Conformational partitioning of the fusion peptide of HIV-1 gp41 and its structural analogs in bilayer membranes

Solid-state nuclear magnetic resonance measurements of HIV fusion peptide 13CO to lipid 31P proximities support similar partially inserted membrane locations of the α helical and β sheet peptide structures

Fusion of the human immunodeficiency virus (HIV) membrane and the host cell membrane is an initial step of infection of the host cell. Fusion is catalyzed by gp41, which is an integral membrane protein of HIV. The fusion peptide (FP) is the ∼25 N-terminal residues of gp41 and is a domain of gp41 that plays a key role in fusion catalysis likely through interaction with the host cell membrane. Much of our understanding of the FP domain has been accomplished with studies of "HFP", i.e., a ∼25-residue peptide composed of the FP sequence but lacking the rest of gp41. HFP catalyzes fusion between membrane vesicles and serves as a model system to understand fusion catalysis. HFP binds to membranes and the membrane location of HFP is likely a significant determinant of fusion catalysis perhaps because the consequent membrane perturbation reduces the fusion activation energy. In the present study, many HFPs were synthesized and differed in the residue position that was (13)CO backbone labeled. Samples were then prepared that each contained a singly (13)CO labeled HFP incorporated into membranes that lacked cholesterol. HFP had distinct molecular populations with either α helical or oligomeric β sheet structure. Proximity between the HFP (13)CO nuclei and (31)P nuclei in the membrane headgroups was probed by solid-state NMR (SSNMR) rotational-echo double-resonance (REDOR) measurements. For many samples, there were distinct (13)CO shifts for the α helical and β sheet structures so that the proximities to (31)P nuclei could be determined for each structure. Data from several differently labeled HFPs were then incorporated into a membrane location model for the particular structure. In addition to the (13)CO labeled residue position, the HFPs also differed in sequence and/or chemical structure. "HFPmn" was a linear peptide that contained the 23 N-terminal residues of gp41. "HFPmn_V2E" contained the V2E mutation that for HIV leads to greatly reduced extent of fusion and infection. The present study shows that HFPmn_V2E induces much less vesicle fusion than HFPmn. "HFPtr" contained three strands with HFPmn sequence that were chemically cross-linked near their C-termini. HFPtr mimics the trimeric topology of gp41 and induces much more rapid and extensive vesicle fusion than HFPmn. For HFPmn and HFPtr, well-resolved α and β peaks were observed for A6-, L9-, and L12-labeled samples. For each of these samples, there were similar HFP (13)CO to lipid (31)P proximities in the α and β structures, which evidenced comparable membrane locations of the HFP in either structure including insertion into a single membrane leaflet. The data were also consistent with deeper insertion of HFPtr relative to HFPmn in both the α and β structures. The results supported a strong correlation between the membrane insertion depth of the HFP and its fusogenicity. More generally, the results supported membrane location of the HFP as an important determinant of its fusogenicity. The deep insertion of HFPtr in both the α and β structures provides the most relevant membrane location of the FP for HIV gp41-catalyzed membrane fusion because HIV gp41 is natively trimeric. Well-resolved α and β signals were observed in the HFPmn_V2E samples with L9- and L12- but not A6-labeling. The α signals were much more dominant for L9- and L12-labeled HFPmn_V2E than the corresponding HFPmn or HFPtr. The structural model for the less fusogenic HFPmn_V2E includes a shorter helix and less membrane insertion than either HFPmn or HFPtr. This greater helical population and different helical structure and membrane location could result in less membrane perturbation and lower fusogenicity of HFPmn_V2E and suggest that the β sheet fusion peptide is the most functionally relevant structure of HFPmn, HFPtr, and gp41.

Single-particle tracking reveals switching of the HIV fusion peptide between two diffusive modes in membranes

Fusion of the HIV membrane with that of a target T cell is an essential first step in the viral infection process. Here we describe single-particle tracking (SPT) studies of a 16-amino-acid peptide derived from the HIV fusion protein (FP16), as it interacts with a supported lipid bilayer. FP16 was found to spontaneously insert into and move within the bilayer with two different modes of diffusion, a fast mode with a diffusion coefficient typical of protein motion in membranes and a much slower one. We observed transitions between the two modes: slow peptides were found to speed up, and fast peptides could slow down. Hidden Markov model analysis was employed as a method for the identification of the two modes in single-molecule trajectories and analysis of their interconversion rates. Surprisingly, the diffusion coefficients of the two modes were found to depend differently on solution viscosity. Thus, whereas the fast diffusive mode behaved as predicted by the Saffman-Delbrück theory, the slow mode behaved according to the Stokes-Einstein relation. To further characterize the two diffusive modes, FP16 molecules were studied in bilayers cooled through their liquid crystalline-to-gel phase transition. Our analysis suggested that the slow diffusive mode might originate from the formation of large objects, such as lipid domains or local protrusions, which are induced by the peptides and move together with them.

HIV-1 gp41 transmembrane domain interacts with the fusion peptide: implication in lipid mixing and inhibition of virus-cell fusion

Fusion of the human immunodeficiency virus (HIV) with target cells is mediated by the gp41 subunit of the envelope protein. Mutation and deletion studies within the transmembrane domain (TMD) of intact gp41 influenced its fusion activity. In addition, current models suggest that the TMD is in proximity with the fusion peptide (FP) at the late fusion stages, but there are no direct experimental data to support this hypothesis. Here, we investigated the TMD focusing on two regions: the N-terminal containing the GxxxG motif and the C-terminal containing the GLRI motif, which is conserved among the TMDs of HIV and the T-cell receptor. Studies utilizing the ToxR expression system combined with synthetic peptides and their fluorescent analogues derived from TMD revealed that the GxxxG motif is important for TMD self-association, whereas the C-terminal region is for its heteroassociation with FP. Functionally, all three TMD peptides induced lipid mixing that was enhanced significantly upon mixing with FP. Furthermore, the TMD peptides inhibited virus-cell fusion apparently through their interaction with their endogenous counterparts. Notably, the R2E mutant (in the GLRI) was significantly less potent than the two others. Overall, our findings provide experimental evidence that HIV-1 TMD contributes to membrane assembly and function of the HIV-1 envelope. Owing to similarities between functional domains within viruses, these findings suggest that the TMDs and FPs may contribute similarly in other viruses as well.

Effect of the HIV-1 fusion peptide on the mechanical properties and leaflet coupling of lipid bilayers

The fusion peptide (FP) of the human immunodeficiency virus (HIV) is part of the N-terminus of the viral envelope glycoprotein gp41 and is believed to play an important role in the viral entry process. To understand the immediate effect of this peptide on the cell membrane, we have studied the influence of the synthetic FP sequence FP23 on the mechanical properties of model lipid bilayers. For this purpose, giant unilamellar vesicles were prepared from the unsaturated lipid dioleoylphosphatidylcholine mixed in various molar ratios with FP23. The bending stiffness of the vesicles was measured with two different methods: fluctuation analysis and aspiration with micropipettes. The data obtained from both of these approaches show that the bending stiffness of the membrane decreases gradually with increasing concentration of the FP23 in the bilayer. Low concentrations of only a few mol% FP23 are sufficient to decrease the bending stiffness of the lipid bilayer by about a factor of 2. Finally, data obtained for the stretching elasticity modulus of the membrane suggest that the peptide insertion decreases the coupling between the two leaflets of the bilayer.

Irregular structure of the HIV fusion peptide in membranes demonstrated by solid-state NMR and MD simulations

To better understand peptide-induced membrane fusion at a molecular level, we set out to determine the structure of the fusogenic peptide FP23 from the HIV-1 protein gp41 when bound to a lipid bilayer. An established solid-state (19)F nuclear magnetic resonance (NMR) approach was used to collect local orientational constraints from a series of CF(3)-phenylglycine-labeled peptide analogues in macroscopically aligned membranes. Fusion assays showed that these (19)F-labels did not significantly affect peptide function. The NMR spectra were characteristic of well-behaved samples, without any signs of heterogeneity or peptide aggregation at 1:300 in 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC). We can conclude from these NMR data that FP23 has a well-defined (time-averaged) conformation and undergoes lateral diffusion in the bilayer plane, presumably as a monomer or small oligomer. Attempts to evaluate its conformation in terms of various secondary structures, however, showed that FP23 does not form any type of regular helix or β-strand. Therefore, all-atom molecular dynamics (MD) simulations were carried out using the orientational NMR constraints as pseudo-forces to drive the peptide into a stable alignment and structure. The resulting picture suggests that FP23 can adopt multiple β-turns and insert obliquely into the membrane. Such irregular conformation explains why the structure of the fusion peptide could not be reliably determined by any biophysical method so far.

HIV fusion peptide penetrates, disorders, and softens T-cell membrane mimics

This work investigates the interaction of N-terminal gp41 fusion peptide (FP) of human immunodeficiency virus type 1 (HIV-1) with model membranes in order to elucidate how FP leads to fusion of HIV and T-cell membranes. FP constructs were (i) wild-type FP23 (23 N-terminal amino acids of gp41), (ii) water-soluble monomeric FP that adds six lysines on the C-terminus of FP23 (FPwsm), and (iii) the C-terminus covalently linked trimeric version (FPtri) of FPwsm. Model membranes were (i) LM3 (a T-cell mimic), (ii) 1,2-dioleoyl-sn-glycero-3-phosphocholine, (iii) 1,2-dioleoyl-sn-glycero-3-phosphocholine/30 mol% cholesterol, (iv) 1,2-dierucoyl-sn-glycero-3-phosphocholine, and (v) 1,2-dierucoyl-sn-glycero-3-phosphocholine/30 mol% cholesterol. Diffuse synchrotron low-angle x-ray scattering from fully hydrated samples, supplemented by volumetric data, showed that FP23 and FPtri penetrate into the hydrocarbon region and cause membranes to thin. Depth of penetration appears to depend upon a complex combination of factors including bilayer thickness, presence of cholesterol, and electrostatics. X-ray data showed an increase in curvature in hexagonal phase 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine, which further indicates that FP23 penetrates into the hydrocarbon region rather than residing in the interfacial headgroup region. Low-angle x-ray scattering data also yielded the bending modulus K(C), a measure of membrane stiffness, and wide-angle x-ray scattering yielded the S(xray) orientational order parameter. Both FP23 and FPtri decreased K(C) and S(xray) considerably, while the weak effect of FPwsm suggests that it did not partition strongly into LM3 model membranes. Our results are consistent with the HIV FP disordering and softening the T-cell membrane, thereby lowering the activation energy for viral membrane fusion.

A strong correlation between fusogenicity and membrane insertion depth of the HIV fusion peptide

Fusion between the membrane of HIV and the membrane of a host cell is a crucial step in HIV infection and is catalyzed by the binding of the fusion peptide domain (HFP) of the HIV gp41 protein to the host cell membrane. The HFP by itself induces vesicle fusion and is a useful model system to understand the fusion peptide/host cell membrane interaction. This article reports an experimental correlation between the membrane locations of different HFP constructs and their fusogenicities. The constructs were the HFP monomer with Val-2 to Glu-2 mutation (HFPmn_mut), wild type HFP monomer (HFPmn), and wild type HFP trimer (HFPtr). All constructs have predominant beta sheet structure in membranes with physiologically relevant cholesterol content. HFPmn_mut does not fuse vesicles, HFPmn has moderate fusion rate, and HFPtr has the putative oligomerization state of HIV gp41 and a very rapid fusion rate. The HFP membrane locations were probed with solid-state NMR measurements of distances between labeled carbonyl ((13)CO) nuclei in the HFP backbone and lipid nuclei in the surface or interior regions of the membrane bilayer. HFPmn_mut is located at the membrane surface, HFPmn is inserted into a single membrane leaflet, and HFPtr is the most deeply inserted construct with contact with the center of the membrane. These results show a clear positive correlation between the insertion depths and the fusion activities of the HFP constructs. Other disease-causing enveloped viruses contain fusion peptides and this correlation may be a general structure-function model for these peptides.

HIV fusion peptide and its cross-linked oligomers: efficient syntheses, significance of the trimer in fusion activity, correlation of beta strand conformation with membrane cholesterol, and proximity to lipid headgroups

For enveloped viruses such as HIV, an approximately 20-residue N-terminal fusion peptide domain in the envelope protein binds to target cell membranes and plays a key role in fusion between the viral and cellular membranes during infection. The chemically synthesized HIV fusion peptide (HFP) catalyzes fusion between membrane vesicles and is a useful model system for understanding some aspects of HIV fusion. Previous studies have shown a common trimeric state for the envelope protein from several different viruses, including HIV, and in this study, practical high-yield syntheses are reported for HFP monomer (HFPmn) and chemically cross-linked HFP dimer (HFPdm), trimer (HFPtr), and tetramer (HFPte). The vesicle fusion rates per strand were ordered as follows: HFPmn < HFPdm < HFPtr approximately HFPte. This suggested that HFPtr is the smallest catalytically efficient oligomer. Solid-state NMR measurements of (13)CO chemical shifts were carried out in constructs labeled at either Ala-6 or Ala-15. For all constructs associated with cholesterol-containing membranes, the chemical shifts of both residues correlated with beta strand conformation while association with membranes without cholesterol resulted in a mixture of helical and beta strand conformations. The dependence of fusion rate on oligomer size is independent of membrane cholesterol content, so one interpretation of the data is fusion activity of both helical and beta strand conformations. Membrane location may be a determinant of fusion activity, and for all constructs in both conformations, a large fraction of the Ala-15 (13)CO groups were 5-6 A from the (31)P atoms in the lipid headgroups, while the Ala-6 (13)CO groups were more distant.

The Membrane-proximal Fusion Domain of HIV-1 GP41 Reveals Sequence-specific and Fine-tuning Mechanism of Membrane Binding

The membrane interface-partitioning region preceding the transmembrane anchor of the human immunodeficiency virus type 1 (HIV-1) gp41 envelope protein is one of the sites responsible for virus binding to its host cell membrane and subsequent fusion events. Here, we used molecular modeling techniques to assess membrane interactions, structure, and hydrophobic properties of the fusion-active peptide representing this region, several of its homologs from different HIV-1 strains, as well as a peptide - defective gp41 phenotype - unable to mediate cell-cell fusion and virus entry. It is shown that the wild-type peptides bind to the water-membrane interface in alpha-helical conformation, while the mutant adopts partly destabilized helix-break-helix structure on the membrane surface. The wild-type peptides reveal specific "tilted oblique-oriented" pattern of hydrophobicity on their surfaces - the property specific for fusion regions of other viruses. Fusion peptides penetrate into the membrane with their N-termini and reveal "fine-tuning" interactions with membrane and water environments: the shift of this balance (e.g., due to point mutations) may dramatically change the mode of membrane binding, and therefore, may cause loss of fusion activity. The modeling results agree well with experimental data and provide a strategy to delineate fusogenic regions in amino acid sequences of viral proteins.

Chemical shift assignment and structural plasticity of a HIV fusion peptide derivative in dodecylphosphocholine micelles

A "HFPK3" peptide containing the 23 residues of the human immunodeficiency virus (HIV) fusion peptide (HFP) plus three non-native C-terminal lysines was studied in dodecylphosphocholine (DPC) micelles with 2D 1H NMR spectroscopy. The HFP is at the N-terminus of the gp41 fusion protein and plays an important role in fusing viral and target cell membranes which is a critical step in viral infection. Unlike HFP, HFPK3 is monomeric in detergent-free buffered aqueous solution which may be a useful property for functional and structural studies. H alpha chemical shifts indicated that DPC-associated HFPK3 was predominantly helical from I4 to L12. In addition to the highest-intensity crosspeaks used for the first chemical shift assignment (denoted I), there were additional crosspeaks whose intensities were approximately 10% of those used for assignment I. A second assignment (II) for residues G5 to L12 as well as a few other residues was derived from these lower-intensity crosspeaks. Relative to the I shifts, the II shifts were different by 0.01-0.23 ppm with the largest differences observed for HN. Comparison of the shifts of DPC-associated HFPK3 with those of detergent-associated HFP and HFP derivatives provided information about peptide structures and locations in micelles.

Solid-state nuclear magnetic resonance measurements of HIV fusion peptide to lipid distances reveal the intimate contact of beta strand peptide with membranes and the proximity of the Ala-14-Gly-16 region with lipid headgroups

Human immunodeficiency virus (HIV) infection begins with fusion between viral and host cell membranes and is catalyzed by the HIV gp41 fusion protein. The approximately 20 N-terminal apolar residues of gp41 are called the HIV fusion peptide (HFP), interact with the host cell membrane, and play a key role in fusion. In this study, the membrane location of peptides which contained the HFP sequence (AVGIGALFLGFLGAAGSTMGARS) was probed in samples containing either only phospholipids or phospholipids and cholesterol. Four HFPs were examined which each contained 13CO labeling at three sequential residues between G5 and G16. The 13CO chemical shifts indicated that HFP had predominant beta strand conformation over the labeled residues in the samples. The internuclear distances between the HFP 13CO groups and the lipid 31P atoms were measured using solid-state nuclear magnetic resonance rotational-echo double-resonance experiments. The shortest 13CO-31P distances of 5-6 A were observed for HFP labeled between A14 and G16 and correlated with intimate association of beta strand HFP and membranes. These results were confirmed with measurements using HFPs singly labeled with 13CO at A6 or A14. To our knowledge, these data are the first measurements of distances between HIV fusion peptide nuclei and lipid P, and qualitative models of the membrane location of oligomeric beta strand HFP which are consistent with the experimental data are presented. Observation of intimate contact between beta strand HFP and membranes provides a rationale for further investigation of the relationship between structure and fusion activity for this conformation.

Conformational flexibility and strand arrangements of the membrane-associated HIV fusion peptide trimer probed by solid-state NMR spectroscopy

The human immunodeficiency virus (HIV) fusion peptide (HFP) is the N-terminal apolar region of the HIV gp41 fusion protein and interacts with target cell membranes and promotes membrane fusion. The free peptide catalyzes vesicle fusion at least to the lipid mixing stage and serves as a useful model fusion system. For gp41 constructs which lack the HFP, high-resolution structures show trimeric protein and suggest that at least three HFPs interact with the membrane with their C-termini in close proximity. In addition, previous studies have demonstrated that HFPs which are cross-linked at their C-termini to form trimers (HFPtr) catalyze fusion at a rate which is 15-40 times greater than that of non-cross-linked HFP. In the present study, the structure of membrane-associated HFPtr was probed with solid-state nuclear magnetic resonance (NMR) methods. Chemical shift and intramolecular (13)CO-(15)N distance measurements show that the conformation of the Leu-7 to Phe-11 region of HFPtr has predominant helical conformation in membranes without cholesterol and beta strand conformation in membranes containing approximately 30 mol % cholesterol. Interstrand (13)CO-(13)CO and (13)CO-(15)N distance measurements were not consistent with an in-register parallel strand arrangement but were consistent with either (1) parallel arrangement with adjacent strands two residues out-of-register or (2) antiparallel arrangement with adjacent strand crossing between Phe-8 and Leu-9. Arrangement 1 could support the rapid fusion rate of HFPtr because of placement of the apolar N-terminal regions of all strands on the same side of the oligomer while arrangement 2 could support the assembly of multiple fusion protein trimers.

The N-terminal 12 Residue Long Peptide of HIV gp41 is the Minimal Peptide Sufficient to Induce Significant T-cell-like Membrane Destabilization in Vitro

Here, we predicted the minimal N-terminal fragment of gp41 required to induce significant membrane destabilization using IMPALA. This algorithm is dedicated to predict peptide interaction with a membrane. We based our prediction of the minimal fusion peptide on the tilted peptide theory. This theory proposes that some protein fragments having a peculiar distribution of hydrophobicity adopt a tilted orientation at a hydrophobic/hydrophilic interface. As a result of this orientation, tilted peptides should disrupt the interface. We analysed in silico the membrane-interacting properties of gp41 N-terminal peptides of different length derived from the isolate BRU and from an alignment of 710 HIV strains available on the Los Alamos National Laboratory. Molecular modelling results indicated that the 12 residue long peptide should be the minimal fusion peptide. We then assayed lipid-mixing and leakage of T-cell-like liposomes with N-terminal peptides of different length as first challenge of our predictions. Experimental results confirmed that the 12 residue long peptide is necessary and sufficient to induce membrane destabilization to the same extent as the 23 residue long fusion peptide. In silico analysis of some fusion-incompetent mutants presented in the literature further revealed that they cannot insert into a modelled membrane correctly tilted. According to this work, the tilted peptide model appears to explain at least partly the membrane destabilization properties of HIV fusion peptide.

Interaction of Fusion Peptides from HIV gp41 with Membranes: A Time-Resolved Membrane Binding, Lipid Mixing, and Structural Study

The interaction of the so-called fusion peptide of the human immunodeficiency virus gp41 envelope glycoprotein with the target cell membrane is believed to trigger the fusion process which allows the entry of the virus into the cell. Many studies on the interaction of the fusion peptide with biological membranes have been carried out using synthetic peptides and model membranes. Due to the variety of experimental systems and sequences used, some controversy exists, concerning mainly the type of structure which triggers membrane destabilization and fusion (alpha helix or beta structure). With the aim of contributing to shed some light on the subject we have undertaken a series of experiments on the interaction of the three most representative fusion sequences with model membranes under the same experimental conditions. The results show that the fusion peptides, which adopt an unordered structure when dissolved in DMSO, form a mixture of aggregated beta and helical + unordered structures in aqueous buffer. Model membranes are shown to enhance the formation of aggregated beta structures. The nature of the membrane binding event, the kinetics of the binding and lipid mixing processes, and the kinetics of the structural changes depend on whether both ends of the fusion sequence or just one bears a positive charge. Analysis of the kinetic data shows that lipid mixing depends on the transformation of unordered + helical structures into aggregated beta structures upon binding to the membrane.

Structure and orientation study of fusion peptide FP23 of gp41 from HIV-1 alone or inserted into various lipid membrane models (mono-, bi- and multibi-layers) by FT-IR spectroscopies and Brewster angle microscopy

In the present work, we study the structure and the orientation of the 23 N-terminal peptide of the HIV-1 gp 41 protein (AVGIGALFLGFLGAAGSTMGARS) called FP23. The behaviour of FP23 was investigated alone at the air/water interface and inserted into various lipid model systems: in monolayer or multibilayers of a DOPC/cholesterol/DOPE/DOPG (6/5/3/2) and in a DMPC bilayer. PMIRRAS and polarized ATR spectroscopy coupled with Brewster angle microscopy and spectral simulations were used to precisely determine the structure and the orientation of the peptide in its environment as well as the lipid perturbations induced by the FP23 insertion. The infra-red results show the structural polymorphism of the FP23 and its ability to transit quasi irreversibly from an alpha-helix to antiparallel beta-sheets. At the air/water interface, the transition is induced by compression of the peptide alone and is modulated by compression and lipid to peptide ratio (Ri) when FP23 is inserted into a lipid monolayer. In multibilayers and in a single bilayer, there is coexistence in quasi equal proportions of alpha-helix and antiparallel beta-sheets of FP23 at low peptide content (Ri=100, 200) while antiparallel beta-sheets are predominant at high FP23 concentration (Ri=50). In (multi)bilayer systems, evaluation of dichroic ratios and sprectral simulations show that both the alpha-helix and the antiparallel beta-sheets are tilted at diluted FP23 concentrations (tilt angle of alpha-helix with respect to the normal of the interface=36.5+/-3.0 degrees for FP23 in multibilayers of DOPC/Chol/DOPE/DOPG at Ri=200 and 39.0+/-5.0 degrees in a single bilayer of DMPC at Ri=100 and tilt angle of the beta-sheets=36.0+/-2.0 degrees for the beta-sheets in multibilayers and 30.0+/-2.0 degrees in the lipid bilayer). In parallel, the FP23 induces an increase of the lipid chain disorder which shows both by an increase of the methylene stretching frequencies and an increase of the average C-C-C angle of the acyl chains. At high FP23 content (Ri=50), the antiparallel beta-sheets induce a complete disorganization of the lipid chains in (multi)bilayers.

Interfacial folding and membrane insertion of designed peptides studied by molecular dynamics simulations

The mechanism of interfacial folding and membrane insertion of designed peptides is explored by using an implicit membrane generalized Born model and replica-exchange molecular dynamics. Folding/insertion simulations initiated from fully extended peptide conformations in the aqueous phase, at least 28 A away from the membrane interface, demonstrate a general mechanism for structure formation and insertion (when it occurs). The predominately hydrophobic peptides from the synthetic WALP and TMX series first become localized at the membrane-solvent interface where they form significant helical secondary structure via a helix-turn-helix motif that inserts the central hydrophobic residues into the membrane interior, and then fluctuations occur that provide a persistent helical structure throughout the peptide and it inserts with its N-terminal end moving across the membrane. More specifically, we observed that: (i) the WALP peptides (WALP16, WALP19, and WALP23) spontaneously insert in the membrane as just noted; (ii) TMX-1 also inserts spontaneously after a similar mechanism and forms a transmembrane helix with a population of approximately 50% at 300 K; and (iii) TMX-3 does not insert, but exists in a fluctuating membrane interface-bound form. These findings are in excellent agreement with available experimental data and demonstrate the potential for new implicit solvent/membrane models together with advanced simulation protocols to guide experimental programs in exploring the nature and mechanism of membrane-associated folding and insertion of biologically important peptides.

Modeling amyloid beta-peptide insertion into lipid bilayers

Inspired by recent suggestions that the Alzheimer's amyloid beta peptide (Abeta) can insert into cell membranes and form harmful ion channels, we model insertion of the 40- and 42-residue forms of the peptide into cell membranes using a Monte Carlo code which is specific at the amino acid level. We examine insertion of the regular Abeta peptide as well as mutants causing familial Alzheimer's disease, and find that all but one of the mutants change the insertion behavior by causing the peptide to spend more simulation steps in only one leaflet of the bilayer. We also find that Abeta42, because of the extra hydrophobic residues relative to Abeta40, is more likely to adopt this conformation than Abeta40 in both wild-type and mutant forms. We argue qualitatively why these effects happen. Here, we present our results and develop the hypothesis that this partial insertion increases the probability of harmful channel formation. This hypothesis can partly explain why these mutations are neurotoxic simply due to peptide insertion behavior. We further apply this model to various artificial Abeta mutants which have been examined experimentally, and offer testable experimental predictions contrasting the roles of aggregation and insertion with regard to toxicity of Abeta mutants. These can be used through further experiments to test our hypothesis.

The interactions of the HIV gp41 fusion peptides with zwitterionic membrane mimics determined by NMR spectroscopy

The wild-type (wt) N-terminal 23-residue fusion peptide (FP) of the human immunodeficiency virus (HIV) fusion protein gp41 and its V2E mutant have been studied by nuclear magnetic resonance (NMR) spectroscopy in dodecylphosphocholine (DPC) micelles as membrane mimics. A number of NMR techniques have been used. Pulsed field-gradient diffusion measurements in DPC and in 4:1 DPC/sodium dodecylsulfate mixed micelles showed that there is no major difference between the partition coefficients of the fusogenic wt peptide and the V2E mutant in these micelles, indicating that there is no correlation between the activity of the fusion peptides and their membrane affinities. The nuclear Overhauser enhancement (NOE) patterns and the chemical shift index for these two peptides indicated that both FP are in an alpha helical conformation between the Ile4 to Leu12 or to Ala15 region. Simulated annealing showed that the helical region extends from Ile4 to Met19. The two FPs share similar conformational characteristics, indicating that the conformation of the FP is not an important factor determining its activity. The spin-label studies, utilizing spin labels 5- and 16-doxystearic acids in the DPC micelles, provided clear indication that the wt FP inserts its N-terminus into the micelles while the V2E mutant does not insert into the micelles. The conclusion from the spin-label results is corroborated by deuterium amide proton exchange experiments. The correlation between the oblique insertion of the FP and its fusogenic activity is in excellent agreement with results from our molecular dynamics simulation and from other previous studies.

Solid-state nuclear magnetic resonance studies of HIV and influenza fusion peptide orientations in membrane bilayers using stacked glass plate samples

The human immunodeficiency virus (HIV) and influenza virus fusion peptides are approximately 20-residue sequences which catalyze the fusion of viral and host cell membranes. The orientations of these peptides in lipid bilayers have been probed with 15N solid-state nuclear magnetic resonance (NMR) spectroscopy of samples containing membranes oriented between stacked glass plates. Each of the peptides adopts at least two distinct conformations in membranes (predominantly helical or beta strand) and the conformational distribution is determined in part by the membrane headgroup and cholesterol composition. In the helical conformation, the 15N spectra suggest that the influenza peptide adopts an orientation approximately parallel to the membrane surface while the HIV peptide adopts an orientation closer to the membrane bilayer normal. For the beta strand conformation, there appears to be a broader peptide orientational distribution. Overall, the data suggest that the solid-state NMR experiments can test models which correlate peptide orientation with their fusogenic function.

Oligomeric beta-structure of the membrane-bound HIV-1 fusion peptide formed from soluble monomers

The human immunodeficiency virus type 1 (HIV-1) fusion peptide serves as a useful model system for understanding viral/target cell fusion, at least to the lipid mixing stage. Previous solid-state NMR studies have shown that the peptide adopts an oligomeric beta-strand structure when associated with a lipid and cholesterol mixture close to that of membranes of host cells of the virus. In this study, this structure was further investigated using four different peptide constructs. In aqueous buffer solution, two of the constructs were primarily monomeric whereas the other two constructs had significant populations of oligomers/aggregates. NMR measurements for all membrane-associated peptide constructs were consistent with oligomeric beta-strand structure. Thus, constructs that are monomeric in solution can be converted to oligomers as a result of membrane association. In addition, samples prepared by very different methods had very similar NMR spectra, which indicates that the beta-strand structure is an equilibrium rather than a kinetically trapped structure. Lipid mixing assays were performed to assess the fusogenicities of the different constructs, and there was not a linear correlation between the solution oligomeric state and fusogenicity. However, the functional assays do suggest that small oligomers may be more fusogenic than either monomers or large aggregates.

Conformational mapping of the N-terminal peptide of HIV-1 gp41 in lipid detergent and aqueous environments using 13C-enhanced Fourier transform infrared spectroscopy

The N-terminal domain of HIV-1 glycoprotein 41,000 (gp41) participates in viral fusion processes. Here, we use physical and computational methodologies to examine the secondary structure of a peptide based on the N terminus (FP; residues 1-23) in aqueous and detergent environments. (12)C-Fourier transform infrared (FTIR) spectroscopy indicated greater alpha-helix for FP in lipid-detergent sodium dodecyl sulfate (SDS) and aqueous phosphate-buffered saline (PBS) than in only PBS. (12)C-FTIR spectra also showed disordered FP conformations in these two environments, along with substantial beta-structure for FP alone in PBS. In experiments that map conformations to specific residues, isotope-enhanced FTIR spectroscopy was performed using FP peptides labeled with (13)C-carbonyl. (13)C-FTIR results on FP in SDS at low peptide loading indicated alpha-helix (residues 5 to 16) and disordered conformations (residues 1-4). Because earlier (13)C-FTIR analysis of FP in lipid bilayers demonstrated alpha-helix for residues 1-16 at low peptide loading, the FP structure in SDS micelles only approximates that found for FP with membranes. Molecular dynamics simulations of FP in an explicit SDS micelle indicate that the fraying of the first three to four residues may be due to the FP helix moving to one end of the micelle. In PBS alone, however, electron microscopy of FP showed large fibrils, while (13)C-FTIR spectra demonstrated antiparallel beta-sheet for FP (residues 1-12), analogous to that reported for amyloid peptides. Because FP and amyloid peptides each exhibit plaque formation, alpha-helix to beta-sheet interconversion, and membrane fusion activity, amyloid and N-terminal gp41 peptides may belong to the same superfamily of proteins.

Interaction of a synthetic peptide corresponding to the N terminus of canine distemper virus fusion protein with phospholipid vesicles: a biophysical study

The F protein of canine distemper virus (CDV) is a classic type I glycoprotein formed by two polypeptides, F1 and F2. The N-terminal regions of the F1 polypeptides of CDV, measles virus and other paramyxoviruses present moderate to high homology, supporting the existence of a high conservation within these structures, which emphasises its role in viral-host cell membrane fusion. This N-terminal polypeptide is usually termed the fusion peptide. The fusion peptides of most viral fusion-mediating glycoproteins contain a high proportion of hydrophobic amino acids, which facilitates its insertion into target membranes during fusion. In this work we report on the interaction of a 31-residue synthetic peptide (FP31) corresponding to the N terminus of CDV F1 protein with phospholipid membranes composed of various phospholipids, as studied by means of various biophysical techniques. FTIR investigation of FP31 secondary structure in aqueous medium and in membranes resulted in a major proportion of alpha-helical structure which increased upon membrane insertion. Differential scanning calorimetry (DSC) showed that the presence of concentrations of FP31 as low as 0.1 mol%, in mixtures with L-alpha-dimyristoylphosphatidylcholine (DMPC), L-alpha-dipalmitoylphosphatidylcholine (DPPC) and L-alpha-distearoylphosphatidylcholine (DSPC), already affected the thermotropic properties of the gel to liquid-crystalline phase transition. In mixtures with the three lipids, increasing the concentration of peptide made the pretransition to disappear, and lowered and broadened the main transition. This effect was slightly stronger as the acyl chain length of the phospholipid grew larger. In the corresponding partial phase diagrams, no immiscibilities or critical points were observed. FTIR showed that incorporation of 1 mol% of peptide in DPPC shifted the antisymmetric and symmetric CH2 stretching bands to higher values, indicating the existence of an additional disordering of the acyl chain region of the fluid bilayer. FTIR studies of the Cz=O stretching band indicated that incorporation of FP31 into phosphatidylcholine membranes produced a strong dehydration of the polar part of the bilayer. In mixtures with L-alpha-dielaidoylphosphatidylethanolamine (DEPE), increasing FP31 concentrations broadened and shifted to lower temperatures the lamellar to hexagonal-HII phase transition, indicating that this peptide destabilized the bilayer and promoted formation of the hexagonal-HII phase. The results are discussed in terms of lipid-peptide hydrophobic mismatch and its influence on the role of the N-terminal polypeptide of CDV F1 protein in viral membrane fusion.

Solid-State Nuclear Magnetic Resonance Evidence for Parallel and Antiparallel Strand Arrangements in the Membrane-Associated HIV-1 Fusion Peptide

The HIV-1 fusion peptide serves as a useful model system for understanding viral/target cell fusion, at least to the lipid-mixing stage. Previous solid-state NMR studies have shown that the membrane-bound HIV-1 fusion peptide adopts an extended conformation in a lipid mixture close to that of host cells of the virus. In the present study, solid-state NMR REDOR methods were applied for detection of oligomeric beta strand structure. The samples were prepared under fusogenic conditions and contained equimolar amounts of two peptides, one with selective [(13)C]carbonyl labeling and the other with selective [(15)N]amide labeling. In the REDOR measurements, observation of reduced (13)C intensity due to hydrogen-bonded amide (15)N provides strong experimental evidence of oligomer formation by the membrane-associated peptide. Comparison of REDOR spectra on samples that were labeled at different residue positions suggests that there are both parallel and antiparallel arrangements of peptide strands. In the parallel arrangement, interpeptide hydrogen bonding decreases toward the C-terminus, while in the antiparallel arrangement, hydrogen bonds are observed along the entire length of residues which was probed (Gly-5 to Gly-16). For the parallel arrangement, these observations are consistent with the model in which the apolar N-terminal and central regions of the peptides penetrate into the membrane and hydrogen bond with one another while the polar C-terminus of the peptide is outside the membrane and hydrogen bonds with water. These measurements show that, at least at the end state of fusion, the peptide can adopt an oligomeric beta strand structure.