Folded monomers and hexamers of the ectodomain of the HIV gp41 membrane fusion protein: potential roles in fusion and synergy between the fusion peptide, hairpin, and membrane-proximal external region

A large HIV gp41 construct with trimer-of-hairpins structure exhibits V2E mutation-dominant attenuation of vesicle fusion and helicity very similar to V2E attenuation of HIV fusion and infection and supports: (1) hairpin stabilization of membrane apposition with larger distance for V2E; and (2) V2E dominance by an antiparallel β sheet with interleaved fusion peptide strands from two gp41 trimers

There is complete attenuation of fusion and infection mediated by HIV gp160 with gp41 subunit with V2E mutation, and also V2E dominance with WT/V2E mixtures. V2E is at the N-terminus of the ∼25-residue fusion peptide (Fp) which likely binds the target membrane. In this study, large V2E attenuation and dominance were observed for vesicle fusion induced by FP_HM, a large gp41 ectodomain construct with Fp followed by hyperthermostable hairpin with N- and C-helices, and membrane-proximal external region (Mper). FP_HM is a trimer-of-hairpins, the final gp41 structure during fusion. Vesicle fusion and helicity were measured for FP_HM using trimers with different fractions (f's) of WT and V2E proteins. Reductions in FP_HM fusion and helicity vs. fV2E were quantitatively-similar to those for gp160-mediated fusion and infection. Global fitting of all V2E data supports 6 WT gp41 (2 trimers) required for fusion. These data are understood by a model in which the ∼25 kcal/mol free energy for initial membrane apposition is compensated by the thermostable hairpin between the Fp in target membrane and Mper/transmembrane domain in virus membrane. The data support a structural model for V2E dominance with a membrane-bound Fp with antiparallel β sheet and interleaved strands from the two trimers. Relative to fV2E = 0, a longer Fp sheet is stabilized with small fV2E because of salt-bridge and/or hydrogen bonds between E2 on one strand and C-terminal Fp residues on adjacent strands, like R22. A longer Fp sheet results in shorter N- and C-helices, and larger separation during membrane apposition which hinders fusion.

Peptide Triazole Thiol Irreversibly Inactivates Metastable HIV-1 Env by Accessing Conformational Triggers Intrinsic to Virus-Cell Entry

KR13, a peptide triazole thiol previously established to inhibit HIV-1 infection and cause virus lysis, was evaluated by flow cytometry against JRFL Env-presenting cells to characterize induced Env and membrane transformations leading to irreversible inactivation. Transiently transfected HEK293T cells were preloaded with calcein dye, treated with KR13 or its thiol-blocked analogue KR13b, fixed, and stained for gp120 (35O22), MPER (10E8), 6-helix-bundle (NC-1), immunodominant loop (50-69), and fusion peptide (VRC34.01). KR13 induced dose-dependent transformations of Env and membrane characterized by transient poration, MPER exposure, and 6-helix-bundle formation (analogous to native fusion events), but also reduced immunodominant loop and fusion peptide exposure. Using a fusion peptide mutant (V504E), we found that KR13 transformation does not require functional fusion peptide for poration. In contrast, simultaneous treatment with fusion inhibitor T20 alongside KR13 prevented membrane poration and MPER exposure, showing that these events require 6-helix-bundle formation. Based on these results, we formulated a model for PTT-induced Env transformation portraying how, in the absence of CD4/co-receptor signaling, PTT may provide alternate means of perturbing the metastable Env-membrane complex, and inducing fusion-like transformation. In turn, the results show that such transformations are intrinsic to Env and can be diverted for irreversible inactivation of the protein complex.

Insights into the mechanism of membrane fusion induced by the plant defense element, plant-specific insert

In plants, many natural defense mechanisms include cellular membrane fusion as a way to resist infection by external pathogens. Several plant proteins mediate membrane fusion, but the detailed mechanism by which they promote fusion is less clear. Understanding this process could provide valuable insights into these proteins' physiological functions and guide bioengineering applications (i.e. the design of antimicrobial proteins). The plant-specific insert (PSI) from Solanum tuberosum can help reduce certain pathogen attack via membrane fusion. To gain new insights into the process of PSI-induced membrane fusion, a combined approach of NMR, FRET, and in silico studies was used. Our results indicate that (i) under acidic conditions, the PSI experiences a monomer-dimer equilibrium, and the dimeric PSI induces membrane fusion below a certain critical pH; (ii) after fusion, the PSI resides in a highly dehydrated environment with limited solvent accessibility, suggesting its capability in reducing repulsive dehydration forces between liposomes to facilitate fusion; and (iii) as shown by molecular dynamics simulations, the PSI dimer can bind stably to membrane surfaces and can bridge liposomes in close proximity, a critical step for the membrane fusion. In summary, this study provides new and unique insights into the mechanisms by which the PSI and similar proteins induce membrane fusion.

2H nuclear magnetic resonance spectroscopy supports larger amplitude fast motion and interference with lipid chain ordering for membrane that contains β sheet human immunodeficiency virus gp41 fusion peptide or helical hairpin influenza virus hemagglutinin fusion peptide at fusogenic pH

Enveloped viruses are surrounded by a membrane which is obtained from an infected host cell during budding. Infection of a new cell requires joining (fusion) of the virus and cell membranes. This process is mediated by a monotopic viral fusion protein with a large ectodomain outside the virus. The ectodomains of class I enveloped viruses have a N-terminal "fusion peptide" (fp) domain that is critical for fusion and binds to the cell membrane. In this study, ²H NMR spectra are analyzed for deuterated membrane with fp from either HIV gp41 (GP) or influenza hemagglutinin (HA) fusion proteins. In addition, the HAfp samples are studied at more fusogenic pH 5 and less fusogenic pH 7. GPfp adopts intermolecular antiparallel β sheet structure whereas HAfp is a monomeric helical hairpin. The data are obtained for a set of temperatures between 35 and 0 °C using DMPC-d54 lipid with perdeuterated acyl chains. The DMPC has liquid-crystalline (L_α) phase with disordered chains at higher temperature and rippled gel (P_β') or gel phase (L_β') with ordered chains at lower temperature. At given temperature T, the no peptide and HAfp, pH 7 samples exhibit similar spectral lineshapes. Spectral broadening with reduced temperature correlates with the transition from L_α to P_β' and then L_β' phases. At given T, the lineshapes are narrower for HAfp, pH 5 vs. no peptide and HAfp, pH 7 samples, and even narrower for the GPfp sample. These data support larger-amplitude fast (>10⁵ Hz) lipid acyl chain motion for samples with fusogenic peptides, and peptide interference with chain ordering. The NMR data of the present paper correlate with insertion of these peptides into the hydrocarbon core of the membrane and support a significant fusion contribution from the resultant lipid acyl chain disorder, perhaps because of reduced barriers between the different membrane topologies in the fusion pathway. Membrane insertion and lipid perturbation appear common to both β sheet and helical hairpin peptides.

Conformational and lipid bilayer-perturbing properties of Marburg virus GP2 segments containing the fusion loop and membrane-proximal external region/transmembrane domain

Fusion of host and viral membranes is a crucial step during infection by enveloped viruses. In the structurally-defined "class I″ viral glycoproteins, the formation of a highly stable α-helical bundle by the ectodomain of the fusion subunit (e.g., GP2 for Marburg virus, MARV) is postulated to provide the energetic driving force to overcome barriers associated with membrane fusion. Upon cell binding, the fusion subunit is proposed to form an extended intermediate that bridges both the viral and host membranes, and collapse of this extended intermediate brings the two membranes into proximity. While there is much high-resolution structural data available for prefusion and post-fusion structures of viral glycoproteins, little information is available about intermediate conformations especially in the context of the fusion loop/peptide (FL or FP) and membrane-proximal external region (MPER)/transmembrane (TM) segments. We present structural and functional studies on segments of MARV GP2 that encompass the FL and MPER/TM in detergent micelles and lipid bicelles. A protein that contains most elements of GP2 ("MGP2-full") is α-helical in membrane-mimicking environments and has pH-dependent membrane lytic activity. MGP2-full is monomeric under such conditions, contrasting with the trimeric species that has been described previously for MARV GP2 ectodomain in aqueous buffer. Variants of MARV GP2 containing the N- and C-terminal halves ("MGP2-FNL" and "MGP2-CMT", respectively) have similar properties. This work provides novel insight into conformational and membrane-perturbing properties of the MARV fusion subunit and how they may relate to viral membrane fusion.

Studies of membranotropic and fusogenic activity of two putative HCV fusion peptides

Over the past decades, membranotropic peptides such as positively charged cell-penetrating peptides (CPPs) or amphipathic antimicrobial peptides (AMPs) have received increasing interest in order to improve therapeutic agent cellular uptake. As far as we are concerned, we were interested in studying HCV fusion peptides as putative anchors. Two peptides, HCV6 and HCV7, were identified and conjugated to a fluorescent tag NBD and tested for their interaction with liposomes as model membranes. DSC and spectrofluorescence analyses demonstrate HCV7 propensity to insert or internalize in vesicles containing anionic lipids DMPG whereas no activity was observed with zwitterionic DMPC. This behavior could be explained by the peptide sequence containing a cationic arginine residue. On the contrary, HCV6 did not exhibit any membranotropic activity but was the only sequence able to induce liposomes' fusion or aggregation monitored by spectrofluorescence and DLS. This two peptides mild activity was related to their inefficient structuration in contact with membrane mimetics, which was demonstrated by CD and NMR experiments. Altogether, our data allowed us to identify two promising membrane-active peptides from E1 and E2 HCV viral proteins, one fusogenic (HCV6) and the other membranotropic (HCV7). The latter was also confirmed by fluorescence microscopy with CHO cells, indicating that HCV7 could cross the plasma membrane via an endocytosis process. Therefore, this study provides new evidences supporting the identification of HCV6 as the HCV fusion peptide as well as insights on a novel membranotropic peptide from the HCV-E2 viral protein.

The Stabilities of the Soluble Ectodomain and Fusion Peptide Hairpins of the Influenza Virus Hemagglutinin Subunit II Protein Are Positively Correlated with Membrane Fusion

Cellular entry of influenza virus is mediated by the viral protein hemagglutinin (HA), which forms an initial complex of three HA1 and three HA2 subunits. Each HA2 includes a fusion peptide (FP), a soluble ectodomain (SE), and a transmembrane domain. HA1 binds to cellular sialic acids, followed by virus endocytosis, pH reduction, dissociation of HA1, and structural rearrangement of HA2 into a final trimer-of-SE hairpins. A decrease in pH also triggers HA2-mediated virus/endosome membrane fusion. SE hairpins have an interior parallel helical bundle and C-terminal strands in the grooves of the exterior of the bundle. FPs are separate helical hairpins. This study compares wild-type HA2 (WT-HA2) with G1E(FP) and I173E(SE strand) mutants. WT-HA2 induces vesicle fusion at pH 5.0, whereas the extent of fusion is greatly reduced for both mutants. Circular dichroism for HA2 and FHA2≡FP+SE constructs shows dramatic losses of stability for the mutants, including a T_m reduced by 40 °C for I173E-FHA2. This is evidence of destabilization of SE hairpins via dissociation of strands from the helical bundle, which is also supported by larger monomer fractions for mutant versus WT proteins. The G1E mutant may have disrupted FP hairpins, with consequent non-native FP binding to dissociated SE strands. It is commonly proposed that free energy released by the HA2 structural rearrangement catalyzes HA-mediated fusion. This study supports an alternate mechanistic model in which fusion is preceded by FP insertion in the target membrane and formation of the final SE hairpin. Less fusion by the mutants is due to the loss of hairpin stability and consequent reduced level of membrane apposition of the virus and target membranes.

HIV-1 gp41 transmembrane oligomerization monitored by FRET and FCS

The HIV-1 envelope gp120/gp41 trimer mediates viral membrane fusion. After cluster of differentiation-4 recognition, gp120 detaches from the virus, exposing gp41 which triggers fusion. During the fusion process, gp41 may not remain trimeric, which could have functional importance. Here, we probe the reversible association of full length gp41 (minus the cytoplasmic domain) in detergent micelles (with probes attached to transmembrane domain) by fluorescence resonance energy transfer (FRET) with a μm dissociation constant. This is compared with other methods. A gp41-targeted fusion inhibitor must interfere with this transition, and monomeric, partially monomeric or trimeric states all present potential binding epitopes. The gp41 self-association is a valid drug target model and FRET, a potential high-throughput assay system, could be used to screen drug libraries.

Efficient Fusion at Neutral pH by Human Immunodeficiency Virus gp41 Trimers Containing the Fusion Peptide and Transmembrane Domains

Human immunodeficiency virus (HIV) is membrane-enveloped, and an initial infection step is joining/fusion of viral and cell membranes. This step is catalyzed by gp41, which is a single-pass integral viral membrane protein. The protein contains an ∼170-residue ectodomain located outside the virus that is important for fusion and includes the fusion peptide (FP), N-helix, loop, C-helix, and viral membrane-proximal external region (MPER). The virion initially has noncovalent complexes between three gp41 ectodomains and three gp120 proteins. A gp120 contains ∼500 residues and functions to identify target T-cells and macrophages via binding to specific protein receptors of the target cell membrane. gp120 moves away from the gp41 ectodomain, and the ectodomain is thought to bind to the target cell membrane and mediate membrane fusion. The secondary and tertiary structures of the ectodomain are different in the initial complex with gp120 and the final state without gp120. There is not yet imaging of gp41 during fusion, so the temporal relationship between the gp41 and membrane structures is not known. This study describes biophysical and functional characterization of large gp41 constructs that include the ectodomain and transmembrane domain (TM). Significant fusion is observed of both neutral and anionic vesicles at neutral pH, which reflects the expected conditions of HIV/cell fusion. Fusion is enhanced by the FP, which in HIV/cell fusion likely contacts the host membrane, and the MPER and TM, which respectively interfacially contact and traverse the HIV membrane. Initial contact with vesicles is made by protein trimers that are in a native oligomeric state that reflects the initial complex with gp120 and also is commonly observed for the ectodomain without gp120. Circular dichroism data support helical structure for the N-helix, C-helix, and MPER and nonhelical structure for the FP and loop. Distributions of monomer, trimer, and hexamer states are observed by size-exclusion chromatography (SEC), with dependences on solubilizing detergent and construct. These SEC and other data are integrated into a refined working model of HIV/cell fusion that includes dissociation of the ectodomain into gp41 monomers followed by folding into hairpins that appose the two membranes, and subsequent fusion catalysis by trimers and hexamers of hairpins. The monomer and oligomer gp41 states may therefore satisfy dual requirements for HIV entry of membrane apposition and fusion.

Molecular and Physicochemical Factors Governing Solubility of the HIV gp41 Ectodomain

The HIV gp41 ectodomain (e-gp41) is an attractive target for the development of vaccines and drugs against HIV because of its crucial role in viral fusion to the host cell. However, because of the high insolubility of e-gp41, most biophysical and structural analyses have relied on the production of truncated versions removing the loop region of gp41 or the utilization of nonphysiological solubilizing conditions. The loop region of gp41 is also known as principal immunodominant domain (PID) because of its high immunogenicity, and it is essential for gp41-mediated HIV fusion. In this study we identify the aggregation-prone regions of the amino acid sequence of the PID and engineer a highly soluble mutant that preserves the trimeric structure of the wild-type e-gp41 under physiological pH. Furthermore, using a reverse mutagenesis approach, we analyze the role of mutated amino acids upon the physicochemical factors that govern solubility of e-gp41. On this basis, we propose a molecular model for e-gp41 self-association, which can guide the production of soluble e-gp41 mutants for future biophysical analyses and biotechnological applications.

Full-length trimeric influenza virus hemagglutinin II membrane fusion protein and shorter constructs lacking the fusion peptide or transmembrane domain: Hyperthermostability of the full-length protein and the soluble ectodomain and fusion peptide make significant contributions to fusion of membrane vesicles

Influenza virus is a class I enveloped virus which is initially endocytosed into a host respiratory epithelial cell. Subsequent reduction of the pH to the 5-6 range triggers a structural change of the viral hemagglutinin II (HA2) protein, fusion of the viral and endosomal membranes, and release of the viral nucleocapsid into the cytoplasm. HA2 contains fusion peptide (FP), soluble ectodomain (SE), transmembrane (TM), and intraviral domains with respective lengths of ∼ 25, ∼ 160, ∼ 25, and ∼ 10 residues. The present work provides a straightforward protocol for producing and purifying mg quantities of full-length HA2 from expression in bacteria. Biophysical and structural comparisons are made between full-length HA2 and shorter constructs including SHA2 ≡ SE, FHA2 ≡ FP+SE, and SHA2-TM ≡ SE+TM constructs. The constructs are helical in detergent at pH 7.4 and the dominant trimer species. The proteins are highly thermostable in decylmaltoside detergent with Tm>90 °C for HA2 with stabilization provided by the SE, FP, and TM domains. The proteins are likely in a trimer-of-hairpins structure, the final protein state during fusion. All constructs induce fusion of negatively-charged vesicles at pH 5.0 with much less fusion at pH 7.4. Attractive protein/vesicle electrostatics play a role in fusion, as the proteins are positively-charged at pH 5.0 and negatively-charged at pH 7.4 and the pH-dependence of fusion is reversed for positively-charged vesicles. Comparison of fusion between constructs supports significant contributions to fusion from the SE and the FP with little effect from the TM.

Complete dissociation of the HIV-1 gp41 ectodomain and membrane proximal regions upon phospholipid binding

The envelope glycoprotein gp41 mediates the process of membrane fusion that enables entry of the HIV-1 virus into the host cell. Strong lipid affinity of the ectodomain suggests that its heptad repeat regions play an active role in destabilizing membranes by directly binding to the lipid bilayers and thereby lowering the free-energy barrier for membrane fusion. In such a model, immediately following the shedding of gp120, the N-heptad and C-heptad helices dissociate and melt into the host cell and viral membranes, respectively, pulling the destabilized membranes into juxtaposition, ready for fusion. Post-fusion, reaching the final 6-helix bundle (6 HB) conformation then involves competition between intermolecular interactions needed for formation of the symmetric 6 HB trimer and the membrane affinity of gp41's ectodomain, including its membrane-proximal regions. Our solution NMR study of the structural and dynamic properties of three constructs containing the ectodomain of gp41 with and without its membrane-proximal regions suggests that these segments do not form inter-helical interactions until the very late steps of the fusion process. Interactions between the polar termini of the heptad regions, which are not associating with the lipid surface, therefore may constitute the main driving force initiating formation of the final post-fusion states. The absence of significant intermolecular ectodomain interactions in the presence of dodecyl phosphocholine highlights the importance of trimerization of gp41's transmembrane helix to prevent complete dissociation of the trimer during the course of fusion.

REDOR solid-state NMR as a probe of the membrane locations of membrane-associated peptides and proteins

Rotational-echo double-resonance (REDOR) solid-state NMR is applied to probe the membrane locations of specific residues of membrane proteins. Couplings are measured between protein (13)CO nuclei and membrane lipid or cholesterol (2)H and (31)P nuclei. Specific (13)CO labeling is used to enable unambiguous assignment and (2)H labeling covers a small region of the lipid or cholesterol molecule. The (13)CO-(31)P and (13)CO-(2)H REDOR respectively probe proximity to the membrane headgroup region and proximity to specific insertion depths within the membrane hydrocarbon core. One strength of the REDOR approach is use of chemically-native proteins and membrane components. The conventional REDOR pulse sequence with 100 kHz (2)H π pulses is robust with respect to the (2)H quadrupolar anisotropy. The (2)H T1's are comparable to the longer dephasing times (τ's) and this leads to exponential rather than sigmoidal REDOR buildups. The (13)CO-(2)H buildups are well-fitted to A×(1-e(-γτ)) where A and γ are fitting parameters that are correlated as the fraction of molecules (A) with effective (13)CO-(2)H coupling d=3γ/2. The REDOR approach is applied to probe the membrane locations of the "fusion peptide" regions of the HIV gp41 and influenza virus hemagglutinin proteins which both catalyze joining of the viral and host cell membranes during initial infection of the cell. The HIV fusion peptide forms an intermolecular antiparallel β sheet and the REDOR data support major deeply-inserted and minor shallowly-inserted molecular populations. A significant fraction of the influenza fusion peptide molecules form a tight hairpin with antiparallel N- and C-α helices and the REDOR data support a single peptide population with a deeply-inserted N-helix. The shared feature of deep insertion of the β and α fusion peptide structures may be relevant for fusion catalysis via the resultant local perturbation of the membrane bilayer. Future applications of the REDOR approach may include samples that contain cell membrane extracts and use of lower temperatures and dynamic nuclear polarization to reduce data acquisition times.

Conditional trimerization and lytic activity of HIV-1 gp41 variants containing the membrane-associated segments

Fusion of host and viral membranes is a critical step during infection by membrane-bound viruses. The HIV-1 glycoproteins gp120 (surface subunit) and gp41 (fusion subunit) represent the prototypic system for studying this process; in the prevailing model, the gp41 ectodomain forms a trimeric six-helix bundle that constitutes a critical intermediate and provides the energetic driving force for overcoming barriers associated with membrane fusion. However, most structural studies of gp41 variants have been performed either on ectodomain constructs lacking one or more of the membrane-associated segments (the fusion peptide, FP, the membrane-proximal external region, MPER, and the transmembrane domain, TM) or on variants consisting of these isolated segments alone without the ectodomain. Several recent reports have suggested that the HIV-1 ectodomain, as well as larger construct containing the membrane-bound segments, dissociates from a trimer to a monomer in detergent micelles. Here we compare the properties of a series of gp41 variants to delineate the roles of the ectodomain, FP, and MPER and TM, all in membrane-mimicking environments. We find that these proteins are prone to formation of a monomer in detergent micelles. In one case, we observed exclusive monomer formation at pH 4 but conditional trimerization at pH 7 even at low micromolar (∼5 μM) protein concentrations. Liposome release assays demonstrate that these gp41-related proteins have the capacity to induce content leakage but that this activity is also strongly modulated by pH with much higher activity at pH 4. Circular dichroism, nuclear magnetic resonance, and binding assays with antibodies specific to the MPER provide insight into the structural and functional roles of the FP, MPER, and TM and their effect on structure within the larger context of the fusion subunit.