Hydration, structure, and molecular interactions in the headgroup region of dioleoylphosphatidylcholine bilayers: an electron spin resonance study

Visualizing intermediate stages of viral membrane fusion by cryo-electron tomography

Protein-mediated membrane fusion is the dynamic process where specialized protein machinery undergoes dramatic conformational changes that drive two membrane bilayers together, leading to lipid mixing and opening of a fusion pore between previously separate membrane-bound compartments. Membrane fusion is an essential stage of enveloped virus entry that results in viral genome delivery into host cells. Recent studies applying cryo-electron microscopy techniques in a time-resolved fashion provide unprecedented glimpses into the interaction of viral fusion proteins and membranes, revealing fusion intermediate states from the initiation of fusion to release of the viral genome. In combination with complementary structural, biophysical, and computation modeling approaches, these advances are shedding new light on the mechanics and dynamics of protein-mediated membrane fusion.

Optical and molecular features of negatively curved surfaces created by POPE lipids: A crucial role of the initial conditions

Membrane fusion is closely related to plasma membrane domains rich in cone-shaped phosphatidylethanolamine (PE) lipids that can reverse membrane curvature under certain conditions. The phase transition of PE-based lipid membranes from the lamellar fluid phase (Lα) to the inverse hexagonal phase (HII) is commonly taken as a general model in reconstructing the membrane fusion pathway, and whose structural features have been mostly described so far using structural and microscopic techniques. The aim of this paper is to decipher the optical and molecular features of Lβ → Lα and especially of Lα → HII transition of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) lipids at pH = 7.0 when they are initially prepared in the form of both multi- and unilamellar liposomes (MLVs and LUVs). The distinction between optical properties of MLS- and LUVs-derived HII phase, provided from turbidity-sensitive temperature-dependent UV-Vis spectra, was attributed to different formation mechanisms of HII phase. Most importantly, from FTIR spectroscopic data of POPE lipids in Lβ (15 °C), Lα (50 °C) and HII (85 °C) phases we identified the changes in molecular features of POPE lipids during phase transitions. Among the latter, by far the most significant is different hydration pattern of POPE lipids in MLVs- and LUVs-derived HII phase which extends from the polar-apolar interface all the way to the terminal amino group of the POPE lipid, along with the changes in the conformation of glycerol backbone as evidenced by the signature of α-methylene groups. Molecular dynamics simulations confirmed higher water penetration in HII phase and provided insight into hydrogen bonding patterns.

A Frame-by-Frame Glance at Membrane Fusion Mechanisms: From Viral Infections to Fertilization

Viral entry and fertilization are distinct biological processes that share a common mechanism: membrane fusion. In viral entry, enveloped viruses attach to the host cell membrane, triggering a series of conformational changes in the viral fusion proteins. This results in the exposure of a hydrophobic fusion peptide, which inserts into the host membrane and brings the viral and host membranes into close proximity. Subsequent structural rearrangements in opposing membranes lead to their fusion. Similarly, membrane fusion occurs when gametes merge during the fertilization process, though the exact mechanism remains unclear. Structural biology has played a pivotal role in elucidating the molecular mechanisms underlying membrane fusion. High-resolution structures of the viral and fertilization fusion-related proteins have provided valuable insights into the conformational changes that occur during this process. Understanding these mechanisms at a molecular level is essential for the development of antiviral therapeutics and tools to influence fertility. In this review, we will highlight the biological importance of membrane fusion and how protein structures have helped visualize both common elements and subtle divergences in the mechanisms behind fusion; in addition, we will examine the new tools that recent advances in structural biology provide researchers interested in a frame-by-frame understanding of membrane fusion.

Lamellarity-Driven Differences in Surface Structural Features of DPPS Lipids: Spectroscopic, Calorimetric and Computational Study

Although single-lipid bilayers are usually considered models of eukaryotic plasma membranes, their research drops drastically when it comes to exclusively anionic lipid membranes. Being a major anionic phospholipid in the inner leaflet of eukaryote membranes, phosphatidylserine-constituted lipid membranes were occasionally explored in the form of multilamellar liposomes (MLV), but their inherent instability caused a serious lack of efforts undertaken on large unilamellar liposomes (LUVs) as more realistic model membrane systems. In order to compensate the existing shortcomings, we performed a comprehensive calorimetric, spectroscopic and MD simulation study of time-varying structural features of LUV made from 1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (DPPS), whereas the corresponding MLV were examined as a reference. A substantial uncertainty of UV/Vis data of LUV from which only T_m was unambiguously determined (53.9 ± 0.8 °C), along with rather high uncertainty on the high-temperature range of DPPS melting profile obtained from DSC (≈50-59 °C), presumably reflect distinguished surface structural features in LUV. The FTIR signatures of glycerol moiety and those originated from carboxyl group serve as a strong support that in LUV, unlike in MLV, highly curved surfaces occur continuously, whereas the details on the attenuation of surface features in MLV were unraveled by molecular dynamics.

Influence of DPPE surface undulations on melting temperature determination: UV/Vis spectroscopic and MD study

One of the most distinguished quantities that describes lipid main phase transition, i.e. the transition from the gel (L_β(')) to the fluid (L_α) phase, is its melting temperature (T_m). Because melting is accompanied by a large change in enthalpy the, L_β(') → L_α transition can be monitored by various calorimetric, structural and spectroscopic techniques and T_m should be the same regardless of the metric monitored or the technique employed. However, in the case of DPPE multilamellar aggregates there is a small but systematic deviation of T_m values determined by DSC and FTIR spectroscopy. The aim of this paper is to explain this discrepancy by combined UV/Vis spectroscopic and MD computational approach. Multivariate analysis performed on temperature-dependent UV/Vis spectra of DPPE suspensions demonstrated that at 55 ± 1 °C certain phenomenon causes a small but detectable change in suspension turbidity, whereas a dominant change in the latter is registered at 63.2 ± 0.4 °C that coincides with T_m value determined from DSC curve. If this effect should be ignored, the overall data give T_m value the same as FTIR spectra data (61.0 ± 0.4 °C). As the classical MD simulations suggest that about 10° below T_m certain undulations appear at the surface of DPPE bilayers, we concluded that certain discontinuities in curvature fluctuations arise at reported temperature which are to some extent coupled with lipid melting. Ultimately, such events and the associated changes in curvature affect T_m value measured by different techniques.

Negatively charged residues in the membrane ordering activity of SARS-CoV-1 and -2 fusion peptides

Entry of coronaviruses into host cells is mediated by the viral spike protein. Previously, we identified the bona fide fusion peptides (FPs) for severe acute respiratory syndrome coronavirus ("SARS-1") and severe acute respiratory syndrome coronavirus-2 ("SARS-2") using electron spin resonance spectroscopy. We also found that their FPs induce membrane ordering in a Ca2+-dependent fashion. Here we study which negatively charged residues in SARS-1 FP are involved in this binding, to build a topological model and clarify the role of Ca2+. Our systematic mutation study on the SARS-1 FP shows that all six negatively charged residues contribute to the FP's membrane ordering activity, with D812 the dominant residue. The corresponding SARS-2 residue D830 plays an equivalent role. We provide a topological model of how the FP binds Ca2+ ions: its two segments FP1 and FP2 each bind one Ca2+. The binding of Ca2+, the folding of FP (both studied by isothermal titration calorimetry experiments), and the ordering activity correlate very well across the mutants, suggesting that the Ca2+ helps the folding of FP in membranes to enhance the ordering activity. Using a novel pseudotyped viral particle-liposome methodology, we monitored the membrane ordering induced by the FPs in the whole spike protein in its trimer form in real time. We found that the SARS-1 and SARS-2 pseudotyped viral particles also induce membrane ordering to the extent that separate FPs do, and mutations of the negatively charged residues also significantly suppress the membrane ordering activity. However, the slower kinetics of the FP ordering activity versus that of the pseudotyped viral particle suggest the need for initial trimerization of the FPs.

Critical Negatively Charged Residues Are Important for the Activity of SARS-CoV-1 and SARS-CoV-2 Fusion Peptides

Coronaviruses are a major infectious disease threat, and include the human pathogens of zoonotic origin SARS-CoV ("SARS-1"), SARS-CoV-2 ("SARS-2") and MERS-CoV ("MERS"). Entry of coronaviruses into host cells is mediated by the viral spike (S) protein. Previously, we identified that the domain immediately downstream of the S2' cleavage site is the bona fide FP (amino acids 798-835) for SARS-1 using ESR spectroscopy technology. We also found that the SARS-1 FP induces membrane ordering in a Ca 2+ dependent fashion. In this study, we want to know which residues are involved in this Ca 2+ binding, to build a topological model and to understand the role of the Ca2+. We performed a systematic mutation study on the negatively charged residues on the SARS-1 FP. While all six negatively charged residues contributes to the membrane ordering activity of the FP to some extent, D812 is the most important residue. We provided a topological model of how the FP binds Ca 2+ ions: both FP1 and FP2 bind one Ca 2+ ion, and there are two binding sites in FP1 and three in FP2. We also found that the corresponding residue D830 in the SARS-2 FP plays a similar critical role. ITC experiments show that the binding energies between the FP and Ca 2+ as well as between the FP and membranes also decreases for all mutants. The binding of Ca 2+ , the folding of FP and the ordering activity correlated very well across the mutants, suggesting that the function of the Ca 2+ is to help to folding of FP in membranes to enhance its activity. Using a novel pseudotyped virus particle (PP)-liposome methodology, we monitored the membrane ordering induced by the FPs in the whole S proteins in its trimer form in real time. We found that the SARS-1 and SARS-2 PPs also induce membrane ordering as the separate FPs do, and the mutations of the negatively charged residues also greatly reduce the membrane ordering activity. However, the difference in kinetic between the PP and FP indicates a possible role of FP trimerization. This finding could lead to therapeutic solutions that either target the FP-calcium interaction or block the Ca 2+ channel to combat the ongoing COVID-19 pandemic.

What Does Time-Dependent Fluorescence Shift (TDFS) in Biomembranes (and Proteins) Report on?

The organization of biomolecules and bioassemblies is highly governed by the nature and extent of their interactions with water. These interactions are of high intricacy and a broad range of methods based on various principles have been introduced to characterize them. As these methods view the hydration phenomena differently (e.g., in terms of time and length scales), a detailed insight in each particular technique is to promote the overall understanding of the stunning “hydration world.” In this prospective mini-review we therefore critically examine time-dependent fluorescence shift (TDFS)—an experimental method with a high potential for studying the hydration in the biological systems. We demonstrate that TDFS is very useful especially for phospholipid bilayers for mapping the interfacial region formed by the hydrated lipid headgroups. TDFS, when properly applied, reports on the degree of hydration and mobility of the hydrated phospholipid segments in the close vicinity of the fluorophore embedded in the bilayer. Here, the interpretation of the recorded TDFS parameters are thoroughly discussed, also in the context of the findings obtained by other experimental techniques addressing the hydration phenomena (e.g., molecular dynamics simulations, NMR spectroscopy, scattering techniques, etc.). The differences in the interpretations of TDFS outputs between phospholipid biomembranes and proteins are also addressed. Additionally, prerequisites for the successful TDFS application are presented (i.e., the proper choice of fluorescence dye for TDFS studies, and TDFS instrumentation). Finally, the effects of ions and oxidized phospholipids on the bilayer organization and headgroup packing viewed from TDFS perspective are presented as application examples.

Membranotropic and biological activities of the membrane fusion peptides from SARS-CoV spike glycoprotein: The importance of the complete internal fusion peptide domain

Fusion peptides (FP) are prominent hydrophobic segments of viral fusion proteins that play critical roles in viral entry. FPs interact with and insert into the host lipid membranes, triggering conformational changes in the viral protein that leads to the viral-cell fusion. Multiple membrane-active domains from the severe acute respiratory syndrome (SARS) coronavirus (CoV) spike protein have been reported to act as the functional fusion peptide such as the peptide sequence located between the S1/S2 and S2' cleavage sites (FP1), the S2'-adjacent fusion peptide domain (FP2), and the internal FP sequence (cIFP). Using a combined biophysical approach, we demonstrated that the α-helical coiled-coil-forming internal cIFP displayed the highest membrane fusion and permeabilizing activities along with membrane ordering effect in phosphatidylcholine (PC)/phosphatidylglycerol (PG) unilamellar vesicles compared to the other two N-proximal fusion peptide counterparts. While the FP1 sequence displayed intermediate membranotropic activities, the well-conserved FP2 peptide was substantially less effective in promoting fusion, leakage, and membrane ordering in PC/PG model membranes. Furthermore, Ca2+ did not enhance the FP2-induced lipid mixing activity in PC/phosphatidylserine/cholesterol lipid membranes, despite its strong erythrocyte membrane perturbation. Nonetheless, we found that the three putative SARS-CoV membrane-active fusion peptide sequences here studied altered the physical properties of model and erythrocyte membranes to different extents. The importance of the distinct membranotropic and biological activities of all SARS-CoV fusion peptide domains and the pronounced effect of the internal fusion peptide sequence to the whole spike-mediated membrane fusion process are discussed.

Highly Basic Clusters in the Herpes Simplex Virus 1 Nuclear Egress Complex Drive Membrane Budding by Inducing Lipid Ordering

During replication of herpesviruses, capsids escape from the nucleus into the cytoplasm by budding at the inner nuclear membrane. This unusual process is mediated by the viral nuclear egress complex (NEC) that deforms the membrane around the capsid by oligomerizing into a hexagonal, membrane-bound scaffold. Here, we found that highly basic membrane-proximal regions (MPRs) of the NEC alter lipid order by inserting into the lipid headgroups and promote negative Gaussian curvature. We also find that the electrostatic interactions between the MPRs and the membranes are essential for membrane deformation. One of the MPRs is phosphorylated by a viral kinase during infection, and the corresponding phosphomimicking mutations block capsid nuclear egress. We show that the same phosphomimicking mutations disrupt the NEC-membrane interactions and inhibit NEC-mediated budding in vitro, providing a biophysical explanation for the in vivo phenomenon. Our data suggest that the NEC generates negative membrane curvature by both lipid ordering and protein scaffolding and that phosphorylation acts as an off switch that inhibits the membrane-budding activity of the NEC to prevent capsid-less budding. IMPORTANCE Herpesviruses are large viruses that infect nearly all vertebrates and some invertebrates and cause lifelong infections in most of the world's population. During replication, herpesviruses export their capsids from the nucleus into the cytoplasm by an unusual mechanism in which the viral nuclear egress complex (NEC) deforms the nuclear membrane around the capsid. However, how membrane deformation is achieved is unclear. Here, we show that the NEC from herpes simplex virus 1, a prototypical herpesvirus, uses clusters of positive charges to bind membranes and order membrane lipids. Reducing the positive charge or introducing negative charges weakens the membrane deforming ability of the NEC. We propose that the virus employs electrostatics to deform nuclear membrane around the capsid and can control this process by changing the NEC charge through phosphorylation. Blocking NEC-membrane interactions could be exploited as a therapeutic strategy.

SARS-CoV-2 Fusion Peptide has a Greater Membrane Perturbating Effect than SARS-CoV with Highly Specific Dependence on Ca2

Coronaviruses are a major infectious disease threat, and include the zoonotic-origin human pathogens SARS-CoV-2, SARS-CoV, and MERS-CoV (SARS-2, SARS-1, and MERS). Entry of coronaviruses into host cells is mediated by the spike (S) protein. In our previous ESR studies, the local membrane ordering effect of the fusion peptide (FP) of various viral glycoproteins including the S of SARS-1 and MERS has been consistently observed. We previously determined that the sequence immediately downstream from the S2' cleavage site is the bona fide SARS-1 FP. In this study, we used sequence alignment to identify the SARS-2 FP, and studied its membrane ordering effect. Although there are only three residue differences, SARS-2 FP induces even greater membrane ordering than SARS-1 FP, possibly due to its greater hydrophobicity. This may be a reason that SARS-2 is better able to infect host cells. In addition, the membrane binding enthalpy for SARS-2 is greater. Both the membrane ordering of SARS-2 and SARS-1 FPs are dependent on Ca2+, but that of SARS-2 shows a greater response to the presence of Ca2+. Both FPs bind two Ca2+ ions as does SARS-1 FP, but the two Ca2+ binding sites of SARS-2 exhibit greater cooperativity. This Ca2+ dependence by the SARS-2 FP is very ion-specific. These results show that Ca2+ is an important regulator that interacts with the SARS-2 FP and thus plays a significant role in SARS-2 viral entry. This could lead to therapeutic solutions that either target the FP-calcium interaction or block the Ca2+ channel.

Application of MCR-ALS with EFA on FT-IR spectra of lipid bilayers in the assessment of phase transition temperatures: Potential for discernment of coupled events

Temperature-dependent transmission FT-IR spectroscopy and DSC measurements were conducted on lipid multibilayers constituted from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. Lipid multibilayers made from 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, which do not form a ripple phase, were examined as a reference. Spectra were analyzed using multivariate curve resolution technique with alternating least squares and evolving factor analysis (MCR-ALS with EFA) and lipid phase transition temperatures were determined. Polar parts of lipid molecules exert greater response on a ripple phase formation than non-polar ones. However, vibrational signatures of hydrocarbon chains with intramolecular origins display certain qualitative differences that pave the way for future work oriented on uncoupling the events that drive ripple phase formation.

Microsecond Exchange Processes Studied by Two-Dimensional ESR at 95 GHz

Exchange processes which include conformational change, protonation/deprotonation, and binding equilibria are routinely studied by 2D exchange NMR techniques, where information about the exchange of nuclei between environments with different NMR shifts is obtained from the development of cross-peaks. Whereas 2D NMR enables the real time study of millisecond and slower exchange processes, 2D ESR in the form of 2D-ELDOR (two-dimensional electron-electron double resonance) has the potential for such studies over the nanosecond to microsecond real time scales. Cross-peak development due to chemical exchange has been seen previously for semiquinones in ESR, but this is not possible for most common ESR probes, such as nitroxides, studied at typical ESR frequencies because, unlike NMR, the exchanging states yield ESR signals that are not resolved from each other within their respective line widths. But at 95 GHz, it becomes possible to resolve them in many cases because of the increased g-factor resolution. The 95 GHz instrumental developments occurring at ACERT now enable such studies. We demonstrate these new capabilities in two studies: (A) the protonation/deprotonation process for a pH-sensitive imidazoline spin label in aqueous solution where the exchange rate and the population ratio of the exchanging states are controlled by the concentration and pH of the buffer solution, respectively, and (B) a nitroxide radical partitioning between polar (aqueous) and nonpolar (phospholipid) environments in multilamellar lipid vesicles, where the cross-peak development arises from the exchange of the nitroxide between the two phases. This work represents the first example of the observation and analysis of cross-peaks arising from chemical exchange processes involving nitroxide spin labels.

Imaging non-classical mechanical responses of lipid membranes using molecular rotors

Lipid packing in cellular membranes has a direct effect on membrane tension and microviscosity, and plays a central role in cellular adaptation, homeostasis and disease. According to conventional mechanical descriptions, viscosity and tension are directly interconnected, with increased tension leading to decreased membrane microviscosity. However, the intricate molecular interactions that combine to build the structure and function of a cell membrane suggest a more complex relationship between these parameters. In this work, a viscosity-sensitive fluorophore ('molecular rotor') is used to map changes in microviscosity in model membranes under conditions of osmotic stress. Our results suggest that the relationship between membrane tension and microviscosity is strongly influenced by the bilayer's lipid composition. In particular, we show that the effects of increasing tension are minimised for membranes that exhibit liquid disordered (Ld) - liquid ordered (Lo) phase coexistence; while, surprisingly, membranes in pure gel and Lo phases exhibit a negative compressibility behaviour, i.e. they soften upon compression.

SARS-CoV-2, Early Entry Events

Viruses are obligate intracellular parasites, and host cell entry is the first step in the viral life cycle. The SARS-CoV-2 (COVID-19) entry process into susceptible host tissue cells is complex requiring (1) attachment of the virus via the conserved spike (S) protein receptor-binding motif (RBM) to the host cell angiotensin-converting-enzyme 2 (ACE2) receptor, (2) S protein proteolytic processing, and (3) membrane fusion. Spike protein processing occurs at two cleavage sites, i.e., S1/S2 and S2'. Cleavage at the S1/S2 and S2' sites ultimately gives rise to generation of competent fusion elements important in the merging of the host cell and viral membranes. Following cleavage, shedding of the S1 crown results in significant conformational changes and fusion peptide repositioning for target membrane insertion and fusion. Identification of specific protease involvement has been difficult due to the many cell types used and studied. However, it appears that S protein proteolytic cleavage is dependent on (1) furin and (2) serine protease transmembrane protease serine 2 proteases acting in tandem. Although at present not clear, increased SARS-CoV-2 S receptor-binding motif binding affinity and replication efficiency may in part account for observed differences in infectivity. Cleavage of the ACE2 receptor appears to be yet another layer of complexity in addition to forfeiture and/or alteration of ACE2 function which plays an important role in cardiovascular and immune function.

Ca2+ Ions Promote Fusion of Middle East Respiratory Syndrome Coronavirus with Host Cells and Increase Infectivity

Fusion with, and subsequent entry into, the host cell is one of the critical steps in the life cycle of enveloped viruses. For Middle East respiratory syndrome coronavirus (MERS-CoV), the spike (S) protein is the main determinant of viral entry. Proteolytic cleavage of the S protein exposes its fusion peptide (FP), which initiates the process of membrane fusion. Previous studies on the related severe acute respiratory syndrome coronavirus (SARS-CoV) FP have shown that calcium ions (Ca²⁺) play an important role in fusogenic activity via a Ca²⁺ binding pocket with conserved glutamic acid (E) and aspartic acid (D) residues. SARS-CoV and MERS-CoV FPs share a high sequence homology, and here, we investigated whether Ca²⁺ is required for MERS-CoV fusion by screening a mutant array in which E and D residues in the MERS-CoV FP were substituted with neutrally charged alanines (A). Upon verifying mutant cell surface expression and proteolytic cleavage, we tested their ability to mediate pseudoparticle (PP) infection of host cells in modulating Ca²⁺ environments. Our results demonstrate that intracellular Ca²⁺ enhances MERS-CoV wild-type (WT) PP infection by approximately 2-fold and that E891 is a crucial residue for Ca²⁺ interaction. Subsequent electron spin resonance (ESR) experiments revealed that this enhancement could be attributed to Ca²⁺ increasing MERS-CoV FP fusion-relevant membrane ordering. Intriguingly, isothermal calorimetry showed an approximate 1:1 MERS-CoV FP to Ca²⁺ ratio, as opposed to an 1:2 SARS-CoV FP to Ca²⁺ ratio, suggesting significant differences in FP Ca²⁺ interactions of MERS-CoV and SARS-CoV FP despite their high sequence similarity.IMPORTANCE Middle East respiratory syndrome coronavirus (MERS-CoV) is a major emerging infectious disease with zoonotic potential and has reservoirs in dromedary camels and bats. Since its first outbreak in 2012, the virus has repeatedly transmitted from camels to humans, with 2,468 confirmed cases causing 851 deaths. To date, there are no efficacious drugs and vaccines against MERS-CoV, increasing its potential to cause a public health emergency. In order to develop novel drugs and vaccines, it is important to understand the molecular mechanisms that enable the virus to infect host cells. Our data have found that calcium is an important regulator of viral fusion by interacting with negatively charged residues in the MERS-CoV FP region. This information can guide therapeutic solutions to block this calcium interaction and also repurpose already approved drugs for this use for a fast response to MERS-CoV outbreaks.

Ca2+ Ions Promote Fusion of Middle East Respiratory Syndrome Coronavirus with Host Cells and Increase Infectivity

Fusion with, and subsequent entry into, the host cell is one of the critical steps in the life cycle of enveloped viruses. For Middle East respiratory syndrome coronavirus (MERS-CoV), the spike (S) protein is the main determinant of viral entry. Proteolytic cleavage of the S protein exposes its fusion peptide (FP), which initiates the process of membrane fusion. Previous studies on the related severe acute respiratory syndrome coronavirus (SARS-CoV) FP have shown that calcium ions (Ca²⁺) play an important role in fusogenic activity via a Ca²⁺ binding pocket with conserved glutamic acid (E) and aspartic acid (D) residues. SARS-CoV and MERS-CoV FPs share a high sequence homology, and here, we investigated whether Ca²⁺ is required for MERS-CoV fusion by screening a mutant array in which E and D residues in the MERS-CoV FP were substituted with neutrally charged alanines (A). Upon verifying mutant cell surface expression and proteolytic cleavage, we tested their ability to mediate pseudoparticle (PP) infection of host cells in modulating Ca²⁺ environments. Our results demonstrate that intracellular Ca²⁺ enhances MERS-CoV wild-type (WT) PP infection by approximately 2-fold and that E891 is a crucial residue for Ca²⁺ interaction. Subsequent electron spin resonance (ESR) experiments revealed that this enhancement could be attributed to Ca²⁺ increasing MERS-CoV FP fusion-relevant membrane ordering. Intriguingly, isothermal calorimetry showed an approximate 1:1 MERS-CoV FP to Ca²⁺ ratio, as opposed to an 1:2 SARS-CoV FP to Ca²⁺ ratio, suggesting significant differences in FP Ca²⁺ interactions of MERS-CoV and SARS-CoV FP despite their high sequence similarity.IMPORTANCE Middle East respiratory syndrome coronavirus (MERS-CoV) is a major emerging infectious disease with zoonotic potential and has reservoirs in dromedary camels and bats. Since its first outbreak in 2012, the virus has repeatedly transmitted from camels to humans, with 2,468 confirmed cases causing 851 deaths. To date, there are no efficacious drugs and vaccines against MERS-CoV, increasing its potential to cause a public health emergency. In order to develop novel drugs and vaccines, it is important to understand the molecular mechanisms that enable the virus to infect host cells. Our data have found that calcium is an important regulator of viral fusion by interacting with negatively charged residues in the MERS-CoV FP region. This information can guide therapeutic solutions to block this calcium interaction and also repurpose already approved drugs for this use for a fast response to MERS-CoV outbreaks.

Ultrafast Dynamics at the Lipid-Water Interface: DMSO Modulates H-Bond Lifetimes

Dimethyl sulfoxide (DMSO) is a common cosolvent and cryopreservation agent used to freeze cells and tissues. DMSO alters the H-bond structure of water, but its interactions with biomolecules and, specifically, with biological interfaces remain poorly understood. Here we investigate the effects of DMSO on the H-bond dynamics at the lipid-water interface using a combination of ultrafast two-dimensional infrared (2D IR) spectroscopy and molecular dynamics simulations. Ester carbonyl absorption spectra show that DMSO dehydrates the interface, and simulations show that the area per lipid is decreased. Ultrafast 2D IR spectra measure the time scales of frequency fluctuations at the ester carbonyl positions located precisely between the hydrophobic and hydrophilic regions of the membrane. 2D IR measurements show that low DMSO concentrations (<10 mol %) induce ∼40% faster H-bond dynamics compared with pure water, whereas increased concentrations (>10-20 mol %) once again slow down the dynamics. This slow-fast-slow trend is described in terms of two different solvation regimes. Below 10 mol %, DMSO weakens the interfacial H bond, leading to faster "bulk-like" dynamics, whereas above 10 mol %, water molecules become "relatively immobilized" as the H-bond networks becoming disrupted by the H-bond donor/acceptor imbalance at the interface. These studies are an important step toward characterizing the environments around lipid membranes, which are essential to numerous biological processes.

Calcium Ions Directly Interact with the Ebola Virus Fusion Peptide To Promote Structure-Function Changes That Enhance Infection

Ebola virus disease is a serious global health concern given its periodic occurrence, high lethality, and the lack of approved therapeutics. Certain drugs that alter intracellular calcium, particularly in endolysosomes, have been shown to inhibit Ebola virus infection; however, the underlying mechanism is unknown. Here, we provide evidence that Zaire ebolavirus (EBOV) infection is promoted in the presence of calcium as a result of the direct interaction of calcium with the EBOV fusion peptide (FP). We identify the glycoprotein residues D522 and E540 in the FP as functionally critical to EBOV's interaction with calcium. We show using spectroscopic and biophysical assays that interactions of the fusion peptide with Ca²⁺ ions lead to lipid ordering in the host membrane during membrane fusion, and these changes are promoted at low pH and can be correlated with infectivity. We further demonstrate using circular dichroism spectroscopy that calcium interaction with the fusion peptide promotes α-helical structure of the fusion peptide, a conformational change that enhances membrane fusion, as validated using functional assays of membrane fusion. This study shows that calcium directly targets the Ebola virus fusion peptide and influences its conformation. As these residues are highly conserved across the Filoviridae, calcium's impact on fusion, and subsequently infectivity, is a key interaction that can be leveraged for developing strategies to defend against Ebola infection. This mechanistic insight provides a rationale for the use of calcium-interfering drugs already approved by the FDA as therapeutics against Ebola and enables further development of novel drugs to combat the virus.

Direct Prediction of EPR Spectra from Lipid Bilayers: Understanding Structure and Dynamics in Biological Membranes

Of the many biophysical techniques now being brought to bear on studies of membranes, electron paramagnetic resonance (EPR) of nitroxide spin probes was the first to provide information about both mobility and ordering in lipid membranes. Here, we report the first prediction of variable temperature EPR spectra of model lipid bilayers in the presence and absence of cholesterol from the results of large scale fully atomistic molecular dynamics (MD) simulations. Three types of structurally different spin probes were employed in order to study different parts of the bilayer. Our results demonstrate very good agreement with experiment and thus confirm the accuracy of the latest lipid force fields. The atomic resolution of the simulations allows the interpretation of the molecular motions and interactions in terms of their impact on the sensitive EPR line shapes. Direct versus indirect effects of cholesterol on the dynamics of spin probes are analysed. Given the complexity of structural organisation in lipid bilayers, the advantage of using a combined MD-EPR simulation approach is two-fold. Firstly, prediction of EPR line shapes directly from MD trajectories of actual phospholipid structures allows unambiguous interpretation of EPR spectra of biological membranes in terms of complex motions. Secondly, such an approach provides an ultimate test bed for the up-to-date MD simulation models employed in the studies of biological membranes, an area that currently attracts great attention.

The SARS-CoV Fusion Peptide Forms an Extended Bipartite Fusion Platform that Perturbs Membrane Order in a Calcium-Dependent Manner

Coronaviruses (CoVs) are a major infectious disease threat and include the pathogenic human pathogens of zoonotic origin: severe acute respiratory syndrome CoV (SARS-CoV) and Middle East respiratory syndrome CoV (MERS-CoV). Entry of CoVs into host cells is mediated by the viral spike (S) protein, which is structurally categorized as a class I viral fusion protein, within the same group as influenza virus and HIV. However, S proteins have two distinct cleavage sites that can be activated by a much wider range of proteases. The exact location of the CoV fusion peptide (FP) has been disputed. However, most evidence suggests that the domain immediately downstream of the S2' cleavage site is the FP (amino acids 798-818 SFIEDLLFNKVTLADAGFMKQY for SARS-CoV, FP1). In our previous electron spin resonance spectroscopic studies, the membrane-ordering effect of influenza virus, HIV, and Dengue virus FPs has been consistently observed. In this study, we used this effect as a criterion to identify and characterize the bona fide SARS-CoV FP. Our results indicate that both FP1 and the region immediately downstream (amino acids 816-835 KQYGECLGDINARDLICAQKF, FP2) induce significant membrane ordering. Furthermore, their effects are calcium dependent, which is consistent with in vivo data showing that calcium is required for SARS-CoV S-mediated fusion. Isothermal titration calorimetry showed a direct interaction between calcium cations and both FPs. This Ca²⁺-dependency membrane ordering was not observed with influenza FP, indicating that the CoV FP exhibits a mechanistically different behavior. Membrane-ordering effects are greater and penetrate deeper into membranes when FP1 and FP2 act in a concerted manner, suggesting that they form an extended fusion "platform."

Thermal Response Analysis of Phospholipid Bilayers Using Ellipsometric Techniques

Biomimetic planar artificial membranes have been widely studied due to their multiple applications in several research fields. Their humectation and thermal response are crucial for reaching stability; these characteristics are related to the molecular organization inside the bilayer, which is affected by the aliphatic chain length, saturations, and molecule polarity, among others. Bilayer stability becomes a fundamental factor when technological devices are developed—like biosensors—based on those systems. Thermal studies were performed for different types of phosphatidylcholine (PC) molecules: two pure PC bilayers and four binary PC mixtures. These analyses were carried out through the detection of slight changes in their optical and structural parameters via Ellipsometry and Surface Plasmon Resonance (SPR) techniques. Phospholipid bilayers were prepared by Langmuir-Blodgett technique and deposited over a hydrophilic silicon wafer. Their molecular inclination degree, mobility, and stability of the different phases were detected and analyzed through bilayer thickness changes and their optical phase-amplitude response. Results show that certain binary lipid mixtures—with differences in its aliphatic chain length—present a co-existence of two thermal responses due to non-ideal mixing.

Non-linear van't Hoff behavior in pulmonary surfactant model membranes

Pulmonary surfactant exhibits phase coexistence over a wide range of surface pressure and temperature. Less is known about the effect of temperature on pulmonary surfactant models. Given the lack of studies on this issue, we used electron paramagnetic resonance (EPR) and nonlinear least-squares (NLLS) simulations to investigate the thermotropic phase behavior of the matrix that mimics the pulmonary surfactant lipid complex, i.e., the lipid mixture composed of dipalmitoyl phosphatidylcholine (DPPC), palmitoyl-oleoyl phosphatidylcholine (POPC) and palmitoyl-oleoyl phosphatidylglycerol (POPG). Irrespective of pH, the EPR spectra recorded from 5°C to 25°C in the DPPC/POPC/POPG (4:3:1) model membrane contain two spectral components corresponding to lipids in gel-like and fluid-like phases, indicating a coexistence of two domains in that range. The temperature dependence of the distribution of spin labels between the domains yielded nonlinear van't Hoff plots. The thermodynamic parameters evaluated were markedly different for DPPC and for the ternary DPPC/POPC/POPG (4:3:1) membranes and exhibited a dependence on chemical environment. While enthalpy and entropy changes for DPPC were always positive and presented a quadratic behavior with temperature, those of the ternary mixture were linearly dependent on temperature and changed from negative to positive values. Despite that, enthalpy-entropy compensation takes place in the two systems. The thermotropic process associated with the coexistence of the two domains is entropically-driven in DPPC and either entropically- or enthalpically-driven in the pulmonary surfactant membrane depending on the pH, ionic strength and temperature. The significance of these results to the structure and function of the pulmonary surfactant lipid matrix is discussed.

Structure-Function Studies Link Class II Viral Fusogens with the Ancestral Gamete Fusion Protein HAP2

The conserved transmembrane protein, HAP2/GCS1, has been linked to fertility in a wide range of taxa and is hypothesized to be an ancient gamete fusogen. Using template-based structural homology modeling, we now show that the ectodomain of HAP2 orthologs from Tetrahymena thermophila and other species adopt a protein fold remarkably similar to the dengue virus E glycoprotein and related class II viral fusogens. To test the functional significance of this predicted structure, we developed a flow-cytometry-based assay that measures cytosolic exchange across the conjugation junction to rapidly probe the effects of HAP2 mutations in the Tetrahymena system. Using this assay, alterations to a region in and around a predicted "fusion loop" in T. thermophila HAP2 were found to abrogate membrane pore formation in mating cells. Consistent with this, a synthetic peptide corresponding to the HAP2 fusion loop was found to interact directly with model membranes in a variety of biophysical assays. These results raise interesting questions regarding the evolutionary relationships of class II membrane fusogens and harken back to a long-held argument that eukaryotic sex arose as the byproduct of selection for the horizontal transfer of a "selfish" genetic element from cell to cell via membrane fusion.

SARS-CoV fusion peptides induce membrane surface ordering and curvature

Viral membrane fusion is an orchestrated process triggered by membrane-anchored viral fusion glycoproteins. The S2 subunit of the spike glycoprotein from severe acute respiratory syndrome (SARS) coronavirus (CoV) contains internal domains called fusion peptides (FP) that play essential roles in virus entry. Although membrane fusion has been broadly studied, there are still major gaps in the molecular details of lipid rearrangements in the bilayer during fusion peptide-membrane interactions. Here we employed differential scanning calorimetry (DSC) and electron spin resonance (ESR) to gather information on the membrane fusion mechanism promoted by two putative SARS FPs. DSC data showed the peptides strongly perturb the structural integrity of anionic vesicles and support the hypothesis that the peptides generate opposing curvature stresses on phosphatidylethanolamine membranes. ESR showed that both FPs increase lipid packing and head group ordering as well as reduce the intramembrane water content for anionic membranes. Therefore, bending moment in the bilayer could be generated, promoting negative curvature. The significance of the ordering effect, membrane dehydration, changes in the curvature properties and the possible role of negatively charged phospholipids in helping to overcome the high kinetic barrier involved in the different stages of the SARS-CoV-mediated membrane fusion are discussed.

The Interaction between Influenza HA Fusion Peptide and Transmembrane Domain Affects Membrane Structure

Viral glycoproteins, such as influenza hemagglutinin (HA) and human immunodeficiency virus gp41, are anchored by a single helical segment transmembrane domain (TMD) on the viral envelope membrane. The fusion peptides (FP) of the glycoproteins insert into the host membrane and initiate membrane fusion. Our previous study showed that the FP or TMD alone perturbs membrane structure. Interaction between the influenza HA FP and TMD has previously been shown, but its role is unclear. We used PC spin labels dipalmitoylphospatidyl-tempo-choline (on the headgroup), 5PC and 14PC (5-C and 14-C positions on the acyl chain) to detect the combined effect of FP-TMD interaction by titrating HA FP to TMD-reconstituted 1,2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol)/cholesterol lipid bilayers using electron spin resonance. We found that the FP-TMD increases the lipid order at all positions, which has a greater lipid ordering effect than the sum of the FP or TMD alone, and this effect reaches deeper into the membranes. Although HA-mediated membrane fusion is pH dependent, this combined effect is observed at both pH 5 and pH 7. In addition to increasing lipid order, multiple components are found for 5PC at increased concentration of FP-TMD, indicating that distinct domains are induced. However, the mutation of Gly1 in the FP and L187 in the TMD eliminates the perturbations, consistent with their fusogenic phenotypes. Electron spin resonance on spin-labeled peptides confirms these observations. We suggest that this interaction may provide a driving force in different stages of membrane fusion: initialization, transition from hemifusion stalk to transmembrane contact, and fusion pore formation.

Cholesterol enhances surface water diffusion of phospholipid bilayers

Elucidating the physical effect of cholesterol (Chol) on biological membranes is necessary towards rationalizing their structural and functional role in cell membranes. One of the debated questions is the role of hydration water in Chol-embedding lipid membranes, for which only little direct experimental data are available. Here, we study the hydration dynamics in a series of Chol-rich and depleted bilayer systems using an approach termed (1)H Overhauser dynamic nuclear polarization (ODNP) NMR relaxometry that enables the sensitive and selective determination of water diffusion within 5-10 Å of a nitroxide-based spin label, positioned off the surface of the polar headgroups or within the nonpolar core of lipid membranes. The Chol-rich membrane systems were prepared from mixtures of Chol, dipalmitoyl phosphatidylcholine and/or dioctadecyl phosphatidylcholine lipid that are known to form liquid-ordered, raft-like, domains. Our data reveal that the translational diffusion of local water on the surface and within the hydrocarbon volume of the bilayer is significantly altered, but in opposite directions: accelerated on the membrane surface and dramatically slowed in the bilayer interior with increasing Chol content. Electron paramagnetic resonance (EPR) lineshape analysis shows looser packing of lipid headgroups and concurrently tighter packing in the bilayer core with increasing Chol content, with the effects peaking at lipid compositions reported to form lipid rafts. The complementary capability of ODNP and EPR to site-specifically probe the hydration dynamics and lipid ordering in lipid membrane systems extends the current understanding of how Chol may regulate biological processes. One possible role of Chol is the facilitation of interactions between biological constituents and the lipid membrane through the weakening or disruption of strong hydrogen-bond networks of the surface hydration layers that otherwise exert stronger repulsive forces, as reflected in faster surface water diffusivity. Another is the concurrent tightening of lipid packing that reduces passive, possibly unwanted, diffusion of ions and water across the bilayer.

A time-resolved study on the interaction of oppositely charged bicelles--implications on the charged lipid exchange kinetics

Time-resolved small-angle X-ray scattering was applied to study charged lipid exchange between oppositely charged disc-shaped bicelles. The exchange of charged lipids gradually reduces the surface charge density and weakens the electrostatic attraction between the oppositely charged bicelles which form alternately stacked aggregates upon mixing. Initially, at a high surface charge density with almost no free water layer between the stacked bicelles, fast exchange kinetics dominate the exchange process. At a later stage with a lower surface charge density and a larger water gap between the stacked bicelles, slow exchange kinetics take over. The fast exchange kinetics are correlated with the close contact of the bicelles when there is almost no free water layer between the tightly bound bicelles with a charged lipid exchange time constant as short as 20-40 min. When the water gap becomes large enough to have a free water layer between the stacked bicelles, the fast lipid exchange kinetics are taken over by slow lipid exchange kinetics with time constants around 200-300 min, which are comparable to the typical time constant of lipid exchange between vesicles in aqueous solution. These two kinds of exchange mode fit well with the lipid exchange models of transient hemifusion for the fast mode and monomer exchange for the slow mode.

Hydration Forces Between Lipid Bilayers: A Theoretical Overview and a Look on Methods Exploring Dehydration

Although, many biological systems fulfil their functions under the condition of excess hydration, the behaviour of bound water as well as the processes accompanying dehydration are nevertheless important to investigate. Dehydration can be a result of applied mechanical pressure, lowered humidity or cryogenic conditions. The effort required to dehydrate a lipid membrane at relatively low degree of hydration can be described by a disjoining pressure which is called hydration pressure or hydration force. This force is short-ranging (a few nm) and is usually considered to be independent of other surface forces, such as ionic or undulation forces. Different theories were developed to explain hydration forces that are usually not consistent with each other and which are also partially in conflict with experimental or numerical data.Over the last decades it has been more and more realised that one experimental method alone is not capable of providing much new insight into the world of such hydration forces. Therefore, research requires the comparison of results obtained from the different methods. This chapter thus deals with an overview on the theory of hydration forces, ranging from polarisation theory to protrusion forces, and presents a selection of experimental techniques appropriate for their characterisation, such as X-ray diffraction, atomic force microscopy and even calorimetry.

Membrane Hydration: A Hint to a New Model for Biomembranes

The classical view of a biological membrane is based on the Singer-Nicholson mosaic fluid model in which the lipid bilayer is the structural backbone. Under this paradigm, many studies of biological processes such as, permeability, active transport, enzyme activity and adhesion and fusion processes have been rationalized considering the lipid membrane as a low dielectric slab of hydrocarbon chains with polar head groups exposed to water at each side in which oil/water partition prevails. In spite of several analyses and evidence available in relation to membrane hydration, water is not taken into account as a functional component. For this purpose, new insights in the water organization in restricted environments and the thermodynamical and mechanical properties emerging from them are specifically analysed and correlated.This chapter summarizes the progress of the studies of water in membranes along the book in order to give a more realistic structural and dynamical picture accounting for the membrane functional properties.

Interactions of the antimalarial amodiaquine with lipid model membranes

A detailed molecular description of the mechanism of action of the antimalarial drug amodiaquine (AQ) is still an open issue. To gain further insights on that, we studied the interactions of AQ with lipid model membranes composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylserine (DPPS) by spin labeling electron spin resonance (ESR) and differential scanning calorimetry (DSC). Both techniques indicate a coexistence of an ordered DPPS-rich domain with a disordered DPPC-rich domain in the binary DPPC/DPPS system. We found that AQ slightly lowered the melting transition temperatures associated to both domains and significantly increased the enthalpy change of the whole DPPC/DPPS phase transition. DSC and ESR data also suggest that AQ increases the number of DPPC molecules in the DPPC-rich domains. AQ also causes opposing ordering effects on different regions of the bilayer: while the drug increases the ordering of the lipid acyl chains from carbon 7 to 16, it decreases the order parameter of the lipid head group and of carbon 5. The gel phase was mostly affected by the presence of AQ, suggesting that AQ is able to influence more organized lipid domains. Moreover, the effects of AQ and cholesterol on lipid acyl chain ordering and mobility were compared at physiological temperature and, in a general way, they are similar. Our results suggest that the quinoline ring of AQ is located completely inside the lipid bilayers with its phenol ring and the tertiary amine directed towards the head group region. The nonspecific interaction between AQ and DPPC/DPPS bilayers is a combination of electrostatic and hydrophobic interactions.

HIV gp41 fusion peptide increases membrane ordering in a cholesterol-dependent fashion

Fusion between viral envelopes and host cell membranes, which is mediated by special glycoproteins anchored on the viral membrane, is required for HIV viral entry and infection. The HIV gp41 fusion peptide (FP), which initiates membrane fusion, adopts either an α-helical or β-sheeted structure depending on the cholesterol concentration. We used phosphocholine spin labels on the lipid headgroup and different positions on the acyl chain to detect its perturbation on lipid bilayers containing different cholesterol concentrations by electron-spin resonance. Our findings were as follows. 1), gp41 FP affects the lipid order in the same manner as previously shown for influenza hemagglutinin FP, i.e., it has a cooperative effect versus the peptide/lipid ratio, supporting our hypothesis that membrane ordering is a common prerequisite for viral membrane fusion. 2), gp41 FP induces membrane ordering in all lipid compositions studied, whereas a nonfusion mutant FP perturbs lipid order to a significantly smaller extent. 3), In high-cholesterol-containing lipid bilayers, where gp41 FP is in the β-aggregation conformation, its effect on the lipid ordering reaches deeper into the bilayer. The different extent to which the two conformers perturb is correlated with their fusogenicity. The possible role of the two conformers in membrane fusion is discussed.

Using spin-label W-band EPR to study membrane fluidity profiles in samples of small volume

Conventional and saturation-recovery (SR) EPR at W-band (94GHz) using phosphatidylcholine spin labels (labeled at the alkyl chain [n-PC] and headgroup [T-PC]) to obtain profiles of membrane fluidity has been demonstrated. Dimyristoylphosphatidylcholine (DMPC) membranes with and without 50 mol% cholesterol have been studied, and the results have been compared with similar studies at X-band (9.4 GHz) (L. Mainali, J.B. Feix, J.S. Hyde, W.K. Subczynski, J. Magn. Reson. 212 (2011) 418-425). Profiles of the spin-lattice relaxation rate (T(1)(-1)) obtained from SR EPR measurements for n-PCs and T-PC were used as a convenient quantitative measure of membrane fluidity. Additionally, spectral analysis using Freed's MOMD (microscopic-order macroscopic-disorder) model (E. Meirovitch, J.H. Freed J. Phys. Chem. 88 (1984) 4995-5004) provided rotational diffusion coefficients (R(perpendicular) and R(||)) and order parameters (S(0)). Spectral analysis at X-band provided one rotational diffusion coefficient, R(perpendicular). T(1)(-1), R(perpendicular), and R(||) profiles reflect local membrane dynamics of the lipid alkyl chain, while the order parameter shows only the amplitude of the wobbling motion of the lipid alkyl chain. Using these dynamic parameters, namely T(1)(-1), R(perpendicular), and R(||), one can discriminate the different effects of cholesterol at different depths, showing that cholesterol has a rigidifying effect on alkyl chains to the depth occupied by the rigid steroid ring structure and a fluidizing effect at deeper locations. The nondynamic parameter, S(0), shows that cholesterol has an ordering effect on alkyl chains at all depths. Conventional and SR EPR measurements with T-PC indicate that cholesterol has a fluidizing effect on phospholipid headgroups. EPR at W-band provides more detailed information about the depth-dependent dynamic organization of the membrane compared with information obtained at X-band. EPR at W-band has the potential to be a powerful tool for studying membrane fluidity in samples of small volume, ~30 nL, compared with a representative sample volume of ~3 μL at X-band.

Spatial distribution of lipid headgroups and water molecules at membrane/water interfaces visualized by three-dimensional scanning force microscopy

At biological interfaces, flexible surface structures and mobile water interact with each other to present non-uniform three-dimensional (3D) distributions. In spite of their impact on biological functions, molecular-scale understanding of such phenomena has remained elusive. Here we show direct visualization of such interfacial structures with subnanometer-scale resolution by 3D scanning force microscopy (3D-SFM). We measured a 3D force distribution at an interface between a model biological membrane and buffer solution by scanning a sharp tip within the 3D interfacial space. We found that vertical cross sections of the 3D image taken along a specific lateral direction show characteristic molecular-scale contrasts tilted at 30° to the membrane surface. Detailed analysis of the 3D image reveals that the tilted contrast corresponds to the time-averaged conformation of fluctuating lipid headgroups. On the basis of the obtained results, we discuss the relationships among the hydration structure, headgroup fluctuation, molecular fluidity, and mechanical strength of the membrane. The results demonstrate that 3D-SFM is capable of visualizing averaged 3D distribution of fluctuating surface structures as well as that of mobile water (i.e., hydration structure) at interfaces between biological systems and water.

Two conserved residues are important for inducing highly ordered membrane domains by the transmembrane domain of influenza hemagglutinin

The interaction with lipids of a synthetic peptide corresponding to the transmembrane domain of influenza hemagglutinin was investigated by means of electron spin resonance. A detailed analysis of the electron spin resonance spectra from spin-labeled phospholipids revealed that the major effect of the peptide on the dynamic membrane structure is to induce highly ordered membrane domains that are associated with electrostatic interactions between the peptide and negatively charged lipids. Two highly conserved residues in the peptide were identified as being important for the membrane ordering effect. Aggregation of large unilamellar vesicles induced by the peptide was also found to be correlated with the membrane ordering effect of the peptide, indicating that an increase in membrane ordering, i.e., membrane dehydration, is important for vesicle aggregation. The possibility that hydrophobic interaction between the highly ordered membrane domains plays a role in vesicle aggregation and viral fusion is discussed.

Liposome fusion rates depend upon the conformation of polycation catalysts

Cryo-TEM and NaCl-leakage experiments demonstrated that the cationic polymer polylysine induces fusion of anionic liposomes but that the cationic polymer poly(N-ethyl-4-vinylpyridinium bromide) (PEVP) does not, although both polymers bind strongly to the liposomes. The difference was traced to the thickness of the coatings at constant charge coverage. Polylysine is believed to form planar β-sheets that are sufficiently thin to allow membrane fusion. In contrast, looping and disorganization among adsorbed PEVP molecules physically prevent fusion. A similar effect is likely to be applicable to important polycation-induced fusion of cell membranes.

On the surface properties of oleate micelles and oleic acid/oleate vesicles studied by spin labeling

Dilute aqueous systems composed of sodium oleate micelles and sodium oleate/oleic acid vesicles were investigated as a function of pH by electron spin resonance spectroscopy with TEMPO-stearate TEMPO-stearamide as well as with a positively charged water soluble spin label, TEMPO-choline. The dynamics of the three TEMPO-spin labels were found to be sensitive to changes in the interfacial region of the aggregates as a function of pH. The results obtained are consistent with the formation of a hydrogen bond network (RCOO(-)↔HOOCR) at the surface of the sodium oleate/oleic acid system in the course of the transformation of micelles into the closed bilayers (vesicles). Vesicles formation below pH 10 was determined independently with a spin labeled glucose derivative.

Local Water Dynamics in Coacervated Polyelectrolytes Monitored Through Dynamic Nuclear Polarization-Enhanced H NMR

We present the first study of quantifying the diffusion coefficient of interfacial water on polyelectrolyte surfaces of systems fully dispersed in bulk water under ambient conditions. Such measurements were made possible through the implementation of a recently introduced Dynamic Nuclear Polarization (DNP) technique to selectively amplify the nuclear magnetic resonance (NMR) signal of hydration water that is interacting with specifically located spin labels on polyelectrolyte surfaces. The merit of this novel capability is demonstrated in this report through the measurement of solvent microvisosity on the surface of two types of oppositely charged polyelectrolytes, when freely dissolved versus when complexed to form a liquid-liquid colloidal phase called complex coacervates. These complex coacervates were formed through electrostatic complexation between the imidazole-based cationic homopolymer poly(N-vinylimidazole) (PVIm), and anionic polypeptide polyaspartate (PAsp) in the pH range of 4.5 - 6.0, under which conditions the coacervate droplets are highly fluidic yet densely packed with polyelectrolytes. We also investigated the rotational diffusion coefficients of the spin labels covalently bound to the polyelectrolyte chains for both PVIm and PAsp, showing a 5 fold change in the rotational correlation time as well as anisotropy parameter upon coacervation, which represents a surprisingly small decrease given the high polymer concentration inside the dense microdroplets. For both DNP and ESR experiments, the polymers were covalently tagged with stable nitroxide radical spin labels (∼1 wt %) to probe the local solvent and polymer segment dynamics. We found that the surface water diffusion coefficients near uncomplexed PVIm and PAsp at pH 8 differ, and are around D∼1.3×10(-9)m(2) / s. In contrast, inside the complex coacervate phase, the water diffusion coefficient in the immediate vicinity of either polyelectrolyte was D∼ 0.25×10(-9)m(2) / s, which is about an order of magnitude smaller than the bulk water self diffusion coefficient, and yet orders of magnitude greater than that of associated, bound, hydration water. This observation suggests the existence of measurable water inside complex coacervates with relatively high diffusion and exchange dynamics, implying that water moves in nanometer-scale pore spaces as opposed to being structurally bound or even absent. We infer from our observation that the PVIm and PAsp chains are undergoing roughly pairwise association, so that largely charge neutralized species compose the concentrated, yet fluidic and partially hydrated coacervate cores.

Fusion peptide from influenza hemagglutinin increases membrane surface order: an electron-spin resonance study

A spin-labeling study of interactions of a fusion peptide from the hemagglutinin of the influenza virus, wt20, and a fusion-inactive mutant DeltaG1 with dimyristoylphosphatidylcholine (DMPC) and 1-palmitoyl-2-oleoyl-phosphatdylcholine bilayers was performed. We found that upon binding of wt20, the ordering of headgroups and the ordering of acyl chains near the headgroup increased significantly, in a manner consistent with a cooperative phenomenon. However, changes in the order at the end of the acyl chains were negligible. The ordering effect of wt20 on the headgroup was much stronger at pH 5 than at pH 7. No effect of DeltaG1 binding on the order of bilayers was evident. We also found that 1-palmitoyl-2-hydroxyl phosphatidylcholine, a membrane-fusion inhibitor, decreased the ordering of DMPC headgroups, whereas arachidonic acid, a membrane-fusion promoter, increased the ordering of DMPC headgroups. These results suggest that increases in headgroup ordering may be important for membrane fusion. We propose that upon binding of wt20, which is known to affect only the outer leaflet of the bilayer, this outer leaflet becomes more ordered, and thus more solid-like. Then the coupling between the hardened outer leaflet and the softer inner leaflet generates bending stresses in the bilayer, which tend to increase the negative curvature of the bilayer. We suggest that the increased ordering in the headgroup region enhances dipolar interactions and lowers electrostatic energy, which may provide an energy source for membrane fusion. Possible roles of bending stresses in promoting membrane fusion are discussed.

Configuration of carbonyl groups at the lipid interphases of different topological arrangements of lipid dispersions

The purpose of this work is to analyze the conformation of the carbonyl groups of acyl phospholipids at the hydrocarbon-water interphase in different topological ensembles and phase states, such as micelles and bilayers. The separation of the band components in lipids dispersed in D(2)O is compared with that of PCs in a low hydrated state. When hydrated, the differences in the frequencies of the band components corresponding to the carbonyl groups identified as low hydrated and hydrated populations increase when dimyristoylphosphatidylcholine (DMPC) bilayers go from the lamellar gel to the ripple corrugated phase at the pretransition temperature. Below the pretransition, at which the membrane in the gel state is planar, the two components overlap making the deconvolution unreliable. A further analysis shows that the frequency of the highly hydrated population increases more noticeable than that corresponding to the low hydrated one following the sequence: micelles, fluid phase, ripple gel phase, and lamellar gel phase. This is confirmed by the increase in the separation of the band components when the liposomes are subjected to an osmotic dehydration suggesting that the hydrated population loses water and the dehydrated one partially hydrates. It is concluded that this behavior is a feature conferred by hydration of the different topological arrangements. The relevance of these results on the interphase properties of lipid membranes is discussed.

Infrared monitoring of interlayer water in stacks of purple membranes

The thermodynamic behavior of films of hydrated purple membranes from Halobacterium salinarum and the water confined in it was studied by Fourier transform infrared spectroscopy in the 180-280 K range. Unlike bulk water, water in the thin layers sandwiched between the biological membranes does not freeze at 273 K but will be supercooled to approximately 256 K. The melting point is unaffected, leading to hysteresis between 250 and 273 K. In its heating branch, a gradually increasing light-scattering by ice is observed with rate-limiting kinetics of tens of minutes. Infrared (IR) spectra decomposition provided extinction coefficients for the confined water vibrational bands and their changes upon freezing. Because of the hysteresis, at any given temperature in the 255-270 K range, the interbilayer water could be either liquid or frozen, depending on thermal history. We find that this difference affects the dynamics of the bacteriorhodopsin photocycle in the hysteresis range: the decay of the M and N states and the redistribution between them are different depending on whether or not the water was initially precooled to below the freezing point. However, freezing of interbilayer water does block the M to N transition. Unlike the water, the purple membrane lipids do not undergo any IR-detectable phase transition in the 180-280 K range.

Dynamic nuclear polarization enhanced nuclear magnetic resonance and electron spin resonance studies of hydration and local water dynamics in micelle and vesicle assemblies

We present a unique analysis tool for the selective detection of local water inside soft molecular assemblies (hydrophobic cores, vesicular bilayers, and micellar structures) suspended in bulk water. Through the use of dynamic nuclear polarization (DNP), the (1)H NMR signal of water is amplified, as it interacts with stable radicals that possess approximately 658 times higher spin polarization. We utilized stable nitroxide radicals covalently attached along the hydrophobic tail of stearic acid molecules that incorporate themselves into surfactant-based micelle or vesicle structures. Here, we present a study of local water content and fluid viscosity inside oleate micelles and vesicles and Triton X-100 micelles to serve as model systems for soft molecular assemblies. This approach is unique because the amplification of the NMR signal is performed in bulk solution and under ambient conditions with site-specific spin labels that only detect the water that is directly interacting with the localized spin labels. Continuous wave (cw) electron spin resonance (ESR) analysis provides rotational dynamics of the spin-labeled molecular chain segments and local polarity parameters that can be related to hydration properties, whereas we show that DNP-enhanced (1)H NMR analysis of fluid samples directly provides translational water dynamics and permeability of the local environment probed by the spin label. Our technique therefore has the potential to become a powerful analysis tool, complementary to cw ESR, to study hydration characteristics of surfactant assemblies, lipid bilayers, or protein aggregates, where water dynamics is a key parameter of their structure and function. In this study, we find that there is significant penetration of water inside the oleate micelles with a higher average local water viscosity (approximately 1.8 cP) than in bulk water, and Triton X-100 micelles and oleate vesicle bilayers mostly exclude water while allowing for considerable surfactant chain motion and measurable water permeation through the soft structure.

Structural and functional properties of hydration and confined water in membrane interfaces

The scope of the present review focuses on the interfacial properties of cell membranes that may establish a link between the membrane and the cytosolic components. We present evidences that the current view of the membrane as a barrier of permeability that contains an aqueous solution of macromolecules may be replaced by one in which the membrane plays a structural and functional role. Although this idea has been previously suggested, the present is the first systematic work that puts into relevance the relation water-membrane in terms of thermodynamic and structural properties of the interphases that cannot be ignored in the understanding of cell function. To pursue this aim, we introduce a new definition of interphase, in which the water is organized in different levels on the surface with different binding energies. Altogether determines the surface free energy necessary for the structural response to changes in the surrounding media. The physical chemical properties of this region are interpreted in terms of hydration water and confined water, which explain the interaction with proteins and could affect the modulation of enzyme activity. Information provided by several methodologies indicates that the organization of the hydration states is not restricted to the membrane plane albeit to a region extending into the cytoplasm, in which polar head groups play a relevant role. In addition, dynamic properties studied by cyclic voltammetry allow one to deduce the energetics of the conformational changes of the lipid head group in relation to the head-head interactions due to the presence of carbonyls and phosphates at the interphase. These groups are, apparently, surrounded by more than one layer of water molecules: a tightly bound shell, that mostly contributes to the dipole potential, and a second one that may be displaced by proteins and osmotic stress. Hydration water around carbonyl and phosphate groups may change by the presence of polyhydroxylated compounds or by changing the chemical groups esterified to the phosphates, mainly choline, ethanolamine or glycerol. Thus, surface membrane properties, such as the dipole potential and the surface pressure, are modulated by the water at the interphase region by changing the structure of the membrane components. An understanding of the properties of the structural water located at the hydration sites and the functional water confined around the polar head groups modulated by the hydrocarbon chains is helpful to interpret and analyze the consequences of water loss at the membranes of dehydrated cells. In this regard, a correlation between the effects of water activity on cell growth and the lipid composition is discussed in terms of the recovery of the cell volume and their viability. Critical analyses of the properties of water at the interface of lipid membranes merging from these results and others from the literature suggest that the interface links the membrane with the aqueous soluble proteins in a functional unit in which the cell may be considered as a complex structure stabilized by water rather than a water solution of macromolecules surrounded by a semi permeable barrier.

Spin labelling study of interfacial properties of egg-phosphatidylcholine liposomes as a function of cholesterol concentrations

In order to gain insight into interfacial properties of liposomes composed of egg-phosphatidylcholine (egg-PC) and dihexadecyl-phosphate (DHP) as a function of 0, 8, 15, 29, 38, 45mol% of cholesterol, dynamic properties of two long-chain spin labels: TEMPO-stearate (2,2,6,6-tetramethylpiperidine-1-oxyl-4-yl)-octa-decanoate) and TEMPO-stearamide (2,2,6,6-tetramethylpiperidine-1-oxyl-4-yl)-octa-decanamide) were studied by CW-ESR spectroscopy. These spin labels reflect motional properties in the region of phospholipid head-groups. Two different environments of TEMPO-stearate were determined at 29, 38 and 45mol% of cholesterol. In the newly formed domain above 29mol%, N-O moiety of the spin label was surrounded by larger amount of bound water and experienced slower motion than in the cholesterol poor domain. The fraction of the second more hydrophilic environment of the spin label increased with cholesterol concentration. TEMPO-stearamide, a hydrogen-bond donor, reported more polar environment and slower motion than TEMPO-stearate even in the absence of cholesterol. Only one spin label environment was determined for all cholesterol concentrations. Slowing down of the TEMPO-stearamide motion was obtained even at 8mol% of cholesterol.