Insights into the mechanism of HIV-1 envelope induced membrane fusion as revealed by its inhibitory peptides

Structural and Mechanistic Evidence for Calcium Interacting Sites in the HIV Transmembrane Protein gp41 Involved in Membrane Fusion

The HIV envelope protein gp160 comprises two subunits, gp120 and gp41, responsible for receptor binding and membrane fusion during viral entry, respectively. In the course of the membrane fusion process, gp41 undergoes a conformational change, leading to the formation of a six-helix bundle (SHB), which ultimately drives membrane fusion. The gp41 C-terminal and N-terminal heptad repeats (CHR and NHR) interact with one another to form the SHB, and this step can be targeted by peptide inhibitors, which are used in the clinic to mitigate HIV infection. Here, we discover the calcium interaction motifs (CIMs) in the gp41 CHR and NHR regions via NMR spectroscopy. We find that the assembly of the CHR-NHR SHB is facilitated in Ca2+-containing media and impaired in CIM mutants. Of note, the clinically approved, gp41-derived fusion inhibitor T20, which does not contain the CIM motif, exhibits reduced inhibitory efficiency when challenged with calcium. This finding could have important implications for the development of better fusion inhibitors for HIV.

Design and Biological Evaluation of m-Xylene Thioether-Stapled Short Helical Peptides Targeting the HIV-1 gp41 Hexameric Coiled-Coil Fusion Complex

Short peptide-based inhibition of fusion remains an attractive goal in antihuman immunodeficiency virus (HIV) research based on its potential for the development of technically and economically desirable antiviral agents. Herein, we report the use of the dithiol bisalkylation reaction to generate a series of m-xylene thioether-stapled 22-residue α-helical peptides that have been identified as fusion inhibitors targeting HIV-1 glycoprotein 41 (gp41). The peptide sequence is based on the helix-zone binding domain of the gp41 C-terminal heptad repeat region. We found that one of these stapled peptides, named hCS6ERE, showed promising inhibitory potency against HIV-1 Env-mediated cell-cell fusion and viral replication at a level comparable to the clinically used 36-mer peptide T20. Furthermore, combining hCS6ERE with a fusion inhibitor having a different target site, such as HP23, produced synergistic anti-HIV-1 activity. Collectively, our study offers new insight into the design of anti-HIV peptides with short sequences.

Six-helix bundle completion in the distal C-terminal heptad repeat region of gp41 is required for efficient human immunodeficiency virus type 1 infection

Background

The native pre-fusion structure of gp120/gp41 complex of human immunodeficiency virus type 1 was recently revealed. In the model, the helices of gp41 (α6, α7, α8, and α9) form a four-helix collar underneath trimeric gp120. Gp41 is a class I fusion protein and mediates membrane fusion by forming a post-fusion structure called the six-helix bundle (6HB). The comparison of the pre- and post-fusion structures revealed the large conformational changes in gp41 during the antiparallel packing of the N- and C-terminal heptad repeats (NHRs and CHRs) in membrane fusion. Several mutagenesis studies of gp41 performed in the past were interpreted based on 6HB, the only available structure at that time. To obtain an insight about the current pre-fusion structural model and conformational changes during membrane fusion, alanine insertion mutagenesis of the NHR, CHR and connecting loop regions of HXB2 gp41 was performed. The effects of mutations on biosynthesis and membrane fusion were analyzed by immunoblotting and fusion assays, respectively. The extent of membrane fusion was evaluated by split luciferase-based pore formation and syncytia formation assays, respectively.

Results

Consistent with the current structural model, drastic negative effects of mutations on biosynthesis and membrane fusion were observed for NHR, loop, and proximal regions of CHR (up to amino acid position 643). The insertions in α9 after it leaves the four-helix collar were tolerable for biosynthesis. These CHR mutants showed varying effects on membrane fusion. Insertion at position 644 or 645 resulted in poor pore and syncytia formation. Efficient pore and syncytia formation almost similar to that of the wild type was observed for insertion at position 647, 648 or 649. However, recovery of virus infectivity was only observed for the insertions beyond position 648.

Conclusions

The mutagenesis data for HXB2 gp41 is in agreement with the recent pre-fusion structure model. The virus infection data suggested that fusion pores sufficiently large enough for the release of the virus genome complex are formed after the completion of 6HB beyond position 648.

Electronic supplementary material

The online version of this article (10.1186/s12977-018-0410-9) contains supplementary material, which is available to authorized users.

Customizable de novo design strategies for DOCK: Application to HIVgp41 and other therapeutic targets

De novo design can be used to explore vast areas of chemical space in computational lead discovery. As a complement to virtual screening, from-scratch construction of molecules is not limited to compounds in pre-existing vendor catalogs. Here, we present an iterative fragment growth method, integrated into the program DOCK, in which new molecules are built using rules for allowable connections based on known molecules. The method leverages DOCK's advanced scoring and pruning approaches and users can define very specific criteria in terms of properties or features to customize growth toward a particular region of chemical space. The code was validated using three increasingly difficult classes of calculations: (1) Rebuilding known X-ray ligands taken from 663 complexes using only their component parts (focused libraries), (2) construction of new ligands in 57 drug target sites using a library derived from ∼13M drug-like compounds (generic libraries), and (3) application to a challenging protein-protein interface on the viral drug target HIVgp41. The computational testing confirms that the de novo DOCK routines are robust and working as envisioned, and the compelling results highlight the potential utility for designing new molecules against a wide variety of important protein targets. © 2017 Wiley Periodicals, Inc.

Structure-based identification of inhibitors targeting obstruction of the HIVgp41 N-heptad repeat trimer

The viral protein HIVgp41 is an attractive and validated drug target that proceeds through a sequence of conformational changes crucial for membrane fusion, which facilitates viral entry. Prior work has identified inhibitors that interfere with the formation of a required six-helix bundle, composed of trimeric C-heptad (CHR) and N-heptad (NHR) repeat elements, through blocking association of an outer CHR helix or obstructing formation of the inner NHR trimer itself. In this work, we employed similarity-based scoring to identify and experimentally characterize 113 compounds, related to 2 small-molecule inhibitors recently reported by Allen et al. (Bioorg. Med. Chem Lett.2015, 25 2853-59), proposed to act via the NHR trimer obstruction mechanism. The compounds were first tested in an HIV cell-cell fusion assay with the most promising evaluated in a second, more biologically relevant viral entry assay. Of the candidates, compound #11 emerged as the most promising hit (IC50=37.81µM), as a result of exhibiting activity in both assays with low cytotoxicity, as was similarly seen with the known control peptide inhibitor C34. The compound also showed no inhibition of VSV-G pseudotyped HIV entry compared to a control inhibitor suggesting it was specific for HIVgp41. Molecular dynamics simulations showed the predicted DOCK pose of #11 interacts with HIVgp41 in an energetic fashion (per-residue footprints) similar to the four native NHR residues (IQLT) which candidate inhibitors were intended to mimic.

Mapping out the intricate relationship of the HIV envelope protein and the membrane environment

The HIV gp160 envelope fusion protein is situated in the viral membrane and mediates virus entry into its host cell. Increasing evidence suggests that virtually all parts of the HIV envelope are structurally and functionally dependent on membranes. Protein-lipid interactions and membrane properties influence the dynamics of a manifold of gp160 biological activities such as membrane fusion, immune suppression and gp160 incorporation into virions during HIV budding and assembly. In the following we will summarize our current understanding of this interdependence between membrane interaction, structural conformation and functionality of the different gp160 domains. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

The HIV gp41 pocket binding domain enables C-terminal heptad repeat transition from mediating membrane fusion to immune modulation

We found that the HIV gp41 pocket binding domain (PBD) promotes the transition from fusion facilitation to an interaction with the T-cell receptor (TCR) leading to T-cell inhibition by stabilizing an α-helical conformation in the membrane.

Small molecule inhibitors of HIVgp41 N-heptad repeat trimer formation

Identification of mechanistically novel anti-HIV fusion inhibitors was accomplished using a computer-aided structure-based design approach with the goal of blocking the formation of the N-heptad repeat (NHR) trimer of the viral protein gp41. A virtual screening strategy that included per-residue interaction patterns (footprints) was employed to identify small molecules compatible with putative binding pockets at the internal interface of the NHR helices at the core native viral six-helix bundle. From a screen of ∼2.8 million compounds using the DOCK program, 120 with favorable energetic and footprint overlap characteristics were purchased and experimentally tested leading to two compounds with favorable cell-cell fusion (IC50) and cytotoxicity profiles. Importantly, both hits were identified on the basis of scores containing footprint overlap terms and would not have been identified using the standard DOCK energy function alone. To our knowledge, these compounds represent the first reported small molecules that inhibit viral entry via the proposed NHR-trimer obstruction mechanism.

Recombinant expression, purification, and biophysical characterization of the transmembrane and membrane proximal domains of HIV-1 gp41

The transmembrane subunit (gp41) of the envelope glycoprotein of HIV-1 associates noncovalently with the surface subunit (gp120) and together they play essential roles in viral mucosal transmission and infection of target cells. The membrane proximal region (MPR) of gp41 is highly conserved and contains epitopes of broadly neutralizing antibodies. The transmembrane (TM) domain of gp41 not only anchors the envelope glycoprotein complex in the viral membrane but also dynamically affects the interactions of the MPR with the membrane. While high-resolution X-ray structures of some segments of the MPR were solved in the past, they represent the post-fusion forms. Structural information on the TM domain of gp41 is scant and at low resolution. Here we describe the design, expression and purification of a protein construct that includes MPR and the transmembrane domain of gp41 (MPR-TMTEV-6His), which reacts with the broadly neutralizing antibodies 2F5 and 4E10 and thereby may represent an immunologically relevant conformation mimicking a prehairpin intermediate of gp41. The expression level of MPR-TMTEV-6His was improved by fusion to the C-terminus of Mistic protein, yielding ∼ 1 mg of pure protein per liter. The isolated MPR-TMTEV-6His protein was biophysically characterized and is a monodisperse candidate for crystallization. This work will enable further investigation into the structure of MPR-TMTEV-6His, which will be important for the structure-based design of a mucosal vaccine against HIV-1.

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.

Structure of an HIV-1-neutralizing antibody target, the lipid-bound gp41 envelope membrane proximal region trimer

The membrane proximal external region (MPER) of HIV-1 glycoprotein (gp) 41 is involved in viral-host cell membrane fusion. It contains short amino acid sequences that are binding sites for the HIV-1 broadly neutralizing antibodies 2F5, 4E10, and 10E8, making these binding sites important targets for HIV-1 vaccine development. We report a high-resolution structure of a designed MPER trimer assembled on a detergent micelle. The NMR solution structure of this trimeric domain, designated gp41-M-MAT, shows that the three MPER peptides each adopt symmetric α-helical conformations exposing the amino acid side chains of the antibody binding sites. The helices are closely associated at their N termini, bend between the 2F5 and 4E10 epitopes, and gradually separate toward the C termini, where they associate with the membrane. The mAbs 2F5 and 4E10 bind gp41-M-MAT with nanomolar affinities, consistent with the substantial exposure of their respective epitopes in the trimer structure. The traditional structure determination of gp41-M-MAT using the Xplor-NIH protocol was validated by independently determining the structure using the DISCO sparse-data protocol, which exploits geometric arrangement algorithms that guarantee to compute all structures and assignments that satisfy the data.

The Type VI secretion system spike protein VgrG5 mediates membrane fusion during intercellular spread by pseudomallei group Burkholderia species

Pseudomallei group Burkholderia species are facultative intracellular parasites that spread efficiently from cell to cell by a mechanism involving the fusion of adjacent cell membranes. Intercellular fusion requires the function of the cluster 5 type VI secretion system (T6SS-5) and its associated valine-glycine repeat protein, VgrG5. Here we show that VgrG5 alleles are conserved and functionally interchangeable between Burkholderia pseudomallei and its relatives B. mallei, B. oklahomensis, and B. thailandensis. We also demonstrate that the integrity of the VgrG5 C-terminal domain is required for fusogenic activity, and we identify sequence motifs, including two hydrophobic segments, that are important for fusion. Mutagenesis and secretion experiments using B. pseudomallei strains engineered to express T6SS-5 in vitro show that the VgrG5 C-terminal domain is dispensable for T6SS-mediated secretion of Hcp5, demonstrating that the ability of VgrG5 to mediate membrane fusion can be uncoupled from its essential role in type VI secretion. We propose a model in which a unique fusogenic activity at the C terminus of VgrG5 facilitates intercellular spread by B. pseudomallei and related species following injection across the plasma membranes of infected cells.

A fusion-inhibiting peptide against Rift Valley fever virus inhibits multiple, diverse viruses

For enveloped viruses, fusion of the viral envelope with a cellular membrane is critical for a productive infection to occur. This fusion process is mediated by at least three classes of fusion proteins (Class I, II, and III) based on the protein sequence and structure. For Rift Valley fever virus (RVFV), the glycoprotein Gc (Class II fusion protein) mediates this fusion event following entry into the endocytic pathway, allowing the viral genome access to the cell cytoplasm. Here, we show that peptides analogous to the RVFV Gc stem region inhibited RVFV infectivity in cell culture by inhibiting the fusion process. Further, we show that infectivity can be inhibited for diverse, unrelated RNA viruses that have Class I (Ebola virus), Class II (Andes virus), or Class III (vesicular stomatitis virus) fusion proteins using this single peptide. Our findings are consistent with an inhibition mechanism similar to that proposed for stem peptide fusion inhibitors of dengue virus in which the RVFV inhibitory peptide first binds to both the virion and cell membranes, allowing it to traffic with the virus into the endocytic pathway. Upon acidification and rearrangement of Gc, the peptide is then able to specifically bind to Gc and prevent fusion of the viral and endocytic membranes, thus inhibiting viral infection. These results could provide novel insights into conserved features among the three classes of viral fusion proteins and offer direction for the future development of broadly active fusion inhibitors.

Entry into a cell is an essential stage of the viral replication cycle. Enveloped viruses require fusion of viral and cellular membranes for the viral genome to enter the cell cytoplasm. This entry is mediated by a viral fusion protein. Here, we synthesized peptides based on the Rift Valley fever virus (RVFV) fusion protein stem region and tested the peptides for their ability to prevent RVFV infection of cell cultures. We found that one of these peptides was able to inhibit RVFV infectivity by preventing the fusion process and that this peptide had broad activity against other RNA viruses including Ebola, Andes, and vesicular stomatitis viruses.

Biological and biochemical characterization of HIV-1 Gag/dgp41 virus-like particles expressed in Nicotiana benthamiana

The transmembrane HIV-1 envelope protein gp41 has been shown to play critical roles in the viral mucosal transmission and infection of CD4⁺ cells. Gag is a structural protein configuring the enveloped viral particles and has been suggested to constitute a target of the cellular immunity that may control viral load. We hypothesized that HIV enveloped virus-like particles (VLPs) consisting of Gag and a deconstructed form of gp41 comprising the membrane proximal external, transmembrane and cytoplasmic domains (dgp41) could be expressed in plants. To this end, plant-optimized HIV-1 genes were constructed and expressed in Nicotiana benthamiana by stable transformation, or transiently using a Tobamovirus-based expression system or a combination of both. Our results of biophysical, biochemical and electron microscopy characterization demonstrates that plant cells could support not only the formation of enveloped HIV-1 Gag VLPs, but also the accumulation of VLPs that incorporated dgp41. These findings provide further impetus for the journey towards a broadly efficacious and inexpensive subunit vaccine against HIV-1.

Small molecule fusion inhibitors: design, synthesis and biological evaluation of (Z)-3-(5-(3-benzyl-4-oxo-2-thioxothiazolidinylidene)methyl)-N-(3-carboxy-4-hydroxy)phenyl-2,5-dimethylpyrroles and related derivatives targeting HIV-1 gp41

By a scaffold elongation strategy, a series of (Z)-3-(5-(3-benzyl-4-oxo-2-thioxothiazolidinylidene)methyl)-N-(3-carboxy-4-hydroxy)phenyl-2,5-dimethylpyrroles and related derivatives with a linear multi-aromatic-ring skeleton were designed, synthesized, and evaluated in HIV-1 gp41 and cellular assays. Among them, the most active compounds, 12e, 12g, and 12k with a one-carbon linker (n=1) between the rhodanine (C) and phenyl (D) rings, exhibited very promising inhibitory potency with IC50 values of 1.8-2.6 μM and EC50 values of 0.3-1.5 μM against gp41 6-HB formation and HIV-1 replication in MT-2 cells, respectively. Additionally, they were almost equally effective against both T20-sensitive and resistant strains. The related SAR studies and molecular modeling results provided potential for further developing a new class of non-peptide small molecular fusion inhibitors targeting the HIV-1 gp41.

Design, synthesis, and biological evaluation of highly potent small molecule-peptide conjugates as new HIV-1 fusion inhibitors

The small molecule fusion inhibitors N-(4-carboxy-3-hydroxyphenyl)-2,5-dimethylpyrrole (NB-2) and N-(3-carboxy-4-hydroxyphenyl)-2,5-dimethylpyrrole (A12) target a hydrophobic pocket of HIV-1 gp41 and have moderate anti-HIV-1 activity. In this paper, we report the design, synthesis, and structure-activity relationship of a group of hybrid molecules in which the pocket-binding domain segment of the C34 peptide was replaced with NB-2 and A12 derivatives. In addition, the synergistic effect between the small molecule and peptide moieties was analyzed, and lead compounds with a novel scaffold were discovered. We found that either the nonpeptide or peptide part alone showed weak activity against HIV-1-mediated cell-cell fusion, but the conjugates properly generated a strong synergistic effect. Among them, conjugates Aoc-βAla-P26 and Noc-βAla-P26 exhibited a low nanomolar IC50 in the cell-cell fusion assay and effectively inhibited T20-sensitive and -resistant HIV-1 strains. Furthermore, the new molecules exhibited better stability against proteinase K digestion than T20 and C34.

Intercellular adhesion molecule 1 promotes HIV-1 attachment but not fusion to target cells

Incorporation of intercellular adhesion molecule 1 (ICAM-1) into HIV-1 particles is known to markedly enhance the virus binding and infection of cells expressing lymphocyte function-associated antigen-1 (LFA-1). At the same time, ICAM-1 has been reported to exert a less pronounced effect on HIV-1 fusion with lymphoid cells. Here we examined the role of ICAM-1/LFA-1 interactions in productive HIV-1 entry into lymphoid cells using a direct virus-cell fusion assay. ICAM-1 promoted HIV-1 attachment to cells in a temperature-dependent manner. It exerted a marginal effect on virus binding in the cold, but enhanced binding up to 4-fold at physiological temperature. ICAM-1-independent attachment in the cold was readily reversible upon subsequent incubation at elevated temperature, whereas ICAM-1-bearing particles were largely retained by cells. The better virus retention resulted in a proportional increase in HIV-1 internalization and fusion, suggesting that ICAM-1 did not specifically accelerate endocytosis or fusion steps. We also measured the rates of CD4 engagement, productive endocytosis and HIV-endosome fusion using specific fusion inhibitors. These rates were virtually independent of the presence of ICAM-1 in viral particles. Importantly, irrespective of the presence of ICAM-1, HIV-1 escaped from the low temperature block, which stopped virus endocytosis and fusion, much later than from a membrane-impermeant fusion inhibitor targeting surface-accessible particles. This result, along with the complete inhibition of HIV-1 fusion by a small molecule dynamin inhibitor, implies this virus enters lymphoid cells used in this study via endocytosis and that this pathway is not altered by the viral ICAM-1. Our data highlight the role of ICAM-1 in stabilizing the HIV-1 attachment to LFA-1 expressing cells, which leads to a proportional enhancement of the receptor-mediated uptake and fusion with endosomes.

A GxxxG-like motif within HIV-1 fusion peptide is critical to its immunosuppressant activity, structure, and interaction with the transmembrane domain of the T-cell receptor

To thrive in the human body, HIV fuses to its target cell and evades the immune response via several mechanisms. The fusion cascade is initiated by the fusion peptide (FP), which is located at the N-terminal of gp41, the transmembrane protein of HIV. Recently, it has been shown that the HIV-1 FP, particularly its 5-13 amino acid region (FP(5-13)), suppresses T-cell activation and interacts with the transmembrane domain (TMD) of the T-cell receptor (TCR) complex. Specific amino acid motifs often contribute to such interactions in TMDs of membrane proteins. Using bioinformatics and experimental studies, we report on a GxxxG-like motif (AxxxG), which is conserved in the FP throughout different clades and strains of HIV-1. Biological activity studies and FTIR spectroscopy revealed that HIV FP(5-13)-derived peptides, in which the motif was altered either by randomization or by a single amino acid shift, lost their immunosuppressive activity concomitant with a loss of the β-sheet structure in a membranous environment. Furthermore, fluorescence studies revealed that the inactive mutants lost their ability to interact with their target site, namely, the TMD of TCRα, designated CP. Importantly, lipotechoic acid activated macrophages (lacking TCR) were not affected by FP, further demonstrating the specificity of the immunosuppressant activity of CP. Finally, although the AxxxG WT and the GxxxG analog both associated with the CP and immunosuppressed T-cells, the AxxxG WT but not the GxxxG analog induced lipid mixing. Overall, the data support an important role for the AxxxG motif in the function of FP and might explain the natural selection of the AxxxG motif rather than the classical GxxxG motif in FP.

Structural basis of potent and broad HIV-1 fusion inhibitor CP32M

CP32M is a newly designed peptide fusion inhibitor possessing potent anti-HIV activity, especially against T20-resistant HIV-1 strains. In this study, we show that CP32M can efficiently inhibit a large panel of diverse HIV-1 variants, including subtype B', CRF07_BC, and CRF01_AE recombinants and naturally occurring or induced T20-resistant viruses. To elucidate its mechanism of action, we determined the crystal structure of CP32M complexed with its target sequence. Differing from its parental peptide, CP621-652, the (621)VEWNEMT(627) motif of CP32M folds into two α-helix turns at the N terminus of the pocket-binding domain, forming a novel layer in the six-helix bundle structure. Prominently, the residue Asn-624 of the (621)VEWNEMT(627) motif is engaged in the polar interaction with a hydrophilic ridge that borders the hydrophobic pocket on the N-terminal coiled coil. The original inhibitor design of CP32M provides several intra- and salt bridge/hydrogen bond interactions favoring the stability of the helical conformation of CP32M and its interactions with N-terminal heptad repeat (NHR) targets. We identified a novel salt bridge between Arg-557 on the NHR and Glu-648 of CP32M that is critical for the binding of CP32M and resistance against the inhibitor. Therefore, our data present important information for developing novel HIV-1 fusion inhibitors for clinical use.

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

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

Viral envelope protein folding and membrane hemifusion are enhanced by the conserved loop region of HIV-1 gp41

Fusion of human immunodeficiency virus (HIV-1) with target cells is mediated by the gp41 transmembrane envelope protein. The loop region within gp41 contains 2 crucial cysteines that play an unknown role in HIV-cell fusion. On the basis of cell-cell fusion assay, using human T-cell lines [Jurkat E6-1 and Jurkat HXBc2(4)], and virus-cell fusion assay, using fully infectious HIV-1 HXBc2 virus and TZM-bl human cell line, we provide evidence that the oxidation state of the disulfide bond within a loop domain peptide determines its activity. The oxidized (closed) form inhibits fusion, while the reduced (opened) form enhances hemifusion. These opposite activities reach 60% difference in viral fusion. Both forms of the loop domain interact with gp41: the opened form enhances gp41 folding into a bundle, whereas the closed form inhibits this folding. Therefore, the transformation of the cysteines from a reduced to an oxidized state enables the loop to convert from opened to closed conformations, which assists gp41 to fold and induces hemifusion. The significant conservation of the loop region within many envelope proteins suggests a general mechanism, which is exploited by viruses to enhance entry into their host cells.

A novel chimeric protein-based HIV-1 fusion inhibitor targeting gp41 glycoprotein with high potency and stability

T20 (enfuvirtide, Fuzeon) is the first generation HIV-1 fusion inhibitor approved for salvage therapy of HIV-1-infected patients refractory to current antiretroviral drugs. However, its application is limited by the high cost of peptide synthesis, rapid proteolysis, and poor efficacy against emerging drug-resistant strains. Here we reported the design of a novel chimera protein-based fusion inhibitor targeting gp41, TLT35, that uses a flexible 35-mer linker to couple T20 and T1144, the first and next generation HIV-1 fusion inhibitors, respectively. TLT35, which was expressed in Escherichia coli with good yield, showed low nm activity against HIV-1-mediated cell-cell fusion and infection by laboratory-adapted HIV-1 strains (X4 or R5), including T20-resistant variants and primary HIV-1 isolates of clades A to G and group O (R5 or X4R5). TLT35 was stable in human sera and in peripheral blood mononuclear cell culture and was more resistant to proteolysis than either T20 or T1144 alone. Circular dichroism spectra showed that TLT35 folded into a thermally stable conformation with high α-helical content and T(m) value in aqueous solution. It formed a highly stable complex with gp41 N-terminal heptad repeat peptide and blocked formation of the gp41 six-helix-bundle core. These merits combined with an anticipated low production cost for expression of TLT35 in E. coli make this novel protein-based fusion inhibitor a promising candidate for further development as an anti-HIV-1 microbicide or therapeutic for the prevention and treatment of HIV-1 infection.