetics at medically relevant peptide concentrations (10 nM to 1 μM) and unusually low peptide-to-lipid (P/L) ratio (~1/1000). Importantly, the preference of the AH peptide to selectively rupture virions with sub-100 nm diameters appears to be related to membrane strain-dependent pore formation rather than to previously observed nanoscale membrane curvature facilitated binding of AH peptides. Compared to other known proteins and peptides, the combination of low effective P/L ratio and high specificity for nm-sized membrane curvature lends this particular AH peptide great potential to serve as a framework for developing a highly specific and potent antiviral agent for prophylactic and therapeutic applications while avoiding toxic side effects against host cell membranes.

Biomimetic supported lipid bilayers with high cholesterol content formed by α-helical peptide-induced vesicle fusion

In this study, we present a technique to create a complex, high cholesterol-containing supported lipid bilayers (SLBs) using α-helical (AH) peptide-induced vesicle fusion. Vesicles consisting of POPC : POPE : POPS : SM : Chol (9.35 : 19.25 : 8.25 : 18.15 : 45.00) were used to form a SLB that models the native composition of the human immunodeficiency virus-1 (HIV-1) lipid envelope. In the absence of AH peptides, these biomimetic vesicles fail to form a complete SLB. We verified and characterized AH peptide-induced vesicle fusion by quartz crystal microbalance with dissipation monitoring, neutron reflectivity, and atomic force microscopy. Successful SLB formation entailed a characteristic frequency shift of -35.4 ± 2.0 Hz and a change in dissipation energy of 1.91 ± 0.52 × 10-6. Neutron reflectivity measurements determined the SLB thickness to be 49.9 +1.9-1.5 Å, and showed the SLB to be 100 +0.0-0.1% complete and void of residual AH peptide after washing. Atomic force microscopy imaging confirmed complete SLB formation and revealed three distinct domains with no visible defects. This vesicle fusion technique gives researchers access to a complex SLB composition with high cholesterol content and thus the ability to better recapitulate the native HIV-1 lipid membrane.

Role for TBC1D20 and Rab1 in hepatitis C virus replication via interaction with lipid droplet-bound nonstructural protein 5A

Replication and assembly of hepatitis C virus (HCV) depend on the host's secretory and lipid-biosynthetic machinery. Viral replication occurs on endoplasmic reticulum (ER)-derived modified membranes, while viral assembly is thought to occur on lipid droplets (LDs). A physical association and coordination between the viral replication and assembly complexes are prerequisites for efficient viral production. Nonstructural protein 5A (NS5A), which localizes both to the ER and LDs, is an ideal candidate for this function. Here, the interaction of NS5A with host cell membranes and binding partners was characterized in living cells. The binding of NS5A to LDs is apparently irreversible, both in HCV-infected cells and when ectopically expressed. In HCV-infected cells, NS5A fluorescence was observed around the LDs and in perinuclear structures that were incorporated into a highly immobile platform superimposed over the ER membrane. Moreover, TBC1D20 and its cognate GTPase Rab1 are recruited by NS5A to LDs. The NS5A-TBC1D20 interaction was shown to be essential for the viral life cycle. In cells, expression of the Rab1 dominant negative (Rab1DN) GTPase mutant abolished steady-state LDs. In infected cells, Rab1DN induced the elimination of NS5A from viral replication sites. Our results demonstrate the significance of the localization of NS5A to LDs and support a model whereby its interaction with TBC1D20 and Rab1 affects lipid droplet metabolism to promote the viral life cycle.

Fabrication of a planar zwitterionic lipid bilayer on titanium oxide

There is great demand to fabricate planar phospholipid bilayers on biocompatible materials. The preferred method of forming bilayers on these substrates is the spontaneous adsorption and rupture of phospholipid vesicles. However, in the case of titanium oxide, model vesicles composed solely of zwitterionic phospholipids do not follow this self-assembly pathway under physiological conditions, prompting the use of complex bilayer materials and less-facile methods. Herein, we report a novel pH-based strategy for fabricating zwitterionic bilayers on titanium oxide in a simple and robust manner. Depending on the pH conditions under which lipid vesicles adsorb onto titanium oxide, quartz crystal microbalance-dissipation (QCM-D) monitoring demonstrated that the self-assembly pathway can in fact result in planar bilayer formation. The pH of the solution could then be adjusted to physiological levels with no effect on the mass and viscoelastic properties of the bilayer. Moreover, fluorescence recovery after photobleaching (FRAP) measurements indicated a high degree of lateral lipid diffusivity within the bilayer at physiological pH, commensurate with its role as a cell membrane mimic. Compared to existing protocols, this strategy permits the fabrication of a more diverse array of planar bilayers on titanium oxide by tuning the self-assembly pathway of lipid vesicle adsorption onto solid substrates.

Type I collagen-functionalized supported lipid bilayer as a cell culture platform

The supported phospholipid bilayer serves as an important biomimetic model for the cell membrane in both basic and applied scientific research. We have constructed a biomimetic platform based on a supported phospholipid bilayer that is functionalized with type I collagen to serve as a substrate for cell culture. To create the type I collagen-functionalized lipid bilayer assembly, a simple chemical approach was employed: lipid vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(glutaryl) (DP-NGPE), a carboxylic acid-functionalized phospholipid, were prepared and then fused onto an SiO(2) substrate to form a supported lipid bilayer. Subsequently, type I collagen molecules were introduced to form stable collagen-lipid conjugates via amide linkages with activated DP-NGPE lipids. The binding kinetics of the conjugation process and the resultant changes in film thickness and viscoelasticity were followed using the quartz crystal microbalance with dissipation (QCM-D) monitoring. The morphology of the conjugated collagen adlayer was investigated with atomic force microscopy (AFM). We observed that the adsorbed collagen molecules tended to self-assemble into fibrillar structures. Fluorescence recovery after photobleaching (FRAP) was utilized to estimate lateral lipid mobility, which was reduced by up to 20% after the coupling of type I collagen to the underlying lipid bilayer. As a cell culture platform, the collagen-conjugated supported lipid bilayer showed promising results. Smooth muscle cells (A10) retained normal growth behavior on the collagen-functionalized platform, unlike the bare POPC lipid bilayer and the POPC/DG-NGPE bilayer without collagen. The biomimetic functionalized lipid system presented here is a simple, yet effective approach for constructing a cell culture platform to explore the interactions between extracellular matrix components and cells.

Quartz crystal microbalance with dissipation monitoring of supported lipid bilayers on various substrates

Supported lipid bilayers (SLBs) mimic biological membranes and are a versatile platform for a wide range of biophysical research fields including lipid-protein interactions, protein-protein interactions and membrane-based biosensors. The quartz crystal microbalance with dissipation monitoring (QCM-D) has had a pivotal role in understanding SLB formation on various substrates. As shown by its real-time kinetic monitoring of SLB formation, QCM-D can probe the dynamics of biomacromolecular interactions. We present a protocol for constructing zwitterionic SLBs supported on silicon oxide and titanium oxide, and discuss technical issues that need to be considered when working with charged lipid compositions. Furthermore, we explain a recently developed strategy that uses an amphipathic, alpha-helical (AH) peptide to form SLBs on gold and titanium oxide substrates. The protocols can be completed in less than 3 h.

Biological activities and molecular interactions of the C-terminal residue of thrombospondin-4, an epitome of acidic amphipathic peptides

C21, the C-terminal residue of thrombospondin-4 (TSP-4), was identified as a peptide growth factor during an investigation concerning erythropoietin-dependent, erythroid stimulating factors of endothelial origin. It is active in cultures of several human hematopoietic stem cells, skin fibroblasts and kidney epithelial cells and stimulates red cell formation in anemic mice. A method of affinity chromatography in the presence of high concentrations of Triton X-100, previously developed for identifying proteins associated with the TSP-1 receptor CD47, was utilized for the detection of C21 binding molecules and their detergent-resistant, associated partners. These experiments helped to delineate two different mechanisms of C21 action, which are compatible with its cell proliferating activity. As a cell matrix peptide, C21 binds to the osteopontin receptor CD44 and could act as an osteopontin antagonist, preventing the inhibition of primitive hematopoietic stem cell proliferation. TSP-1, another matrix protein, binds to C21 and could indirectly act as an antagonist, by shunting C21-CD44 interactions. The second mechanism is a direct effect of C21 on cell proliferation. The extremely rapid internalization and nuclear localization of the peptide could be explained by CD44-mediated internalization, followed by a microtubule-mediated transport towards the nucleus, or, eventually, direct membrane insertion. These alternative hypotheses are supported by previously observed membrane insertion of similar synthetic and viral acidic amphipathic peptides, the presence of microtubule-associated protein 1B (MAP1B) and dynactin in the triton-soluble complexes associated with C21 and the presence in such complexes of dual compartment proteins for nuclei and plasma membranes, such as MAP1B, AHNAK and CD44.

Interfacial binding dynamics of bee venom phospholipase A2 investigated by dynamic light scattering and quartz crystal microbalance

Bee venom phospholipase A(2) (bvPLA(2)) is part of the secretory phospholipase A(2) (sPLA(2)) family whose members are active in biological processes such as signal transduction and lipid metabolism. While controlling sPLA(2) activity is of pharmaceutical interest, the relationship between their mechanistic actions and physiological functions is not well understood. Therefore, we investigated the interfacial binding process of bvPLA(2) to characterize its biophysical properties and gain insight into how membrane binding affects interfacial activation. Attention was focused on the role of membrane electrostatics in the binding process. Although dynamic light scattering experiments indicated that bvPLA(2) does not lyse lipid vesicles, a novel, nonhydrolytic activity was discovered. We employed a supported lipid bilayer platform on the quartz crystal microbalance with dissipation sensor to characterize this bilayer-disrupting behavior and determined that membrane electrostatics influence this activity. The data suggest that (1) adsorption of bvPLA(2) to model membranes is not primarily driven by electrostatic interactions; (2) lipid desorption can follow bvPLA(2) adsorption, resulting in nonhydrolytic bilayer-disruption; and (3) this desorption is driven by electrostatic interactions. Taken together, these findings provide evidence that interfacial binding of bvPLA(2) is a dynamic process, shedding light on how membrane electrostatics can modulate interfacial activation.

Mechanism of an amphipathic alpha-helical peptide's antiviral activity involves size-dependent virus particle lysis

The N-terminal region of the hepatitis C virus (HCV) nonstructural protein NS5A contains an amphipathic alpha-helix that is necessary and sufficient for NS5A membrane association. A synthetic peptide (AH) comprising this amphipathic helix is able to lyse lipid vesicles that serve as a model system for virus particles. Based on quartz crystal microbalance-dissipation (QCM-D) experiments, the degree of vesicle rupturing was found to be inversely related to vesicle size, with maximal activity in the size range of several medically important viruses. In order to confirm and further study vesicle rupture, dynamic light scattering (DLS) and atomic force microscopy (AFM) experiments were also performed. The size dependence of vesicle rupturing helps explain the peptide's observed effect on the infectivity of a wide range of viruses. Further, in vitro studies demonstrated that AH peptide treatment significantly decreased the infectivity of HCV particles. Thus, the AH peptide might be used to rupture HCV particles extra-corporally (for HCV prevention) and within infected individuals (for HCV therapy).

Crystal structure of a novel dimeric form of NS5A domain I protein from hepatitis C virus

A new protein expression vector design utilizing an N-terminal six-histidine tag and tobacco etch virus protease cleavage site upstream of the hepatitis C virus NS5A sequence has resulted in a more straightforward purification method and improved yields of purified NS5A domain I protein. High-resolution diffracting crystals of NS5A domain I (amino acids 33 to 202) [NS5A(33-202)] were obtained by using detergent additive crystallization screens, leading to the structure of a homodimer which is organized differently from that published previously (T. L. Tellinghuisen, J. Marcotrigiano, and C. M. Rice, Nature 435:374-379, 2005) yet is consistent with a membrane association model for NS5A. The monomer-monomer interface of NS5A(33-202) features an extensive buried surface area involving the most-highly conserved face of each monomer. The two alternate structural forms of domain I now available may be indicative of the multiple roles emerging for NS5A in viral RNA replication and viral particle assembly.

TBC1D20 is a Rab1 GTPase-activating protein that mediates hepatitis C virus replication

Like other viruses, productive hepatitis C virus (HCV) infection depends on certain critical host factors. We have recently shown that an interaction between HCV nonstructural protein NS5A and a host protein, TBC1D20, is necessary for efficient HCV replication. TBC1D20 contains a TBC (Tre-2, Bub2, and Cdc16) domain present in most known Rab GTPase-activating proteins (GAPs). The latter are master regulators of vesicular membrane transport, as they control the activity of membrane-associated Rab proteins. To better understand the role of the NS5A-TBC1D20 interaction in the HCV life cycle, we used a biochemical screen to identify the TBC1D20 Rab substrate. TBC1D20 was found to be the first known GAP for Rab1, which is implicated in the regulation of anterograde traffic between the endoplasmic reticulum and the Golgi complex. Mutation of amino acids implicated in Rab GTPase activation by other TBC domain-containing GAPs abrogated the ability of TBC1D20 to activate Rab1 GTPase. Overexpression of TBC1D20 blocked the transport of exogenous vesicular stomatitis virus G protein from the endoplasmic reticulum, validating the involvement of TBC1D20 in this pathway. Rab1 depletion significantly decreased HCV RNA levels, suggesting a role for Rab1 in HCV replication. These results highlight a novel mechanism by which viruses can hijack host cell machinery and suggest an attractive model whereby the NS5A-TBC1D20 interaction may promote viral membrane-associated RNA replication.