Interactions between HIV-1 gp41 core and detergents and their implications for membrane fusion

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.

Kinetically coupled folding of a single HIV-1 glycoprotein 41 complex in viral membrane fusion and inhibition

HIV-1 glycoprotein 41 (gp41) mediates viral entry into host cells by coupling its folding energy to membrane fusion. Gp41 folding is blocked by fusion inhibitors, including the commercial drug T20, to treat HIV/AIDS. However, gp41 folding intermediates, energy, and kinetics are poorly understood. Here, we identified the folding intermediates of a single gp41 trimer-of-hairpins and measured their associated energy and kinetics using high-resolution optical tweezers. We found that folding of gp41 hairpins was energetically independent but kinetically coupled: Each hairpin contributed a folding energy of ∼-23 kBT, but folding of one hairpin successively accelerated the folding rate of the next one by ∼20-fold. Membrane-mimicking micelles slowed down gp41 folding and reduced the stability of the six-helix bundle. However, the stability was restored by cooperative folding of the membrane-proximal external region. Surprisingly, T20 strongly inhibited gp41 folding by actively displacing the C-terminal hairpin strand in a force-dependent manner. The inhibition was abolished by a T20-resistant gp41 mutation. The energetics and kinetics of gp41 folding established by us provides a basis to understand viral membrane fusion, infection, and therapeutic intervention.

P20A inhibits HIV-1 fusion through its electrostatic interaction with the distal region of the gp41 fusion core

We previously identified an HIV-1 fusion inhibitor P20A targeting HIV-1 gp41 6-HB fusion core. Using alanine scanning mutagenesis, we investigated the effect of 6-HB surface residue mutations on the binding affinity between P20A and 6-HB. Substitution of positively or negatively charged residues in the distal region of 6-HB with alanines resulted in significant decrease or increase of its binding affinity to P20A, respectively. The 6-HB with E630K, D632K, or E634K mutation exhibited enhanced binding affinity with P20A, suggesting that P20A blocks HIV-1 fusion through electrostatic interaction with the positively charged residues in the distal region of the gp41 fusion core.

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

HIV is an enveloped virus and fusion between the HIV and host cell membranes is catalyzed by the ectodomain of the HIV gp41 membrane protein. Both the N-terminal fusion peptide (FP) and C-terminal membrane-proximal external region (MPER) are critical for fusion and are postulated to bind to the host cell and HIV membranes, respectively. Prior to fusion, the gp41 on the virion is a trimer in noncovalent complex with larger gp120 subunits. The gp120 bind host cell receptors and move away or dissociate from gp41 which subsequently catalyzes fusion. In the present work, large gp41 ectodomain constructs were produced and biophysically and structurally characterized. One significant finding is observation of synergy between the FP, hairpin, and MPER in vesicle fusion. The ectodomain-induced fusion can be very efficient with only ∼15 gp41 per vesicle, which is comparable to the number of gp41 on a virion. Conditions are found with predominant monomer or hexamer but not trimer and these may be oligomeric states during fusion. Monomer gp41 ectodomain is hyperthermostable and has helical hairpin structure. A new HIV fusion model is presented where (1) hemifusion is catalyzed by folding of gp41 ectodomain monomers into hairpins and (2) subsequent fusion steps are catalyzed by assembly into a hexamer with FPs in an antiparallel β sheet. There is also significant interest in the gp41 MPER because it is the epitope of several broadly neutralizing antibodies. Two of these antibodies bind our gp41 ectodomain constructs and support investigation of the gp41 ectodomain as an immunogen in HIV vaccine development.

Computer-Aided Approaches for Targeting HIVgp41

Virus-cell fusion is the primary means by which the human immunodeficiency virus-1 (HIV) delivers its genetic material into the human T-cell host. Fusion is mediated in large part by the viral glycoprotein 41 (gp41) which advances through four distinct conformational states: (i) native, (ii) pre-hairpin intermediate, (iii) fusion active (fusogenic), and (iv) post-fusion. The pre-hairpin intermediate is a particularly attractive step for therapeutic intervention given that gp41 N-terminal heptad repeat (NHR) and C-terminal heptad repeat (CHR) domains are transiently exposed prior to the formation of a six-helix bundle required for fusion. Most peptide-based inhibitors, including the FDA-approved drug T20, target the intermediate and there are significant efforts to develop small molecule alternatives. Here, we review current approaches to studying interactions of inhibitors with gp41 with an emphasis on atomic-level computer modeling methods including molecular dynamics, free energy analysis, and docking. Atomistic modeling yields a unique level of structural and energetic detail, complementary to experimental approaches, which will be important for the design of improved next generation anti-HIV drugs.

Dissociation of the trimeric gp41 ectodomain at the lipid-water interface suggests an active role in HIV-1 Env-mediated membrane fusion

The envelope glycoprotein gp41 mediates the process of membrane fusion that enables entry of the HIV-1 virus into the host cell. The actual fusion process involves a switch from a homotrimeric prehairpin intermediate conformation, consisting of parallel coiled-coil helices, to a postfusion state where the ectodomains are arranged as a trimer of helical hairpins, adopting a six-helix bundle (6HB) state. Here, we show by solution NMR spectroscopy that a water-soluble 6HB gp41 ectodomain binds to zwitterionic detergents that contain phosphocholine or phosphatidylcholine head groups and phospholipid vesicles that mimic T-cell membrane composition. Binding results in the dissociation of the 6HB and the formation of a monomeric state, where its two α-helices, N-terminal heptad repeat (NHR) and C-terminal heptad repeat (CHR), become embedded in the lipid-water interface of the virus and host cell. The atomic structure of the gp41 ectodomain monomer, based on NOE distance restraints and residual dipolar couplings, shows that the NHR and CHR helices remain mostly intact, but they completely lose interhelical contacts. The high affinity of the ectodomain helices for phospholipid surfaces suggests that unzippering of the prehairpin intermediate leads to a state where the NHR and CHR helices become embedded in the host cell and viral membranes, respectively, thereby providing a physical force for bringing these membranes into close juxtaposition before actual fusion.

HIV-1 envelope glycoprotein structure

The trimeric envelope glycoprotein of HIV-1, composed of gp120 and gp41 subunits, remains a major target for vaccine development. The structures of the core regions of monomeric gp120 and gp41 have been determined previously by X-ray crystallography. New insights into the structure of trimeric HIV-1 envelope glycoproteins are now coming from cryo-electron tomographic studies of the gp120/gp41 trimer as displayed on intact viruses and from cryo-electron microscopic studies of purified, soluble versions of the ectodomain of the trimer. Here, we review recent developments in these fields as they relate to our understanding of the structure and function of HIV-1 envelope glycoproteins.

HIV-1 gp41 ectodomain enhances Cryptococcus neoformans binding to human brain microvascular endothelial cells via gp41 core-induced membrane activities

Cryptococcus neoformans causes life-threatening meningoencephalitis, particularly prevalent in AIDS patients. The interrelationship between C. neoformans and HIV-1 is intriguing, as both pathogens elicit severe neuropathological complications. We have previously demonstrated that the HIV-1 gp41 ectodomain fragments gp41-I33 (amino acids 579-611) and gp41-I90 (amino acids 550-639) can enhance C. neoformans binding to HBMECs (human brain microvascular endothelial cells). Both peptides contain the loop region of gp41. In the present study, we used immunofluorescence microscopy and transmission and scanning electron microscopy to explore the underlying mechanisms. Our findings indicated that both C. neoformans and gp41-I90 up-regulated ICAM-1 (intercellular adhesion molecule 1) on the HBMECs and elicited membrane ruffling on the surface of HBMECs. The HIV-1 gp41 ectodomain could also induce CD44 and β-actin redistribution to the membrane lipid rafts, but it could not enhance PKCα (protein kinase Cα) phosphorylation like C. neoformans. Instead, gp41-I90 was able to induce syncytium formation on HBMECs. The results of the present study suggest HIV-1 gp41-enhanced C. neoformans binding to HBMECs via gp41 core domain-induced membrane activities, revealing a potential mechanism of invasion for this pathogenic fungus into the brain tissues of HIV-1-infected patients.

HIV-1 envelope accessible surface and polarity: clade, blood, and brain

The human immunodeficiency virus type-1 (HIV-1) gp160 (gp120-gp41 complex) trimer envelope (ENV) protein is a potential vaccine candidate for HIV/AIDS. HIV-1 vaccine development has been problematic and charge polarity as well as sequence variation across clades may relate to the difficulties. Further obstacles are caused by sequence variation between blood and brain-derived sequences, since the brain is a separate compartment for HIV-1 infection. We utilize a threedimensional residue measure of solvent exposure, accessible surface area (ASA), which shows that major segments of gp120 and gp41 known structures are solvent exposed across clades. We demonstrate a large percent sequence polarity for solvent exposed residues in gp120 and gp41. The range of sequence polarity varies across clades, blood, and brain from different geographical locations. Regression analysis shows that blood and brain gp120 and gp41 percent sequence polarity range correlate with mean Shannon entropy. These results point to the use of protein modifications to enhance HIV-1 ENV vaccines across multiple clades, blood, and brain. It should be noted that we do not address the issue of protein glycosylation here; however, this is an important issue for vaccine design and development.

ABBREVIATIONS

HIV-1 - human immunodeficiency virus type 1, AIDS - acquired immunodeficiency syndrome, ENV - envelope, gp160 - 160,000d glycoprotein, gp120 - 120,000d glycoprotein, gp41 - 41,000d glycoprotein, LANL - Los Alamos National Laboratories, PDB - Protein Data Bank, HVTN - STEP HIV vaccine trial, AA - amino acids, MSA - multiple sequence alignment, ASA - accessible surface area, SNPs- single nucleotide polymorphisms, HAART - Highly Active Antiretroviral Therapy, CCR5 - C-C chemokine receptor type 5, CNS - central nervous system, HIVE - HIV encephalitis, P - polarity, NP - non-polarity, CTL - cytotoxic T lymphocyte, NIAID - National Institute of Allergy and Infectious Diseases.

HIV infection upregulates caveolin 1 expression to restrict virus production

Caveolin 1 (Cav-1) is a major protein of a specific membrane lipid raft known as caveolae. Cav-1 interacts with the gp41 of the human immunodeficiency virus (HIV) envelope, but the role of Cav-1 in HIV replication and pathogenesis is not known. In this report, we demonstrate that HIV infection in primary human monocyte-derived macrophages (MDMs), THP-1 macrophages, and U87-CD4 cells results in a dramatic upregulation of Cav-1 expression mediated by HIV Tat. The activity of p53 is essential for Tat-induced Cav-1 expression, as our findings show enhanced phosphorylation of serine residues at amino acid positions 15 and 46 in the presence of Tat with a resulting Cav-1 upregulation. Furthermore, inhibition of p38 mitogen-activated protein kinase (MAPK) blocked phosphorylation of p53 in the presence of Tat. Infection studies of Cav-1-overexpressing cells reveal a significant reduction of HIV production. Taken together, these results suggest that HIV infection enhances the expression of Cav-1, which subsequently causes virus reduction, suggesting that Cav-1 may contribute to persistent infection in macrophages.

Structure-based design, synthesis and biological evaluation of new N-carboxyphenylpyrrole derivatives as HIV fusion inhibitors targeting gp41

A new series of N-carboxyphenylpyrrole ligands were designed using GeometryFit based on an X-ray crystal structure of gp41. The synthesized ligands showed significant inhibitory activities against HIV gp41 6-helix bundle formation, HIV-1 mediated cell-cell fusion and HIV-1 replication.

Hairpin folding of HIV gp41 abrogates lipid mixing function at physiologic pH and inhibits lipid mixing by exposed gp41 constructs

Conformational changes in the HIV gp41 protein are directly correlated with fusion between the HIV and target cell plasma membranes, which is the initial step of infection. Key gp41 fusion conformations include an early extended conformation termed prehairpin which contains exposed regions and a final low-energy conformation termed hairpin which has a compact six-helix bundle structure. Current fusion models debate the roles of hairpin and prehairpin conformations in the process of membrane merger. In the present work, gp41 constructs have been engineered which correspond to fusion relevant parts of both prehairpin and hairpin conformations and have been analyzed for their ability to induce lipid mixing between membrane vesicles. The data correlate membrane fusion function with the prehairpin conformation and suggest that one of the roles of the final hairpin conformation is sequestration of membrane-perturbing gp41 regions with consequent loss of the membrane disruption induced earlier by the prehairpin structure. To our knowledge, this is the first biophysical study to delineate the membrane fusion potential of gp41 constructs modeling key fusion conformations.

Conformational stability and membrane interaction of the full-length ectodomain of HIV-1 gp41: implication for mode of action

Membrane fusion between the human immunodeficiency virus (HIV) and the target cell plasma membrane is correlated with conformational changes in the HIV gp41 glycoprotein, which include an early exposed conformation (prehairpin) and a late low energy six helix bundle (SHB) conformation also termed hairpin. Peptides resembling regions from the exposed prehairpin have been previously studied for their interaction with membranes. Here we report on the expression, purification, SHB stability, and membrane interaction of the full-length ectodomain of the HIV gp41 and its two deletion mutants, all in their SHB-folded state. The interaction of the proteins with zwitterionic and negatively charged membranes was examined by using various biophysical methods including circular dichroism spectroscopy, differential scanning calorimetry, lipid mixing of large unilamellar vesicles, and atomic force microscopy (AFM). All experiments were done in an acidic environment in which the protein remains in its soluble trimeric state. The data reveal that all three proteins fold into a stable coiled-coil core in aqueous solution and retain a stable helical fold with reduced coiled-coil characteristics in a zwitterionic and negatively charged membrane mimetic environment. Furthermore, in contrast with the extended exposed N-terminal domain, the folded gp41 ectodomain does not induce lipid mixing of zwitterionic membranes. However, it disrupts and induces lipid mixing of negatively charged phospholipid membranes (approximately 100-fold more effective than fusion peptide alone), which are known to be expressed more in HIV-1-infected T cells or macrophages. The results support the emerging model in which one of the roles of gp41 folding into the SHB conformation is to slow down membrane disruption effects induced by early exposed gp41. However, it can further affect membrane morphology once exposed to negatively charged membranes during late stages.

Conserved salt bridge between the N- and C-terminal heptad repeat regions of the human immunodeficiency virus type 1 gp41 core structure is critical for virus entry and inhibition

The fusogenic human immunodeficiency virus type 1 (HIV-1) gp41 core structure is a stable six-helix bundle formed by its N- and C-terminal heptad repeat sequences. Notably, the negatively charged residue Asp(632) located at the pocket-binding motif in the C-terminal heptad repeat interacts with the positively charged residue Lys(574) in the pocket formation region of the N-terminal heptad repeat to form a salt bridge. We previously demonstrated that the residue Lys(574) plays an essential role in six-helix bundle formation and virus infectivity and is a key determinant of the target for anti-HIV fusion inhibitors. In this study, the functionality of residue Asp(632) has been specifically characterized by mutational analysis and biophysical approaches. We show that Asp(632) substitutions with positively charged residues (D632K and D632R) or a hydrophobic residue (D632V) could completely abolish Env-mediated viral entry, while a protein with a conserved substitution (D632E) retained its activity. Similar to the Lys(574) mutations, nonconserved substitutions of Asp(632) also severely impaired the alpha-helicity, stability, and conformation of six-helix bundles as shown by N36 and C34 peptides as a model system. Furthermore, nonconserved substitutions of Asp(632) significantly reduced the potency of C34 to sequestrate six-helix bundle formation and to inhibit HIV-1-mediated cell-cell fusion and infection, suggesting its importance for designing antiviral fusion inhibitors. Taken together, these data suggest that the salt bridge between the N- and C-terminal heptad repeat regions of the fusion-active HIV-1 gp41 core structure is critical for viral entry and inhibition.

Identification of critical antibody-binding sites in the HIV-1 gp41 six-helix bundle core as potential targets for HIV-1 fusion inhibitors

Formation of the six-helix bundle (6-HB) core between the N- and C-terminal heptad repeats (NHR and CHR) regions of the HIV-1 envelope glycoprotein (Env) transmembrane subunit gp41 is a critical step during the process of virus and target cell membrane fusion. In the present study, we generated a panel of five monoclonal antibodies (mAbs) which specifically recognized the HIV-1 gp41 6-HB formed by the NHR-peptide N36 and CHR-peptide C34 mixture, but did not react with the isolated peptides N36 and C34. These mAbs did not block the HIV-1 Env-mediated cell-cell fusion at physiological temperature (37 degrees C), but inhibited the HIV-1 Env-mediated cell-cell fusion at suboptimal temperature (31.5 degrees C), under which condition the fusion process is slowed down and the viral 6-HB becomes accessible. The fusion inhibitory activity of the mAbs is correlated with their binding affinity with the 6-HB core. By screening 24 6-HB variants with single mutations at the b, c, and f positions in the helical wheels, we found that the critical binding sites of these mAbs were localized in the N-terminal region of the NHR and the C-terminal region of the CHR. These sites may serve as targets for design of small molecule HIV fusion inhibitors, e.g., organic compounds, peptides, and low molecular weight proteins.

Interaction of HIV-1 gp41 core with NPF motif in Epsin: implication in endocytosis of HIV

The human immunodeficiency virus, type 1 (HIV-1), gp41 core plays an important role in fusion between viral and target cell membranes. We previously identified an HIV-1 gp41 core-binding motif HXXNPF (where X is any amino acid residue). In this study, we found that Asn, Pro, and Phe were the key residues for gp41 core binding. There are two NPF motifs in Epsin-1-(470-499), a fragment of Epsin, which is an essential accessory factor of endocytosis that can dock to the plasma membrane by interacting with the lipid. Epsin-1-(470-499) bound significantly to the gp41 core formed by the polypeptide N36(L8)C34 and interacted with the recombinant soluble gp41 containing the core structure. A synthetic peptide containing the Epsin-1-(470-499) sequence could effectively block entry of HIV-1 virions into SupT1 T cells via the endocytosis pathway. These results suggest that interaction between Epsin and the gp41 core, which may be present in the target cell membrane, is probably essential for endocytosis of HIV-1, an alternative pathway of HIV-1 entry into the target cell.

Design and evaluation of sifuvirtide, a novel HIV-1 fusion inhibitor

Enfuvirtide (T20) is the first and only HIV-1 fusion inhibitor approved for clinical use, but it can easily induce drug resistance limiting its practical application. A novel anti-HIV peptide, termed sifuvirtide, was designed based on the three-dimensional structure of the HIV-1 gp41 fusogenic core conformation. Here we report its in vitro anti-HIV potency, its mechanism of action, as well as the results from Phase Ia clinical studies. We demonstrated that sifuvirtide inhibited HIV-1-mediated cell-cell fusion in a dose-dependent manner and exhibited high potency against infections by a wide range of primary and laboratory-adapted HIV-1 isolates from multiple genotypes with R5 or X4 phenotypes. Notably, sifuvirtide was also highly effective against T20-resistant strains. Unlike T20, sifuvirtide could efficiently block six-helix bundle formation in a dominant negative fashion. These results suggest that sifuvirtide has a different mechanism of action from that of T20. Phase Ia clinical studies of sifuvirtide (FS0101) in 60 healthy individuals demonstrated good safety, tolerability, and pharmacokinetic profiles. A single dose regimen (5, 10, 20, 30, and 40 mg) by subcutaneous injection once daily at abdominal sites was well tolerated without serious adverse events. Pharmacokinetic studies of single and multiple administration of sifuvirtide showed that its decay half-lives were 20.0 +/- 8.6 h and 26.0 +/- 7.9 h, respectively. In summary, sifuvirtide has potential to become an ideal fusion inhibitor for treatment of HIV/AIDS patients, including those with HIV-1 strains resistant to T20.

Peptide mimic of the HIV envelope gp120-gp41 interface

The human immunodeficiency virus envelope glycoprotein (Env) is composed of surface (gp120) and transmembrane (gp41) subunits, which are noncovalently associated on the viral surface. Human immunodeficiency virus Env mediates viral entry after undergoing a complex series of conformational changes induced by interaction with cellular CD4 and a chemokine coreceptor. These changes propagate from gp120 to gp41 via the gp120-gp41 interface, ultimately exposing gp41 and allowing it to form the trimer-of-hairpins structure that provides the driving force for membrane fusion. Key unresolved questions about the gp120-gp41 interface include the specific regions of gp41 and gp120 involved, the mechanism by which receptor and coreceptor-binding-induced conformational changes in gp120 are communicated to gp41, how trimer-of-hairpins formation is prevented in the prefusogenic gp120-gp41 complex, and, ultimately, the structure of the prefusion gp120-gp41 complex. Here, we develop a biochemical model system that mimics a key portion of the gp120-gp41 interface in the prefusogenic state. We find that a gp41 fragment containing the disulfide bond loop and C-peptide region binds primarily to the gp120 C5 region and that this interaction is incompatible with trimer-of-hairpins formation. Based on these data, we propose that in prefusogenic Env, gp120 sequesters the gp41 C-peptide region away from the N-trimer region, preventing trimer-of-hairpins formation until coreceptor binding disrupts this interface. This model system is a valuable tool for studying the gp120-gp41 complex, conformational changes induced by CD4 and coreceptor binding, and the mechanism of membrane fusion.

HIV gp41 C-terminal heptad repeat contains multifunctional domains. Relation to mechanisms of action of anti-HIV peptides

T20 (Fuzeon), a novel anti-human immunodeficiency virus (HIV) drug, is a peptide derived from HIV-1 gp41 C-terminal heptad repeat (CHR). Its mechanism of action has not yet been defined. We applied Pepscan strategy to determine the relationship between functional domains and mechanisms of action of five 36-mer overlapping peptides with a shift of five amino acids (aa): CHR-1 (aa 623-658), C36 (aa 628-663), CHR-3 (aa 633-668), T20 (aa 638-673), and CHR-5 (aa 643-678). C36 is a peptide with addition of two aa to the N terminus of C34. Peptides CHR-1 and C36 contain N-terminal heptad repeat (NHR)- and pocket-binding domains. They inhibited HIV-1 fusion by interacting with gp41 NHR, forming stable six-helix bundles and blocking gp41 core formation. Peptide T20 containing partial NHR- and lipid-binding domains, but lacking pocket-binding domain, blocked viral fusion by binding its N- and C-terminal sequences with gp41 NHR and cell membrane, respectively. Peptide CHR-3, which is located in the middle between C36 and T20, overlaps >86% of the sequences of these two peptides, and lacks pocket- and lipid-binding domains, exhibited marginal anti-HIV-1 activity. These results suggest that T20 and C36 contain different functional domains, through which they inhibit HIV-1 entry with distinct mechanisms of action. The multiple functional domains in gp41 CHR and their binding partners may serve as targets for rational design of new anti-HIV-1 drugs and vaccines.

Membrane perturbing actions of HIV type 1 glycoprotein 41 domains are inhibited by helical C-peptides

To study the membrane actions of various domains of HIV-1 glycoprotein 41,000 (gp41), synthetic peptides were prepared corresponding to the N-terminal fusion region (FP; gp41 residues 519-541), the nearby N-leucine zipper domain (N-peptides; DP-107; gp41 residues 560-597), the C-leucine zipper domain (C-peptides; DP-178; gp41 residues 645-680), and the viral envelope adjacent domain that partially overlaps DP-178 (Pre-TM; gp41 residues 671-690). With erythrocytes, FP, DP-107, and Pre-TM induced hemolysis and cell aggregation; the order for hemolytic activity was Pre-TM > FP > DP-107, but each was equally effective in aggregating cells at the highest peptide concentrations tested. DP-178 produced neither hemolysis nor aggregation, but efficiently reduced FP-, DP-107-, and Pre-TM-induced membrane actions. Fourier transform infrared spectroscopy indicated that the membrane perturbations of Pre-TM, as well as the ability of DP-178 to block membrane activities of other gp41 domains, are dependent on Pre-TM and DP-178 each maintaining helical conformations and tryptophans at residues 673, 677, and 679. These results suggest that the corresponding N-terminal fusion, N-leucine zipper, and viral membrane-adjacent regions of HIV-1 gp41 may similarly promote key membrane perturbations underlying the merging of the viral envelope with the cell surface. Further, the antiviral mechanism of exogenous DP-178 (clinically approved enfuvirtide) may be partially explained by its coordinate inhibition of the fusogenic actions of the FP, DP-107, and Pre-TM regions of gp41.

Secondary structure analysis of HIV-1-gp41 in solution and adsorbed to aluminum hydroxide by Fourier transform infrared spectroscopy

The formulation of human vaccines often includes adjuvants such as aluminum hydroxide that are added to enhance the immune responses to vaccine antigens. However, these adjuvants may also affect the conformation of antigenic proteins. Such structural modifications could lead to changes in antigenicity such that suboptimal protective immune responses could be generated relative to those induced by the vaccine antigens alone. Here, we used attenuated total reflectance infrared spectroscopy (ATR-FTIR) to compare the secondary structures of recombinant HIV-1-gp41 (gp41) in solution or adsorbed to aluminum hydroxide. The gp41 secondary structure content was 72% alpha-helices and 28% beta-sheets in 5 mM formate buffer p(2)H 2.5, while it was 66% beta-sheets and 34% random coil in acetonitril/(2)H(2)O (95/5:v/v). A fully reversible conformational change of gp41 in acetonitril/(2)H(2)O (95/5:v/v) was observed upon addition of either 35 mM formate p(2)H 2.5 or 0.1% (w/v) detergent (Tween 20, Hecameg, Brij 35 or beta-d-octyl-glucopyranoside). When gp41 was adsorbed to aluminum hydroxide in the presence of 0.1% (w/v) detergent, in either formate or in acetonitril/(2)H(2)O (95/5:v/v) its secondary structure remained stable and was identical to that of gp41 in 5 mM formate buffer p(2)H 2.5. The method described here could be applied for the characterization of gp41 conformers for use in immunological screening of antigens, and more generally to all antigenic proteins adsorbed to aluminum hydroxide.

Identification of the HIV-1 gp41 core-binding motif in the scaffolding domain of caveolin-1

The human immunodeficiency virus, type 1 (HIV-1) gp41 core plays an important role in fusion between viral and target cell membranes. A single chain polypeptide, N36(L8)C34, which forms a six-helix bundle in physiological solution, can be used as a model of gp41 core. Here we identified from a 12-mer phage peptide library a positive phage clone displaying a peptide sequence with high binding activity to the HIV-1 gp41 core. The peptide sequence contains a putative gp41-binding motif, PhiXXXXPhiXPhi (X is any amino acid residue, and Phi is any one of the aromatic amino acid residues Trp, Phe, or Tyr). This motif also exists in the scaffolding domain of caveolin-1 (Cav-1), a known gp41-binding protein. Cav-1-(61-101) and Cav-1-(82-101), two recombinant fusion proteins containing the Cav-1 scaffolding domain, bound significantly to the gp41 expressed in mammalian cells and interacted with the polypeptide N36(L8)C34. These results suggest that the scaffolding domain of Cav-1 may bind to the gp41 core via the motif. This interaction may be essential for formation of fusion pore or endocytosis of HIV-1 and affect the pathogenesis of HIV-1 infection. Further characterization of the gp41 core-binding motifs may shed light on the alternative mechanism by which HIV-1 enters into the target cell.

Identification of the HIV-1 gp41 core-binding motif--HXXNPF

The HIV-1 gp41 core, a six-helix bundle formed between the N- and C-terminal heptad repeats, plays a critical role in fusion between the viral and target cell membranes. Using N36(L8)C34 as a model of the gp41 core to screen phage display peptide libraries, we identified a common motif, HXXNPF (X is any of the 20 natural amino acid residues). A selected positive phage clone L7.8 specifically bound to N36(L8)C34 and this binding could be blocked by a gp41 core-specific monoclonal antibody (NC-1). JCH-4, a peptide containing HXXNPF motif, effectively inhibited HIV-1 envelope glycoprotein-mediated syncytium-formation. The epitope of JCH-4 was proven to be linear and might locate in the NHR regions of the gp41 core. These data suggest that HXXNPF motif may be a gp41 core-binding sequence and HXXNPF motif-containing molecules can be used as probes for studying the role of the HIV-1 gp41 core in membrane fusion process.

The disintegrin echistatin stabilizes integrin alphaIIbbeta3's open conformation and promotes its oligomerization

We have employed echistatin, a 5.4 kDa snake venom disintegrin, as a model protein to investigate the paradox that small ligand-mimetics can bind to the resting alphaIIbbeta3 integrin while adhesive macromolecules cannot. We characterized the interactions between purified human alphaIIbbeta3 and two recombinant echistatin variants: rEch (1-49) M28L, chosen for its selectivity toward beta3-integrins, and rEch (1-40) M28L, a carboxy-terminal truncation mutant. While both contain an RGD integrin targeting sequence, only rEch (1-49) M28L was an effective inhibitor of alphaIIbbeta3 function. Electron microscopy of rotary shadowed specimens yielded a variety of alphaIIbbeta3 conformers ranging from compact, spherical particles (maximum dimension 22 nm) to the classical "head with two tails" forms (32 nm). The population of larger particles (42-56 nm) increased from 17% to 28% in the presence of rEch (1-49) M28L, indicative of ligand-induced oligomerization. Sedimentation velocity measurements demonstrated that both full length and truncated echistatin perturbed alphaIIbbeta3's solution structure, yielding slower-sedimenting open conformers. Dynamic light scattering showed that rEch (1-49) M28L protected alphaIIbbeta3 from thermal aggregation, raising its transition mid-point from 46 degrees C to 69 degrees C; a smaller shift resulted with rEch (1-40) M28L. Sedimentation equilibrium demonstrated that both echistatin ligands induced substantial alphaIIbbeta3 dimerization. van't Hoff analysis revealed a pattern of entropy/enthalpy compensation similar to tirofiban, a small RGD ligand-mimetic that binds tightly to alphaIIbbeta3, but yields smaller conformational perturbations than echistatin. We propose that echistatin may serve as a paradigm for understanding multidomain adhesive macromolecules because its ability to modulate alphaIIbbeta3's structure resides on an RGD loop, while full disintegrin activity requires an auxiliary site that includes the carboxy-terminal nine amino acid residues.

The Caveolin-1 Binding Domain of HIV-1 Glycoprotein gp41 Is an Efficient B Cell Epitope Vaccine Candidate against Virus Infection

Caveolin-1 is a scaffolding protein that organizes and concentrates specific ligands within the caveolae membranes. We identified a conserved caveolin-1 binding motif in the HIV-1 transmembrane envelope glycoprotein gp41 and designed several synthetic peptides, referred to as CBD1, corresponding to the consensus caveolin-1 binding domain in gp41. In rabbits, these peptides elicit the production of antibodies that inhibit infection of primary CD4(+) T lymphocytes by various primary HIV-1 isolates. Interestingly, gp41 exists as a stable complex with caveolin-1 in HIV-infected cells. Anti-CBD1 peptide antibodies, therefore, might be functional by inhibiting the potential interaction of gp41 with caveolin-1. Because of their capacity to elicit antibodies that inhibit the different clades of HIV-1, CBD1-based peptides may represent a novel synthetic universal B cell epitope vaccine candidate for HIV/AIDS. Moreover, such peptides could also have an application as a therapeutic vaccine since CBD1-specific antibodies are rare in HIV-infected individuals from several geographic origins.

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.

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.

An alanine-zipper structure determined by long range intermolecular interactions

A major challenge in protein folding is to identify and quantify specific structural determinants that allow native proteins to acquire their unique folded structures. Here we report the engineering of a 52-residue protein (Ala-14) that contains exclusively alanine residues at the hydrophobic a and d positions of a natural heptad-repeat sequence. Ala-14 is unfolded under normal solution conditions yet forms a parallel three-stranded alpha-helical coiled coil in crystals. Ala-14 trimers in the solid state associate with each other through the pairing of polar side chains and formation of an extended network of water-mediated hydrogen bonds. In contrast to the classical view that local intramolecular tertiary interactions dictate the three-dimensional structure of small single-domain proteins, Ala-14 shows that long range intermolecular interactions can be essential in determining the metastable alanine-zipper structure. A similar interplay between short range local and longer range global forces may underlie the conformational properties of the growing class of natively unstructured proteins in biological processes.

Enhancement of alpha -helicity in the HIV-1 inhibitory peptide DP178 leads to an increased affinity for human monoclonal antibody 2F5 but does not elicit neutralizing responses in vitro. Implications for vaccine design

The synthetic peptide DP178, derived from the carboxyl-terminal heptad repeat region of human immunodeficiency virus type 1 GP41 protein is a potent inhibitor of viral-mediated fusion and contains the sequence ELDKWA, which constitutes the recognition epitope for the broadly neutralizing human monoclonal antibody 2F5. Efforts at eliciting a 2F5-like immune response by immunization with peptides or fusion proteins containing this sequence have not met with success, possibly because of incorrect structural presentation of the epitope. Although the structure of the carboxyl-terminal heptad repeat on the virion is not known, several recent reports have suggested a propensity for alpha-helical conformation. We have examined DP178 in the context of a model for optimized alpha-helices and show that the native sequence conforms poorly to the model. Solution conformation of DP178 was studied by circular dichroism and NMR spectroscopy and found to be predominantly random, consistent with previous reports. NMR mapping was used to show that the low percentage of alpha-helix present was localized to residues Glu(662) through Asn(671), a region encompassing the 2F5 epitope. Using NH(2)-terminal extensions derived from either GP41 or the yeast GCN4 leucine zipper dimerization domain, we designed peptide analogs in which the average helicity is significantly increased compared with DP178 and show that these peptides exhibit both a modest increase in affinity for 2F5 using a novel competitive solution-based binding assay and an increased ability to inhibit viral entry in a single-cycle infectivity model. Selected peptides were conjugated to carrier protein and used for guinea pig immunizations. High peptide-specific titers were achieved using these immunogens, but the resulting sera were incapable of viral neutralization. We discuss these findings in terms of structural and immunological considerations as to the utility of a 2F5-like response.

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.

The leucine zipper motif of the envelope glycoprotein ectodomain of human immunodeficiency virus type 1 contains conformationally flexible regions as revealed by NMR and circular dichroism studies in different media

A 43-mer peptide derived from the coiled coil domain of the transmembrane glycoprotein, gp41, of human immunodeficiency virus type 1, was synthesized. Light scattering measurements suggested that the peptide molecules likely exist in the aqueous solution in trimeric form. Circular dichroism experiments showed a moderate helix population enhancement for the peptide in 80% methanol solution relative to helicity in sodium dodecyl sulfate micellar suspension. NMR spectroscopy indicated that the N-terminal section of the peptide was conformationally more sensitive to the medium. The conformationally labile regions contain residues implicated in gp41-gp120 association. Our data support the idea that the coiled coil region is responsible for oligomerization of the gp41 ectodomain and suggest a site of conformational isomerization following receptor binding-induced gp120 dissociation from gp41.

Paramyxovirus fusion (F) protein: a conformational change on cleavage activation

The fusion (F) protein of the paramyxovirus SV5 promotes both virus-cell and cell-cell fusion. Recently, the atomic structure at 1.4 A of an extremely thermostable six-helix bundle core complex consisting of two heptad repeat regions of the F protein has been described (K. A. Baker, R. E. Dutch, R. A. Lamb, and T. S Jardetsky, Mol. Cell 3, 309-319, 1999). To analyze the conformations of the F protein at various stages of the membrane fusion process and to understand further the role of formation of the six-helix bundle core complex in promotion of membrane fusion, antibodies to peptides corresponding to regions of the F protein were obtained. Major changes in F protein antibody recognition were found after cleavage of the precursor protein F(0) to the fusogenically active disulfide-linked heterodimer, F(1) + F(2), and antibodies directed against the heptad repeat regions recognized only the uncleaved form. A monoclonal antibody directed against the F protein showed increased recognition at the cell surface of the cleaved form of the F protein as compared to uncleaved F protein, again indicating changes in conformation between the uncleaved and cleaved forms of the F protein. Anti-peptide antibodies specific for the heptad repeat regions were unable to precipitate a synthetic protein that consisted of the heptad repeat regions separated only by a small spacer, suggesting that the antibodies are unable to recognize their target regions when the heptad repeats are present in the six-helix bundle core complex. Taken together, these data indicate that the six-helix bundle core complex is not present in the precursor molecule F(0) and that significant conformational changes occur subsequent to cleavage of the F protein.

Membrane-induced conformational change during the activation of HIV-1 gp41

The human immunodeficiency virus type 1 gp41 ectodomain forms a three-hairpin protease-resistant core in the absence of membranes, namely, the putative gp41 fusion-active state. Here, we show that recombinant proteins corresponding to the ectodomain of gp41, but lacking the fusion peptide, bind membranes and consequently undergo a major conformational change. As a result, the protease-resistant core becomes susceptible to proteolytic digestion. Accordingly, synthetic peptides corresponding to the segments that construct this core bind the membrane. It is remarkable that the hetero-oligomer formed by these peptides dissociates upon binding to the membrane. These results are consistent with a model in which, after the three-hairpin conformation is formed, membrane binding induces opening of the gp41 core complex. We speculate that binding of the segments that constructed the core to the viral and cellular membranes could bring the membranes closer together and facilitate their merging.