Three-dimensional solution structure of the 44 kDa ectodomain of SIV gp41

Sedimentation velocity studies of the high-molecular weight aggregates of the HIV gp41 ectodomain

Accumulation of the HIV envelope protein gp41 is observed in the brain tissues of patients suffering from HIV-associated dementia. Previously, we have shown by electron microscopy that the extracellular domain of SIV gp41, which is analogous to that of HIV, forms high-molecular weight aggregates in vitro, and we have postulated that such high-molecular weight aggregates are responsible for neurological damage in a manner similar to protein deposition diseases such as Alzheimer's and the prion diseases. In this manuscript, we have characterized the self-association of the HIV gp41 ectodomain by sedimentation velocity. We show that discreet species of the gp41 high-molecular weight aggregates are present. The maximum population occurs at 20 S, which corresponds to ~5 trimers of gp41, suggesting that five trimers are required for nucleation of the high-molecular weight aggregates. The concentration dependence of the gp41 self-association indicates that it occurs by mass action. The temperature dependence of gp41 self-association suggests that it is driven by entropy, indicating that intermolecular hydrophobic interactions between trimers of gp41 are driving the association.

Temperature-dependent intermediates in HIV-1 envelope glycoprotein-mediated fusion revealed by inhibitors that target N- and C-terminal helical regions of HIV-1 gp41

Peptides derived from the N- (N-HR) and C- (C-HR) terminal heptad repeat regions adjacent to the fusion peptide and transmembrane domains, respectively, of human immunodeficiency virus (HIV)-1 gp41 inhibit HIV-1 viral envelope glycoproteins (Env)-mediated cell fusion specifically. The mechanism of HIV-1 Env-mediated cell fusion and its inhibition by agents that target the N- and C-HR regions was investigated. Priming experiments with Env-expressing cells indicate that the N-HR region but not the C-HR region is exposed by treatment with sCD4 at 31 degrees C, whereas both the N- and C-HR regions are exposed at 37 degrees C.

Peptide inhibitors of virus-cell fusion: enfuvirtide as a case study in clinical discovery and development

The peptidic antiretroviral enfuvirtide (Fuzeon) is the first clinically approved antiviral fusion inhibitor and the first antiretroviral that must routinely be administered parenterally. Its extracellular activity results both in activity against current drug-resistant strains of HIV-1 and a low potential for systemic toxicities. As a peptide, enfuvirtide also exhibits few interactions with other antiretrovirals and concomitant medications used in HIV disease. Enfuvirtide shows potent antiretroviral activity and significantly improves medical outcomes in highly treatment-experienced patients with HIV-1 infection, but like other antiretrovirals must be given as part of a carefully selected combination regimen to minimise the risk of emergent drug resistance. Despite its subcutaneous route of administration, clinical data indicate that most patients can accept long-term enfuvirtide treatment with little difficulty or impact on daily activities. The only common adverse event associated with enfuvirtide use is injection-site reactions of generally mild-to-moderate severity, which are seldom treatment-limiting.

Expression and biochemical analysis of the entire HIV-2 gp41 ectodomain: determinants of stability map to N- and C-terminal sequences outside the 6-helix bundle core

The folding of HIV gp41 into a 6-helix bundle drives virus-cell membrane fusion. To examine the structural relationship between the 6-helix bundle core domain and other regions of gp41, we expressed in Escherichia coli, the entire ectodomain of HIV-2(ST) gp41 as a soluble, trimeric maltose-binding protein (MBP)/gp41 chimera. Limiting proteolysis indicated that the Cys-591-Cys-597 disulfide-bonded region is outside a core domain comprising two peptides, Thr-529-Trp-589 and Val-604-Ser-666. A biochemical examination of MBP/gp41 chimeras encompassing these core peptides indicated that the N-terminal polar segment, 521-528, and C-terminal membrane-proximal segment, 658-666, cooperate in stabilizing the ectodomain. A functional interaction between sequences outside the gp41 core may contribute energy to membrane fusion.

The stability of the intact envelope glycoproteins is a major determinant of sensitivity of HIV/SIV to peptidic fusion inhibitors

C-peptides derived from the HIV envelope glycoprotein transmembrane subunit gp41 C-terminal heptad repeat (C-HR) region are potent HIV fusion inhibitors. These peptides interact with the gp41 N-terminal heptad repeat (N-HR) region and block the gp41 six-helix bundle formation that is required for fusion. However, the parameters that govern this inhibition have yet to be elucidated. We address this issue by comparing the ability of C34, derived from HIV-1, HIV-2 and SIV gp41, to inhibit HIV-1, HIV-2 and SIV envelope-mediated fusion and the ability of these peptides to form stable six-helix bundles with N36 peptides derived from gp41 of these three viruses. The ability to form six-helix bundles was examined by circular dichroism spectroscopy, and HIV/SIV Env-mediated membrane fusion was monitored by a dye transfer assay. HIV-1 N36 formed stable helix bundles with HIV-1, HIV-2 and SIV C34, which all inhibited HIV-1 Env-mediated fusion at IC(50)<10nM. The three C34 peptides were poor inhibitors of HIV-2 and SIV fusion (IC(50)>100nM), although HIV-2 and SIV N36 formed stable helix bundles with SIV C34. Priming experiments with sCD4 indicate that, in contrast to HIV-1, HIV-2 and SIV Env do not expose their N-HR region to SIV C34 following CD4 binding, but rapidly proceed to co-receptor engagement and six-helix bundle formation resulting in fusion. Our results suggest that several factors, including six-helix bundle stability and the ability of CD4 to destabilize the envelope glycoprotein, serve as determinants of sensitivity to entry inhibitors.

Role of hydrophobic residues in the central ectodomain of gp41 in maintaining the association between human immunodeficiency virus type 1 envelope glycoprotein subunits gp120 and gp41

The interaction between the gp120 and gp41 subunits of the human immunodeficiency virus envelope glycoprotein serves to stabilize the virion form of the complex and to transmit receptor-induced conformational changes in gp120 to trigger the membrane fusion activity of gp41. In this study, we used site-directed mutagenesis to identify amino acid residues in the central ectodomain of gp41 that contribute to the stability of the gp120-gp41 association. We identified alanine mutations at six positions, including four tryptophan residues, which result in mutant envelope glycoprotein complexes that fail to retain gp120 on the cell surface. These envelope glycoproteins readily shed their gp120 and are unable to mediate cell-cell fusion. These findings suggest an important role for the conserved bulky hydrophobic residues in stabilizing the gp120-gp41 complex.

HIV-1 gp41 and gp160 are hyperthermostable proteins in a mesophilic environment. Characterization of gp41 mutants

HIV gp41(24-157) unfolds cooperatively over the pH range of 1.0-4.0 with T(m) values of > 100 degrees C. At pH 2.8, protein unfolding was 80% reversible and the DeltaH(vH)/DeltaH(cal) ratio of 3.7 is indicative of gp41 being trimeric. No evidence for a monomer-trimer equilibrium in the concentration range of 0.3-36 micro m was obtained by DSC and tryptophan fluorescence. Glycosylation of gp41 was found to have only a marginal impact on the thermal stability. Reduction of the disulfide bond or mutation of both cysteine residues had only a marginal impact on protein stability. There was no cooperative unfolding event in the DSC thermogram of gp160 in NaCl/P(i), pH 7.4, over a temperature range of 8-129 degrees C. When the pH was lowered to 5.5-3.4, a single unfolding event at around 120 degrees C was noted, and three unfolding events at 93.3, 106.4 and 111.8 degrees C were observed at pH 2.8. Differences between gp41 and gp160, and hyperthermostable proteins from thermophile organisms are discussed. A series of gp41 mutants containing single, double, triple or quadruple point mutations were analysed by DSC and CD. The impact of mutations on the protein structure, in the context of generating a gp41 based vaccine antigen that resembles a fusion intermediate state, is discussed. A gp41 mutant, in which three hydrophobic amino acids in the gp41 loop were replaced with charged residues, showed an increased solubility at neutral pH.

Structural basis for coronavirus-mediated membrane fusion. Crystal structure of mouse hepatitis virus spike protein fusion core

The surface transmembrane glycoprotein is responsible for mediating virion attachment to cell and subsequent virus-cell membrane fusion. However, the molecular mechanisms for the viral entry of coronaviruses remain poorly understood. The crystal structure of the fusion core of mouse hepatitis virus S protein, which represents the first fusion core structure of any coronavirus, reveals a central hydrophobic coiled coil trimer surrounded by three helices in an oblique, antiparallel manner. This structure shares significant similarity with both the low pH-induced conformation of influenza hemagglutinin and fusion core of HIV gp41, indicating that the structure represents a fusion-active state formed after several conformational changes. Our results also indicate that the mechanisms for the viral fusion of coronaviruses are similar to those of influenza virus and HIV. The coiled coil structure has unique features, which are different from other viral fusion cores. Highly conserved heptad repeat 1 (HR1) and HR2 regions in coronavirus spike proteins indicate a similar three-dimensional structure among these fusion cores and common mechanisms for the viral fusion. We have proposed the binding regions of HR1 and HR2 of other coronaviruses and a structure model of their fusion core based on our mouse hepatitis virus fusion core structure and sequence alignment. Drug discovery strategies aimed at inhibiting viral entry by blocking hairpin formation may be applied to the inhibition of a number of emerging infectious diseases, including severe acute respiratory syndrome.

Identification of membrane-active regions of the HIV-1 envelope glycoprotein gp41 using a 15-mer gp41-peptide scan

The identification of membrane-active regions of the ectodomain of the HIV-1 envelope glycoprotein gp41 has been made by determining the effect on membrane integrity of a 15-mer gp41-derived peptide library. By monitoring the effect of this peptide library on membrane leakage, we have identified three regions on the gp41 ectodomain with membrane-interacting capabilities: Region 1, which would roughly correspond to the polar sequence which follows the fusion domain and extends to the N-terminal heptad repeat region; Region 2, which would correspond to the immunodominant loop; and Region 3, which would correspond to the pre-transmembrane region of gp41. The identification of these three regions supports their direct role in membrane fusion as well as facilitating the future development of HIV-1 entry inhibitors.

Evolution of the HIV-1 envelope glycoproteins with a disulfide bond between gp120 and gp41

BACKGROUND

We previously described the construction of an HIV-1 envelope glycoprotein complex (Env) that is stabilized by an engineered intermolecular disulfide bond (SOS) between gp120 and gp41. The modified Env protein antigenically mimics the functional wild-type Env complex. Here, we explore the effects of the covalent gp120 - gp41 interaction on virus replication and evolution.

RESULTS

An HIV-1 molecular clone containing the SOS Env gene was only minimally replication competent, suggesting that the engineered disulfide bond substantially impaired Env function. However, virus evolution occurred in cell culture infections, and it eventually always led to elimination of the intermolecular disulfide bond. In the course of these evolution studies, we identified additional and unusual second-site reversions within gp41.

CONCLUSIONS

These evolution paths highlight residues that play an important role in the interaction between gp120 and gp41. Furthermore, our results suggest that a covalent gp120 - gp41 interaction is incompatible with HIV-1 Env function, probably because this impedes conformational changes that are necessary for fusion to occur, which may involve the complete dissociation of gp120 from gp41.

Structural characterization of the SARS-coronavirus spike S fusion protein core

The spike (S) glycoprotein of coronaviruses mediates viral entry into host cells. It is a type 1 viral fusion protein that characteristically contains two heptad repeat regions, denoted HR-N and HR-C, that form coiled-coil structures within the ectodomain of the protein. Previous studies have shown that the two heptad repeat regions can undergo a conformational change from their native state to a 6-helix bundle (trimer of dimers), which mediates fusion of viral and host cell membranes. Here we describe the biophysical analysis of the two predicted heptad repeat regions within the severe acute respiratory syndrome coronavirus S protein. Our results show that in isolation the HR-N region forms a stable α-helical coiled coil that associates in a tetrameric state. The HR-C region in isolation formed a weakly stable trimeric coiled coil. When mixed together, the two peptide regions (HR-N and HR-C) associated to form a very stable α-helical 6-stranded structure (trimer of heterodimers). Systematic peptide mapping showed that the site of interaction between the HR-N and HR-C regions is between residues 916–950 of HR-N and residues 1151–1185 of HR-C. Additionally, interchain disulfide bridge experiments showed that the relative orientation of the HR-N and HR-C helices in the complex was antiparallel. Overall, the structure of the hetero-stranded complex is consistent with the structures observed for other type 1 viral fusion proteins in their fusion-competent state.

Role of the ectodomain of the gp41 transmembrane envelope protein of human immunodeficiency virus type 1 in late steps of the membrane fusion process

The membrane fusion process mediated by the gp41 transmembrane envelope glycoprotein of the human immunodeficiency virus type 1 (HIV-1) was addressed by a flow cytometry assay detecting exchanges of fluorescent membrane probes (DiI and DiO) between cells expressing the HIV-1 envelope proteins (Env) and target cells. Double-fluorescent cells were detected when target cells expressed the type of chemokine receptor, CXCR4 or CCR5, matching the type of gp120 surface envelope protein, X4 or R5, respectively. Background levels of double-fluorescent cells were observed when the gp120-receptor interaction was blocked by AMD3100, a CXCR4 antagonist. The L568A mutation in the N-terminal heptad repeat (HR1) of gp41 resulted in parallel inhibition of the formation of syncytia and double-fluorescent cells, indicating that gp41 had a direct role in the exchange of fluorescent probes. In contrast, three mutations in the loop region of the gp41 ectodomain, located on either side of the Cys-(X)(5)-Cys motif (W596 M and W610A) or at the distal end of HR1 (D589L), had limited or no apparent effect on membrane lipid mixing between Env(+) and target cells, while they blocked formation of syncytia and markedly reduced the exchanges of cytoplasmic fluorescent probes. The loop region could therefore have a direct or indirect role in events occurring after the merging of membranes, such as the formation or dilation of fusion pores. Two types of inhibitors of HIV-1 entry, the gp41-derived peptide T20 and the betulinic acid derivative RPR103611, had limited effects on membrane exchanges at concentrations blocking or markedly reducing syncytium formation. This finding confirmed that T20 can inhibit the late steps of membrane fusion (post-lipid mixing) and brought forth an indirect argument for the role of the gp41 loop region in these steps, as mutations conferring resistance to RPR103611V were mapped in this region (I595S or L602H).

Completion of trimeric hairpin formation of influenza virus hemagglutinin promotes fusion pore opening and enlargement

For influenza virus hemagglutinin, an N-cap structure, created at low pH, interacts with membrane-proximal residues (173-178), bringing fusion peptides and membrane-spanning domains close together. Mutational analysis was used to define the role of these interactions in membrane fusion. For all N-cap mutants, both lipid and aqueous dye spread was greatly reduced. Mutation at residues that interact with the N-cap did not reduce levels of fusion, except for substitutions made at residue I173. For N-cap and I173 mutants, the addition of chlorpromazine greatly promoted transfer of aqueous dye. Electrical capacitance measurements confirmed that fusion pores usually did not form for the I173 mutants. Thus, neither N-cap formation nor interactions with segment 173-178 are needed for hemifusion, but are required for reliable formation and enlargement of the fusion pore. It is proposed that binding of I173 into a deep hydrophobic cavity within the coiled-coil promotes the transition from hemifusion to fusion.

Electron tomography analysis of envelope glycoprotein trimers on HIV and simian immunodeficiency virus virions

We used electron tomography to directly visualize trilobed presumptive envelope (env) glycoprotein structures on the surface of negatively stained HIV type 1 (HIV-1) and simian immunodeficiency virus (SIV) virions. Wild-type HIV-1 and SIV virions had an average of 8-10 trimers per virion, consistent with predictions based on biochemical evidence. Mutant SIVs, biochemically demonstrated to contain high levels of the viral env proteins, averaged 70-79 trimers per virion in tomograms. These correlations strongly indicate that the visualized trimers represent env spikes. The env trimers were without obvious geometric distribution pattern or preferred rotational orientation. Combined with biochemical analysis of gag/env ratios in virions, these trimer counts allow calculation of the number of gag molecules per virion, yielding an average value of approximately 1400. Virion and env dimensions were also determined. Image-averaging analysis of SIV env trimers revealed a distinct chirality and strong concordance with recent molecular models. The results directly demonstrate the presence of env trimers on the surface of AIDS virus virions, albeit at numbers much lower than generally appreciated, and have important implications for understanding virion formation, virus interactions with host cells, and virus neutralization.

Role of the Fusion Peptide and Membrane-Proximal Domain in HIV-1 Envelope Glycoprotein-Mediated Membrane Fusion

The N-terminal fusion peptide and the interfacial sequence preceding the transmembrane anchor of HIV-1 gp41 are required for viral fusion. Studies with synthetic peptides indicated that these regions function by destabilizing membranes, which is regarded as a crucial step in the membrane fusion reaction. However, it is not clear whether membrane destabilization is induced by these sequences in the intact gp41. We address this question by examining fusion and destabilization of membranes expressing HIV-1(IIIB) wild-type Env and two mutant Envs. (1) A Glu residue at position 2 of the gp41 fusion peptide is substituted for Val (V2E) to produce one mutant. (2) Residues 665-682 in the membrane-proximal domain are deleted to form the other. The process of membrane destabilization was monitored by the influx of Sytox, an impermeant fluorescent dye, into the Env-expressing cells following the interaction with CD4-CXCR4 complexes, and fusion was monitored by observing dye transfer between Env-expressing cells and appropriate target cells. We also monitored the conformational changes in the Envs following their interactions with CD4 and CXCR4 by immunofluorescence using an anti-gp41 mAb that reacts with the six-helix bundle. In contrast to the wild type, both Env mutants did not mediate cell fusion. The V2E Env did not mediate membrane destabilization. However, the Env with an unmodified fusion peptide but with a deletion of residues 665-682 in the membrane-proximal domain did mediate membrane destabilization. The wild type and both mutant Envs undergo conformational changes detected by the anti-gp41 six-helix bundle mAbs. Our results suggest that in intact HIV-1 Env the membrane-proximal domain is not required for membrane perturbations, but rather enables the bending of gp41 that is required for viral and target membranes to come together. Moreover, the observation that the Delta665-683 Env self-inserts its fusion peptide but does not cause fusion suggests that self-insertion of the fusion peptide is not sufficient for HIV-1 Env-mediated fusion.

A novel monoclonal antibody specific to the C-terminal tail of the gp41 envelope transmembrane protein of human immunodeficiency virus type 1 that preferentially neutralizes virus after it has attached to the target cell and inhibits the production of infectious progeny

SAR1 is a new IgG2a murine monoclonal antibody derived by immunization with a plant virus expressing the sequence GERDRDR from the C-terminal tail of the gp41 transmembrane glycoprotein of human immunodeficiency virus type 1 (HIV-1). SAR1 binds to peptides and proteins carrying the GERDRDR sequence, to some but not all preparations of purified virus, and to cells infected with all viruses tested. In a standard neutralization assay, SAR1 failed to neutralize, or neutralized poorly, a number of T cell line-adapted viruses. However, it was more effective at postattachment neutralization. This was measured by two assays, the inhibition of the syncytium production by input virus, and the inhibition of the production of infectious progeny virus. In general SAR1 was more effective at neutralizing progeny virus than inoculum virus. Fifty percent inhibition of progeny virus production by different HIV-1 strains was obtained with 2-26 microg/ml of SAR1. The SAR1 neutralizing epitope was mapped specifically to the gp41 C-terminal tail. SAR1 is an unusual, if not unique, antibody whose activity supports the view that part of the gp41 C-terminal tail is exposed on the outside of the virion.

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.

Determination of the HIV-1 gp41 fusogenic core conformation modeled by synthetic peptides: applicable for identification of HIV-1 fusion inhibitors

Triggered by receptor binding of gp120, the human immunodeficiency virus type 1 (HIV-1) gp41 changes its conformation to a fusogenic six-helix bundle structure. In the present study, this core conformation modeled by the peptides derived from the gp41 N- and C-terminal heptad repeat regions was determined by fluorescence native polyacrylamide gel electrophoresis and size exclusion high-performance liquid chromatography (HPLC). Two previously described small molecule HIV-1 fusion inhibitors significantly blocked the six-helix bundle formation. It suggests that these biophysical techniques can be used in a novel way to study the conformational change of gp41 during virus entry into cells and to identify HIV-1 fusion inhibitors.

The coronavirus spike protein is a class I virus fusion protein: structural and functional characterization of the fusion core complex

Coronavirus entry is mediated by the viral spike (S) glycoprotein. The 180-kDa oligomeric S protein of the murine coronavirus mouse hepatitis virus strain A59 is posttranslationally cleaved into an S1 receptor binding unit and an S2 membrane fusion unit. The latter is thought to contain an internal fusion peptide and has two 4,3 hydrophobic (heptad) repeat regions designated HR1 and HR2. HR2 is located close to the membrane anchor, and HR1 is some 170 amino acids (aa) upstream of it. Heptad repeat (HR) regions are found in fusion proteins of many different viruses and form an important characteristic of class I viral fusion proteins. We investigated the role of these regions in coronavirus membrane fusion. Peptides HR1 (96 aa) and HR2 (39 aa), corresponding to the HR1 and HR2 regions, were produced in Escherichia coli. When mixed together, the two peptides were found to assemble into an extremely stable oligomeric complex. Both on their own and within the complex, the peptides were highly alpha helical. Electron microscopic analysis of the complex revealed a rod-like structure approximately 14.5 nm in length. Limited proteolysis in combination with mass spectrometry indicated that HR1 and HR2 occur in the complex in an antiparallel fashion. In the native protein, such a conformation would bring the proposed fusion peptide, located in the N-terminal domain of HR1, and the transmembrane anchor into close proximity. Using biological assays, the HR2 peptide was shown to be a potent inhibitor of virus entry into the cell, as well as of cell-cell fusion. Both biochemical and functional data show that the coronavirus spike protein is a class I viral fusion protein.

The 3D structure of the fusion primed Sendai F-protein determined by electron cryomicroscopy

The three dimensional (3D) structure of the ectodomain of the entire fusion mediating F protein from Sendai virus [MW (trimer) approximately 177 kDa] has been determined by cryoelectron microscopy of single molecules and subsequent 3D reconstruction at a resolution of approximately 16 A. The reconstruction, which has been obtained from the native, proteolytic processed fusion primed F1+F2 form, shows the protein protruding approximately 170 A out of the membrane in a homotrimeric association. It consists of a defined approximately 65 A wide distal head and an adjacent neck, which is connected to an 70 A elongated stalk. Although the overall shape appears to be similar to the recently reported X-ray structure of the Newcastle disease virus F protein, a closer comparison reveals structural differences suggesting that the investigated Sendai F structure represents an advanced state towards the fusion active conformation.

Membrane fusion: a structural perspective on the interplay of lipids and proteins

The fusion of biological membranes is governed by the carefully orchestrated interplay of membrane proteins and lipids. Recently determined structures of fusion proteins, individual domains of fusion proteins and their complexes with regulatory proteins and membrane lipids have yielded much suggestive insight into how viral and intracellular membrane fusion might proceed. These structures may be combined with new knowledge on the fusion of pure lipid bilayer membranes in an attempt to begin to piece together the complex puzzle of how biological membrane fusion machines operate on membranes.

Structural analysis of the epitope of the anti-HIV antibody 2F5 sheds light into its mechanism of neutralization and HIV fusion

Inhibition of human immunodeficiency virus (HIV) fusion with the host cell has emerged as a viable therapeutic strategy, and rational design of inhibitors and vaccines, interfering with this process, is a prime target for antiviral research. To advance our knowledge of the structural biology of HIV fusion, we have studied the membrane-proximal region of the fusogenic envelope subunit gp41, which includes the epitope ELDKWA of the broadly neutralizing human antibody 2F5. The structural evidence available for this region is contradictory, with some studies suggesting an overall helical conformation, while the X-ray structure of the ELDKWAS peptide bound to the antibody shows it folded in a type I beta turn. We used a two-step strategy: Firstly, by a competition binding assay, we identified the proper boundaries of the domain recognized by 2F5, which we found considerably larger than the ELDKWAS hexapeptide. Secondly, we studied the structure of the resulting 13 amino acid residue peptide by collecting NMR data and analyzing them by our previously developed statistical method (NAMFIS). Our study revealed that the increase in binding affinity goes in parallel with stabilization of specific local and global conformational propensities, absent from the shorter epitope. When compounded with the available biological evidence, our structural analysis allows us to propose a specific role for the membrane-proximal region during HIV fusion, in terms of a conformational transition between the turn and the helical structure. At the same time, our hypothesis offers a structural explanation for the mechanism of neutralization of mAb 2F5.

Viral fusion proteins: multiple regions contribute to membrane fusion

In recent years, the simple picture of a viral fusion protein interacting with the cell and/or viral membranes by means of only two localized segments (i.e. the fusion peptide and the transmembrane domain) has given way to a more complex picture in which multiple regions from the viral proteins interact with membranes. Indeed, possible roles in membrane binding and/or destabilization have been postulated for the N-terminal heptad repeats, pre-transmembrane segments, and other internal regions of fusion proteins from distant viruses (such as orthomyxo-, retro-, paramyxo-, or flaviviruses). This review focuses on the experimental evidence and functional models postulated so far about the role of these regions in the process of virus-induced membrane fusion.

The HIV Env-mediated fusion reaction

The current general model of HIV viral entry involves the binding of the trimeric viral envelope glycoprotein gp120/gp41 to cell surface receptor CD4 and chemokine co-receptor CXCR4 or CCR5, which triggers conformational changes in the envelope proteins. Gp120 then dissociates from gp41, allowing for the fusion peptide to be inserted into the target membrane and the pre-hairpin configuration of the ectodomain to form. The C-terminal heptad repeat region and the leucine/isoleucine zipper region then form the thermostable six-helix coiled-coil, which drives the membrane merger and eventual fusion. This model needs updating, as there has been a wealth of data produced in the last few years concerning HIV entry, including target cell dependencies, fusion kinetic data, and conformational intermediates. A more complete model must include the involvement of membrane microdomains, actin polymerization, glycosphingolipids, and possibly CD4 and chemokine signaling in entry. In addition, kinetic experiments involving the addition of fusion inhibitors have revealed some of the rate-limiting steps in this process, adding a temporal component to the model. A review of these data that may require an updated version of the original model is presented here.

Structural features of paramyxovirus F protein required for fusion initiation

On the basis of the coordinates of the related Newcastle disease virus (NDV) F protein, Valine-94, a determinant of measles virus (MV) cytopathicity, is predicted to lie in a cylindrical cavity with 10 A diameter located at the F neck. A 16-residue domain around V94 is functionally interchangeable between NDV and MV F, supporting our homology model. Features of the cavity are conserved within the Paramyxovirinae. A hydrophobic base and a hydrophilic residue at the rim are required for surface expression. Small residue substitutions predicted to open the cavity were found to disrupt transport or limit fusogenicity of transport-competent mutants but can be compensated for by simultaneous insertion of larger residues at the opposing wall. Variants containing histidine substitutions mediate fusion at pH 8.5, while at pH 7.2 fusion is blocked, suggesting that functionality requires low charge in the cavity. These results indicate that specific structural features of the cavity are essential for paramyxovirus fusion initiation.

C-terminal octylation rescues an inactive T20 mutant: implications for the mechanism of HIV/SIMIAN immunodeficiency virus-induced membrane fusion

T20, a synthetic peptide corresponding to a C-terminal segment of the envelope glycoprotein (gp41) of human and simian immunodeficiency viruses, is a potent inhibitor of viral infection. We report here that C-terminal octylation of simian immunodeficiency virus gp41-derived T20 induces a significant increase in its inhibitory potency. Furthermore, when C-terminally octylated, an otherwise inactive mutant in which the C-terminal residues GNWF were replaced by ANAA has potency similar to that of the wild type T20. This effect cannot be explained by a trivial inhibitory effect of the octyl group added to the peptides, because the N-terminally octylated peptides have the same activity as the non-octylated parent peptides. The effects caused by octylation on the oligomerization, secondary structure, and membrane-interaction properties of the peptides were investigated. Our results shed light on the mechanism of inhibition by T20 and provide experimental support for the existence of a pre-hairpin intermediate.

Covalent trimers of the internal N-terminal trimeric coiled-coil of gp41 and antibodies directed against them are potent inhibitors of HIV envelope-mediated cell fusion

We have engineered two soluble, covalently linked, trimeric polypeptides, N35CCG-N13 and N34CCG comprising only the internal trimeric coiled-coil of the ectodomain of HIV-1 gp41. Both trimers inhibit human immunodeficiency virus, type 1 (HIV-1) envelope (Env)-mediated cell fusion at nanomolar concentrations by targeting the exposed C-terminal region of the gp41 ectodomain in the prehairpin intermediate state. The IC50 values for N35CCG-N13 and N34CCG are approximately 15 and approximately 95 nM, respectively, in a quantitative vaccinia virus-based reporter gene assay for HIV-1 Env-mediated cell fusion using Env from the T cell tropic strain LAV. Polyclonal antibodies were raised against N35CCG-N13 and a tightly binding fraction of anti-N35CCG-N13 inhibits T cell and macrophage tropic HIV-1 Env-mediated cell fusion with respective IC50 values of approximately 0.5 and approximately 1.5 microg/ml at 37 degrees C. The tightly binding anti-N35CCG-N13 antibody fraction targets the exposed internal trimeric coiled-coil in the prehairpin intermediate state of gp41 in a manner analogous to peptides derived from the C region of the gp41 ectodomain. The potency of the tightly binding anti-N35CCG-N13 antibody fraction in the fusion assay is comparable with that of the broadly neutralizing monoclonal antibody 2G12. These results indicate that N35CCG-N13 is a potential anti-HIV therapeutic agent and represents a suitable immunogen for the generation of neutralizing monoclonal antibodies targeted to the internal trimeric coiled-coil of gp41. The data on the tightly binding anti-N35CCG-N13 antibody fraction demonstrate that the internal trimeric coiled-coil of gp41 in the prehairpin intermediate state is accessible to antibodies and that access is not restricted by either antibody size or the presence of a kinetic barrier.

Molecular features of the broadly neutralizing immunoglobulin G1 b12 required for recognition of human immunodeficiency virus type 1 gp120

IgG1 b12 is a broadly neutralizing antibody against human immunodeficiency virus type 1 (HIV-1). The epitope recognized by b12 overlaps the CD4 receptor-binding site (CD4bs) on gp120 and has been a target for vaccine design. Determination of the three-dimensional structure of immunoglobulin G1 (IgG1) b12 allowed modeling of the b12-gp120 interaction in which the protruding third complementarity-determining region (CDR) of the heavy chain (H3) was crucial for antibody binding. In the present study, extensive mutational analysis of the antigen-binding site of Fab b12 was carried out to investigate the validity of the model and to identify residues important for gp120 recognition and, by inference, key to the anti-HIV-1 activity of IgG1 b12. In all, 50 mutations were tested: 40 in H3, 4 each in H2 and L1, and 2 in L3. The results suggest that the interaction of gp120 with H3 of b12 is crucially dependent not only on a Trp residue at the apex of the H3 loop but also on a number of residues at the base of the loop. The arrangement of these residues, including aromatic side chains and side chains that hydrogen bond across the base of the loop, may rigidify H3 for penetration of the recessed CD4-binding cavity. The results further emphasize the importance to gp120 binding of a Tyr residue at the apex of the H2 loop that forms a second finger-like structure and a number of Arg residues in L1 that form a positively charged, shelf-like structure. In general, the data are consistent with the b12-gp120 interaction model previously proposed. At the gene level, somatic mutation is seen to be crucial for the generation of many of the structural features described. The Fab b12 mutants were also tested against the b12 epitope-mimic peptide B2.1, and the reactivity profile had many similarities but also significant differences from that observed for gp120. The paratope map of b12 may facilitate the design of molecules that are able to elicit b12-like activities.

Membrane fusion induced by vesicular stomatitis virus depends on histidine protonation

Entry of enveloped animal viruses into their host cells always depends on a step of membrane fusion triggered by conformational changes in viral envelope glycoproteins. Vesicular stomatitis virus (VSV) infection is mediated by virus spike glycoprotein G, which induces membrane fusion at the acidic environment of the endosomal compartment. VSV-induced membrane fusion occurs at a very narrow pH range, between 6.2 and 5.8, suggesting that His protonation is required for this process. To investigate the role of His in VSV fusion, we chemically modified these residues using diethylpyrocarbonate (DEPC). We found that DEPC treatment inhibited membrane fusion mediated by VSV in a concentration-dependent manner and that the complete inhibition of fusion was fully reversed by incubation of modified virus with hydroxylamine. Fluorescence measurements showed that VSV modification with DEPC abolished pH-induced conformational changes in G protein, suggesting that His protonation drives G protein interaction with the target membrane at acidic pH. Mass spectrometry analysis of tryptic fragments of modified G protein allowed the identification of the putative active His residues. Using synthetic peptides, we showed that the modification of His-148 and His-149 by DEPC, as well as the substitution of these residues by Ala, completely inhibited peptide-induced fusion, suggesting the direct participation of these His in VSV fusion.

Mutations in the putative HR-C region of the measles virus F2 glycoprotein modulate syncytium formation

The fusion (F) glycoproteins of measles virus strains Edmonston (MV-Edm) and wtF (MV-wtF) confer distinct cytopathic effects and strengths of hemagglutinin (H) interaction on a recombinant MV-Edm virus. They differ in just two amino acids, V94 and V101 in F-Edm versus M94 and F101 in F-wtF, both of which lie in the relatively uncharacterized F(2) domain. By comparing the sequence of MV F with those of the parainfluenza virus SV5 and Newcastle disease virus (NDV) F proteins, the structures of which are known, we show that MV F(2) also possesses a potential heptad repeat (HR) C domain. In NDV, the N-terminal half of HR-C interacts with HR-A in F(1) while the C-terminal half is induced to kink outward by a central proline residue. We found that this proline is part of an LXP motif conserved in all three viruses. Folding and transport of MV F require this motif to be intact and also require covalent interaction of cysteine residues that probably support the potential HR-A-HR-C interaction. Amino acids 94 and 101, both located in "d" positions of the HR-C helical wheel, lie in the potentially outwardly kinked region. We demonstrate that their effect on MV fusogenicity and glycoprotein interaction is mediated solely by amino acid 94. Substitutions at position 94 with polar or charged amino acids are tolerated poorly or not at all, while changes to smaller and more hydrophilic amino acids are tolerated in both transiently expressed F protein and recombinant virus. MV F V94A and MV F V94G viruses induce extensive syncytium formation and are relatively, or almost completely, resistant to a known inhibitor of MV glycoprotein-induced fusion. We propose that the conformational changes in MV F protein required to expose the fusion peptide involve the C-terminal half of the HR-C helix, specifically amino acid 94.

Synthesis, enhanced fusogenicity, and solid state NMR measurements of cross-linked HIV-1 fusion peptides

In the HIV-1 gp41 and other viral fusion proteins, the minimal oligomerization state is believed to be trimeric with three N-terminal fusion peptides inserting into the membrane in close proximity. Previous studies have demonstrated that the fusion peptide by itself serves as a useful model fusion system, at least to the hemifusion stage in which the viral and target cell lipids are mixed. In the present study, HIV-1 fusion peptides were chemically synthesized and cross-linked at their C-termini to form dimers or trimers. C-terminal trimerization is their likely topology in the fusogenic form of the intact gp41 protein. The fusogenicity of the peptides was then measured in an intervesicle lipid mixing assay, and the assay results were compared to those of the monomer. For monomer, dimer, and trimer at peptide strand/lipid mol ratios between 0.0050 and 0.010, the final extent of lipid mixing for the dimer and trimer was 2-3 times greater than for the monomer. These data suggest that the higher local concentration of peptide strands in the cross-linked peptides enhances fusogenicity and that oligomerization of the fusion peptide in gp41 may enhance the rate of viral/target cell membrane fusion. For gp41, this effect is in addition to the role of the trimeric coiled-coil structure in bringing about apposition of viral and target cell membranes. NMR measurements on the membrane-associated dimeric fusion peptide were consistent with an extended structure at Phe-8, which is the same as has been observed for the membrane-bound monomer in the same lipid composition.

The LLSGIV stretch of the N-terminal region of HIV-1 gp41 is critical for binding to a model peptide, T20

A number of peptides and peptide analogs derived from the membrane proximal region of gp41 ectodomain are found to be effective inhibitors of human immunodeficiency virus type 1 (HIV-1)-mediated fusion events. One of them, T20 (aa 638-673), was found disordered and sparingly soluble in water, but became soluble upon mixing with selected, structured peptides from the amino terminal heptad repeat (HR1) region of gp41 using a simple and sensitive method of reduction in the scattering of T20 suspension. From the results on mapping the locus of interaction with T20 by employing partially overlapping peptides derived from HR1, it was concluded that the LLSGIV segment was a critical docking site for the C-terminal peptide of gp41 in its putative inhibitory action consistent with a previous fluorescence study. It was also found that peptides capable of solubilizing T20 dispersion have a high content of helix, as well as beta-strand, conformation in aqueous solution. Specificity of T20/HR1-derived peptide binding was ascertained by using a scrambled sequence of a T20-active peptide and a plateau in scattering reduction of T20 suspension with variation in the concentration of a T20-active HR1 peptide. Implications on the mechanism of T20 inhibition and the sequence of folding of the gp41 core structure are discussed.

On the interaction between gp41 and membranes: the immunodominant loop stabilizes gp41 helical hairpin conformation

gp41 is the protein responsible for the process of membrane fusion that allows primate lentiviruses (HIV and SIV) to enter into their host cells. gp41 ectodomain contains an N-terminal and a C-terminal heptad repeat region (NHR and CHR) connected by an immunodominant loop. In the absence of membranes, the NHR and CHR segments fold into a protease-resistant core with a trimeric helical hairpin structure. However, when the immunodominant loop is not present (either in a complex formed by HIV-1 gp41-derived NHR and CHR peptides or by mild treatment with protease of recombinant constructs of HIV-1 gp41 ectodomain, which also lack the N-terminal fusion peptide and the C-terminal Trp-rich region) membrane binding induces a conformational change in the gp41 core structure. Here, we further investigated whether covalently linking the NHR and CHR segments by the immunodominant loop affects this conformational change. Specifically, we analyzed a construct corresponding to a fragment of SIVmac239 gp41ectodomain (residues 27-149, named e-gp41) by means of surface plasmon resonance, Trp and rhodamine fluorescence, ATR-FTIR spectroscopy, and differential scanning calorimetry. Our results suggest that the presence of the loop stabilizes the trimeric helical hairpin both when e-gp41 is in aqueous solution and when it is bound to the membrane surface. Bearing in mind possible differences between HIV-1 and SIV gp41, and considering that the gp41 ectodomain constructs analyzed to date lack the N-terminal fusion peptide and the C-terminal Trp-rich region, we discuss our observations in relation to the mechanism of virus-induced membrane fusion.

The prefusogenic intermediate of HIV-1 gp41 contains exposed C-peptide regions

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein is composed of a complex between the surface subunit gp120, which binds to cellular receptors, and the transmembrane subunit gp41. Upon activation of the envelope glycoprotein by cellular receptors, gp41 undergoes conformational changes that mediate fusion of the viral and cellular membranes. Prior to formation of a fusogenic "trimer-of-hairpins" structure, gp41 transiently adopts a prefusogenic conformation whose structural features are poorly understood. An important approach toward understanding structural conformations of gp41 during HIV-1 entry has been to analyze the structural targets of gp41 inhibitors. We have constructed epitope-tagged versions of 5-Helix, a designed protein that binds to the C-peptide region of gp41 and inhibits HIV-1 membrane fusion. Using these 5-Helix variants, we examined which conformation of gp41 is the target of 5-Helix. We find that although 5-Helix binds poorly to native gp41, it binds strongly to gp41 activated by interaction of the envelope protein with either soluble CD4 or membrane-bound cellular receptors. This preferential interaction with activated gp41 results in the accumulation of 5-Helix on the surface of activated cells. These results strongly suggest that the gp41 prefusogenic intermediate is the target of 5-Helix and that this intermediate has a remarkably "open" structure, with exposed C-peptide regions. These results provide important structural information about this intermediate that should facilitate the development of HIV-1 entry inhibitors and may lead to new vaccine strategies.

Peptides trap the human immunodeficiency virus type 1 envelope glycoprotein fusion intermediate at two sites

Human immunodeficiency virus type 1 (HIV-1) entry into target cells requires folding of two heptad-repeat regions (N-HR and C-HR) of gp41 into a trimer of N-HR and C-HR hairpins, which brings viral and target cell membranes together to facilitate membrane fusion. Peptides corresponding to the N-HR and C-HR of gp41 are potent inhibitors of HIV infection. Here we report new findings on the mechanism of inhibition of a N-HR peptide and compare these data with inhibition by a C-HR peptide. Using intact envelope glycoprotein (Env) under fusogenic conditions, we show that the N-HR peptide preferentially binds receptor-activated Env and that CD4 binding is sufficient for triggering conformational changes that allow the peptide to bind Env, results similar to those seen with the C-HR peptide. However, activation by both CD4 and chemokine receptors further enhances Env binding by both peptides. We also show that a nonconservative mutation in the N-HR of gp41 abolishes C-HR peptide but not N-HR peptide binding to gp41. These results indicate that there are two distinct sites in receptor-activated Env that are potential targets for drug or vaccine development.

The Xplor-NIH NMR molecular structure determination package

We announce the availability of the Xplor-NIH software package for NMR biomolecular structure determination. This package consists of the pre-existing XPLOR program, along with many NMR-specific extensions developed at the NIH. In addition to many features which have been developed over the last 20 years, the Xplor-NIH package contains an interface with a new programmatic framework written in C++. This interface currently supports the general purpose scripting languages Python and TCL, enabling rapid development of new tools, such as new potential energy terms and new optimization methods. Support for these scripting languages also facilitates interaction with existing external programs for structure analysis, structure manipulation, visualization, and spectral analysis.

A monomeric 3(10)-helix is formed in water by a 13-residue peptide representing the neutralizing determinant of HIV-1 on gp41

The peptide gp41(659-671) (ELLELDKWASLWN) comprises the entire epitope for one of the three known antibodies capable of neutralizing a broad spectrum of primary HIV-1 isolates and is the only such epitope that is sequential. Here we present the NMR structure of gp41(659-671) in water. This peptide forms a monomeric 3(10)-helix stabilized by i,i+3 side chain-side chain interactions favored by its primary sequence. In this conformation the peptide presents an exposed surface, which is mostly hydrophobic and consists of conserved HIV-1 residues. The presence of the 3(10)-helix is confirmed by its characteristic CD pattern. Studies of the 3(10)-helix have been hampered by the absence of a model peptide adopting this conformation. gp41(659-671) can serve as such a model to investigate the spectral characteristics of the 3(10)-helix, the factors that influence its stability, and the propensity of different amino acids to form a 3(10)-helix. The observation that the 3(10)-helical conformation is highly populated in the peptide gp41(659-671) indicates that the corresponding segment in the cognate protein is an autonomous folding unit. As such, it is very likely that the helical conformation is maintained in gp41 throughout the different tertiary structures of the envelope protein that form during the process of viral fusion. However, the exposure of the gp41(659-671) segment may vary, leading to changes in the reactivity of anti-gp41 antibodies in the different stages of viral fusion. Since gp41(659-671) is an autonomous folding unit, peptide immunogens consisting of the complete gp41(659-671) sequence are likely to induce antibodies highly cross-reactive with HIV-1.

The mechanism of inhibition of HIV-1 env-mediated cell-cell fusion by recombinant cores of gp41 ectodomain

N36(L6)C34 is a recombinant protein that forms a six-helix bundle with high thermal stability. It consists of the N-terminal heptad-repeat region (N36 peptide) and the C-terminal heptad-repeat region (C34) of HIV-1 gp41, connected by six polar amino acids. The protein inhibits HIV-1 envelope-induced membrane fusion. Whether inhibition occurs while N36(L6)C34 is in its six-helix bundle configuration was investigated. Mutating a critical residue within the N36 region to promote dissociation of C34 from the grooves of the N36 coiled coil reduced bundle stability and increased the inhibition of fusion. In contrast, mutating a key residue within the C34 region to reduce bundle stability decreased inhibitory potency. The data provide strong evidence that the proteins inhibit fusion while they expose their C34 segments, rather than as six-helix bundles. Thus, despite high thermal stability of the bundle, the recombinants' less folded structures are present in sufficient concentration to inhibit fusion at physiological temperatures.

Solution structure of the HIV gp120 C5 domain

In HIV the viral envelope protein is processed by a host cell protease to form gp120 and gp41. The C1 and C5 domains of gp120 are thought to directly interact with gp41 but are largely missing from the available X-ray structure. Biophysical studies of the HIV gp120 C5 domain (residues 489-511 of HIV-1 strain HXB2), which corresponds to the carboxy terminal region of gp120, have been undertaken. CD studies of the C5 domain suggest that it is unstructured in aqueous solutions but partially helical in trifluoroethanol/aqueous and hexafluoroisopropanol/aqueous buffers. The solution structure of the C5 peptide in 40% trifluoroethanol/aqueous buffer was determined by NMR spectroscopy. The resulting structure is a turn helix structural motif, consistent with the CD results. Fluorescence titration experiments suggest that HIV C5 forms a 1 : 1 complex with the HIV gp41 ectodomain in the presence of cosolvent with an apparent Kd of approximately 1.0 micro m. The absence of complex formation in the absence of cosolvent indicates that formation of the turn-helix structural motif of C5 is necessary for complex formation. Examination of the C5 structure provides insight into the interaction between gp120 and gp41 and provides a possible target site for future drug therapies designed to disrupt the gp120/gp41 complex. In addition, the C5 structure lends insight into the site of HIV envelope protein maturation by the host enzymes furin and PC7, which provides other possible targets for drug therapies.

Stabilization of the soluble, cleaved, trimeric form of the envelope glycoprotein complex of human immunodeficiency virus type 1

The envelope glycoprotein (Env) complex of human immunodeficiency virus type 1 has evolved a structure that is minimally immunogenic while retaining its natural function of receptor-mediated virus-cell fusion. The Env complex is trimeric; its six individual subunits (three gp120 and three gp41 subunits) are associated by relatively weak, noncovalent interactions. The induction of neutralizing antibodies after vaccination with individual Env subunits has proven very difficult, probably because they are inadequate mimics of the native complex. Our hypothesis is that a stable form of the Env complex, perhaps with additional modifications to rationally alter its antigenic structure, may be a better immunogen than the individual subunits. A soluble form of Env, SOS gp140, can be made that has gp120 stably linked to the gp41 ectodomain by an intermolecular disulfide bond. This protein is fully cleaved at the proteolysis site between gp120 and gp41. However, the gp41-gp41 interactions in SOS gp140 are too weak to maintain the protein in a trimeric configuration. Consequently, purified SOS gp140 is a monomer (N. Schülke, M. S. Vesanen, R. W. Sanders, P. Zhu, D. J. Anselma, A. R. Villa, P. W. H. I. Parren, J. M. Binley, K. H. Roux, P. J. Maddon, J. P. Moore, and W. C. Olson, J. Virol. 76:7760-7776, 2002). Here we describe modifications of SOS gp140 that increase its trimer stability. A variant SOS gp140, designated SOSIP gp140, contains an isoleucine-to-proline substitution at position 559 in the N-terminal heptad repeat region of gp41. This protein is fully cleaved, has favorable antigenic properties, and is predominantly trimeric. SOSIP gp140 trimers are noncovalently associated and can be partially purified by gel filtration chromatography. These gp140 trimers are dissociated into monomers by anionic detergents or heat but are relatively resistant to nonionic detergents, high salt concentrations, or exposure to a mildly acidic pH. SOSIP gp140 should be a useful reagent for structural and immunogenicity studies.

Oligomeric and conformational properties of a proteolytically mature, disulfide-stabilized human immunodeficiency virus type 1 gp140 envelope glycoprotein

We describe the further properties of a protein, designated SOS gp140, wherein the association of the gp120 and gp41 subunits of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein is stabilized by an intersubunit disulfide bond. HIV-1(JR-FL) SOS gp140, proteolytically uncleaved gp140 (gp140(UNC)), and gp120 were expressed in stably transfected Chinese hamster ovary cells and analyzed for antigenic and structural properties before and after purification. Compared with gp140(UNC), SOS gp140 reacted more strongly in surface plasmon resonance and radioimmunoprecipitation assays with the neutralizing monoclonal antibodies (MAbs) 2G12 (anti-gp120), 2F5 (anti-gp41), and 17b (to a CD4-induced epitope that overlaps the CCR5-binding site). In contrast, gp140(UNC) displayed the greater reactivity with nonneutralizing anti-gp120 and anti-gp41 MAbs. Immunoelectron microscopy studies suggested a model for SOS gp140 wherein the gp41 ectodomain (gp41(ECTO)) occludes the "nonneutralizing" face of gp120, consistent with the antigenic properties of this protein. We also report the application of Blue Native polyacrylamide gel electrophoresis (BN-PAGE), a high-resolution molecular sizing method, to the study of viral envelope proteins. BN-PAGE and other biophysical studies demonstrated that SOS gp140 was monomeric, whereas gp140(UNC) comprised a mixture of noncovalently associated and disulfide-linked dimers, trimers, and tetramers. The oligomeric and conformational properties of SOS gp140 and gp140(UNC) were largely unaffected by purification. An uncleaved gp140 protein containing the SOS cysteine mutations (SOS gp140(UNC)) was also oligomeric. Surprisingly, variable-loop-deleted SOS gp140 proteins were expressed (although not yet purified) as cleaved, noncovalently associated oligomers that were significantly more stable than the full-length protein. Overall, our findings have relevance for rational vaccine design.

Comparison of predicted scaffold-compatible sequence variation in the triple-hairpin structure of human imunodeficiency virus type 1 gp41 with patient data

It has been proposed that the ectodomain of human immunodeficiency virus type 1 (HIV-1) gp41 (e-gp41), involved in HIV entry into the target cell, exists in at least two conformations, a pre-hairpin intermediate and a fusion-active hairpin structure. To obtain more information on the structure-sequence relationship in e-gp41, we performed in silico a full single-amino-acid substitution analysis, resulting in a Fold Compatible Database (FCD) for each conformation. The FCD contains for each residue position in a given protein a list of values assessing the energetic compatibility (ECO) of each of the 20 natural amino acids at that position. Our results suggest that FCD predictions are in good agreement with the sequence variation observed for well-validated e-gp41 sequences. The data show that at a minECO threshold value of 5 kcal/mol, about 90% of the observed patient sequence variation is encompassed by the FCD predictions. Some inconsistent FCD predictions at N-helix positions packing against residues of the C helix suggest that packing of both peptides may involve some flexibility and may be attributed to an altered orientation of the C-helical domain versus the N-helical region. The permissiveness of sequence variation in the C helices is in agreement with FCD predictions. Comparison of N-core and triple-hairpin FCDs suggests that the N helices may impose more constraints on sequence variation than the C helices. Although the observed sequences of e-gp41 contain many multiple mutations, our method, which is based on single-point mutations, can predict the natural sequence variability of e-gp41 very well.

Structure-affinity relationships in the gp41 ELDKWA epitope for the HIV-1 neutralizing monoclonal antibody 2F5: effects of side-chain and backbone modifications and conformational constraints

The human monoclonal antibody, mAb 2F5, has broad HIV-1 neutralizing activity and binds a conserved linear epitope within the envelope glycoprotein gp41 having a core recognition sequence ELDKWA. In this study, the structural requirements of this epitope for high-affinity binding to mAb 2F5 were explored using peptide synthesis and competitive enzyme-linked immunosorbant assay (ELISA). Expansion of the minimal epitope to an end-capped, linear nonapeptide, Ac-LELDKWASL-amide, was sufficient to attain maximal affinity within the set of native gp41-sequence peptides assayed. Scanning single-residue alanine and d-residue substitutions then confirmed the essential recognition requirements of 2F5 for the central DKW sequence, and also established the importance of the terminal leucine residues in determining high-affinity binding of the linear nonapeptide. Further studies of side-chain and backbone-modified analogs revealed a high degree of structural specificity for the DK sequence in particular, and delineated the steric requirements of the Leu(3) and Trp(6) residues. The nine-residue 2F5 epitope, flanked by pairs of serine residues, retained a high affinity for 2F5 when it was conformationally constrained as a 15-residue, disulfide-bridged loop. However, analogs with smaller or larger loop sizes resulted in lower 2F5 affinities. The conformational effects of the gp41 C-peptide helix immediately adjacent to the N-terminal end of the ELDKWA epitope were examined through the synthesis of helix-initiated analogs. Circular dichroism (CD) studies indicated that the alpha-helical conformation was propagated efficiently into the LELDKWASL epitope, but without any significant effect on its affinity for 2F5. This study should guide the design of a second generation of conformationally constrained ELDKWA analogs that might elicit an immune response that mimics the HIV-neutralizing actions of 2F5.

Potential new therapies for the treatment of HIV-1 infection

The development and clinical use of chemotherapeutic agents for the treatment of persistent HIV-1 infection over the past decade has profoundly and favorably affected the course of HIV-1 disease for many infected individuals. Unfortunately, the long-term use of these therapies is complicated by unwanted metabolic side effects, by issues of adherence, and by the selection of viral variants with reduced susceptibility. These complications have spurred the search for new anti-HIV-1 agents having improved pharmacological properties and expressing activity against viral variants resistant to the currently available agents. This brief review describes the current state of this search as well as potentially novel viral targets for chemotherapeutic intervention.

Design of a novel peptide inhibitor of HIV fusion that disrupts the internal trimeric coiled-coil of gp41

The pre-hairpin intermediate of gp41 from the human immunodeficiency virus (HIV) is the target for two classes of fusion inhibitors that bind to the C-terminal region or the trimeric coiled-coil of N-terminal helices, thereby preventing formation of the fusogenic trimer of hairpins. Using rational design, two 36-residue peptides, N36(Mut(e,g)) and N36(Mut(a,d)), were derived from the parent N36 peptide comprising the N-terminal helix of the gp41 ectodomain (residues 546-581 of HIV-1 envelope), characterized by analytical ultracentrifugation and CD, and assessed for their ability to inhibit HIV fusion using a quantitative vaccinia virus-based fusion assay. N36(Mut(e,g)) contains nine amino acid substitutions designed to disrupt interactions with the C-terminal region of gp41 while preserving contacts governing the formation of the trimeric coiled-coil. N36(Mut(a,d)) contains nine substitutions designed to block formation of the trimeric coiled-coil but retains residues that interact with the C-terminal region of gp41. N36(Mut(a,d)) is monomeric, is largely random coil, does not interact with the C34 peptide derived from the C-terminal region of gp41 (residues 628-661), and does not inhibit fusion. The trimeric coiled-coil structure is therefore a prerequisite for interaction with the C-terminal region of gp41. N36(Mut(e,g)) forms a monodisperse, helical trimer in solution, does not interact with C34, and yet inhibits fusion about 50-fold more effectively than the parent N36 peptide (IC(50) approximately 308 nm versus approximately 16 microm). These results indicate that N36(Mut(e,g)) acts by disrupting the homotrimeric coiled-coil of N-terminal helices in the pre-hairpin intermediate to form heterotrimers. Thus N36(Mut(e,g)) represents a novel third class of gp41-targeted HIV fusion inhibitor. A quantitative model describing the interaction of N36(Mut(e,g)) with the pre-hairpin intermediate is presented.

Mutations that destabilize the gp41 core are determinants for stabilizing the simian immunodeficiency virus-CPmac envelope glycoprotein complex

The human and simian immunodeficiency viruses (HIV and SIV) envelope glycoprotein consists of a trimer of two noncovalently and weakly associated subunits, gp120 and gp41. Upon binding of gp120 to cellular receptors, this labile native envelope complex undergoes conformational changes, resulting in a stable trimer-of-hairpins structure in gp41. Formation of the hairpin structure is thought to mediate membrane fusion by placing the viral and cellular membranes in close proximity. An in vitro-derived variant of SIVmac251, denoted CPmac, has acquired an unusually stable virion-associated gp120-gp41 complex. This unique phenotype is conferred by five amino acid substitutions in the gp41 ectodomain. Here we characterize the structural and physicochemical properties of the N40(L6)C38 model of the CPmac gp41 core. The 1.7-A resolution crystal structure of N40(L6)C38 is very similar to the six-helix bundle structure present in the parent SIVmac251 gp41. In both structures, three N40 peptides form a central three-stranded coiled coil, and three C38 peptides pack in an antiparallel orientation into hydrophobic grooves on the coiled-coil surface. Thermal unfolding studies show that the CPmac mutations destabilize the SIVmac251 six-helix bundle by 15 kJ/mol. Our results suggest that the formation of the gp41 trimer-of-hairpins structure is thermodynamically coupled to the conformational stability of the native envelope glycoprotein and raise the intriguing possibility that introduction of mutations to destabilize the six-helix bundle may lead to the stabilization of the trimeric gp120-gp41 complex. This study suggests a potential strategy for the production of stably folded envelope protein immunogens for HIV vaccine development.

Solid-phase proteoliposomes containing human immunodeficiency virus envelope glycoproteins

The human immunodeficiency virus type 1 (HIV-1) exterior envelope glycoprotein gp120 mediates receptor binding and is the major target for neutralizing antibodies. A broadly neutralizing antibody response is likely to be a critical component of the immune response against HIV-1. Although antibodies against monomeric gp120 are readily elicited in immunized individuals, these antibodies are inefficient in neutralizing primary HIV-1 isolates. As a chronic pathogen, HIV-1 has evolved to avoid an optimal host response by a number of immune escape mechanisms. Monomeric gp120 that has dissociated from the functional trimer presents irrelevant epitopes that are not accessible on functional trimeric envelope glycoproteins. The resulting low level of antigenic cross-reactivity between monomeric gp120 and the functional spike may contribute to the inability of monomeric gp120 to elicit broadly neutralizing antibodies. Attempts to generate native, trimeric envelope glycoproteins as immunogens have been frustrated by both the lability of the gp120-gp41 interaction and the weak association between gp120 subunits. Here, we present solid-phase HIV-1 gp160DeltaCT (cytoplasmic tail-deleted) proteoliposomes (PLs) containing native, trimeric envelope glycoproteins in a physiologic membrane setting. We present data that indicate that the gp160DeltaCT glycoproteins on PLs are trimers and are recognized by several relevant conformational ligands in a manner similar to that for gp160DeltaCT oligomers expressed on the cell surface. The PLs represent a significant advance over present envelope glycoprotein formulations as candidate immunogens for HIV vaccine design and development.