Structural and functional analysis of interhelical interactions in the human immunodeficiency virus type 1 gp41 envelope glycoprotein by alanine-scanning mutagenesis

Natural Variation in HIV-1 Entry Kinetics Map to Specific Residues and Reveal an Interdependence Between Attachment and Fusion

HIV-1 entry kinetics reflect the fluid motion of the HIV envelope glycoprotein through at least three major structural configurations that drive virus-cell membrane fusion. The lifetime of each state is an important component of potency for inhibitors that target them. We used the time-of-addition inhibitor assay and a novel analytical strategy to define the kinetics of pre-hairpin exposure (using T20) and co-receptor engagement (via. maraviroc), through a characteristic delay metric, across a variety of naturally occurring HIV Env isolates. Among 257 distinct HIV-1 envelope isolates we found a remarkable breadth of T20 and maraviroc delays ranging from as early as 30 seconds to as late as 60 minutes. The most extreme delays were observed among transmission-linked clade C isolates. We identified four single-residue determinants of late T20 and maraviroc delays that are associated with either receptor engagement or gp41 function. Comparison of these delays with T20 sensitivity suggest co-receptor engagement and fusogenic activity in gp41 act cooperatively but sequentially to drive entry. Our findings support current models of entry where co-receptor engagement drives gp41 eclipse and have strong implications for the design of entry inhibitors and antibodies that target transient entry states.

Author Summary

The first step of HIV-1 infection is entry, where virus-cell membrane fusion is driven by the HIV-1 envelope glycoprotein through a series of conformational changes. Some of the most broadly active entry inhibitors work by binding conformations that exist only transiently during entry. The lifetimes of these states and the kinetics of entry are important elements of inhibitor activity for which little is known. We demonstrate a remarkable range of kinetics among 257 diverse HIV-1 isolates and find that this phenotype is highly flexible, with multiple single-residue determinants. Examination of the kinetics of two conformational landmarks shed light on novel kinetic features that offer new details about the role of co-receptor engagement and provide a framework to explain entry inhibitor synergy.

Molecular dynamics simulations show how antibodies may rescue HIV-1 mutants incapable of infecting host cells

High mutation and replication rates of HIV-1 result in the continuous generation of variants, allowing it to adapt to changing host environments. Mutations often have deleterious effects, but variants carrying them are rapidly purged. Surprisingly, a particular variant incapable of entering host cells was found to be rescued by host antibodies targeting HIV-1. Understanding the molecular mechanism of this rescue is important to develop and improve antibody-based therapies. To unravel the underlying mechanisms, we performed fully atomistic molecular dynamics simulations of the HIV-1 gp41 trimer responsible for viral entry into host cells, its entry-deficient variant, and its complex with the rescuing antibody. We find that the Q563R mutation, which the entry-deficient variant carries, prevents the native conformation of the gp41 6-helix bundle required for entry and stabilizes an alternative conformation instead. This is the consequence of substantial changes in the secondary structure and interactions between the domains of gp41. Binding of the antibody F240 to gp41 reverses these changes and re-establishes the native conformation, resulting in rescue. To test the generality of this mechanism, we performed simulations with the entry-deficient L565A variant and antibody 3D6. We find that 3D6 binding was able to reverse structural and interaction changes introduced by the mutation and restore the native gp41 conformation. Viral variants may not only escape antibodies but be aided by them in their survival, potentially compromising antibody-based therapies, including vaccination and passive immunization. Our simulation framework could serve as a tool to assess the likelihood of such resistance against specific antibodies.Communicated by Ramaswamy H. SarmaCommunicated by Ramaswamy H. Sarma.

HIV Infection: Shaping the Complex, Dynamic, and Interconnected Network of the Cytoskeleton

HIV-1 has evolved a plethora of strategies to overcome the cytoskeletal barrier (i.e., actin and intermediate filaments (AFs and IFs) and microtubules (MTs)) to achieve the viral cycle. HIV-1 modifies cytoskeletal organization and dynamics by acting on associated adaptors and molecular motors to productively fuse, enter, and infect cells and then traffic to the cell surface, where virions assemble and are released to spread infection. The HIV-1 envelope (Env) initiates the cycle by binding to and signaling through its main cell surface receptors (CD4/CCR5/CXCR4) to shape the cytoskeleton for fusion pore formation, which permits viral core entry. Then, the HIV-1 capsid is transported to the nucleus associated with cytoskeleton tracks under the control of specific adaptors/molecular motors, as well as HIV-1 accessory proteins. Furthermore, HIV-1 drives the late stages of the viral cycle by regulating cytoskeleton dynamics to assure viral Pr55Gag expression and transport to the cell surface, where it assembles and buds to mature infectious virions. In this review, we therefore analyze how HIV-1 generates a cell-permissive state to infection by regulating the cytoskeleton and associated factors. Likewise, we discuss the relevance of this knowledge to understand HIV-1 infection and pathogenesis in patients and to develop therapeutic strategies to battle HIV-1.

Deep Mutational Scanning of Viral Glycoproteins and Their Host Receptors

Deep mutational scanning or deep mutagenesis is a powerful tool for understanding the sequence diversity available to viruses for adaptation in a laboratory setting. It generally involves tracking an in vitro selection of protein sequence variants with deep sequencing to map mutational effects based on changes in sequence abundance. Coupled with any of a number of selection strategies, deep mutagenesis can explore the mutational diversity available to viral glycoproteins, which mediate critical roles in cell entry and are exposed to the humoral arm of the host immune response. Mutational landscapes of viral glycoproteins for host cell attachment and membrane fusion reveal extensive epistasis and potential escape mutations to neutralizing antibodies or other therapeutics, as well as aiding in the design of optimized immunogens for eliciting broadly protective immunity. While less explored, deep mutational scans of host receptors further assist in understanding virus-host protein interactions. Critical residues on the host receptors for engaging with viral spikes are readily identified and may help with structural modeling. Furthermore, mutations may be found for engineering soluble decoy receptors as neutralizing agents that specifically bind viral targets with tight affinity and limited potential for viral escape. By untangling the complexities of how sequence contributes to viral glycoprotein and host receptor interactions, deep mutational scanning is impacting ideas and strategies at multiple levels for combatting circulating and emergent virus strains.

New Biophysical Approaches Reveal the Dynamics and Mechanics of Type I Viral Fusion Machinery and Their Interplay with Membranes

Protein-mediated membrane fusion is a highly regulated biological process essential for cellular and organismal functions and infection by enveloped viruses. During viral entry the membrane fusion reaction is catalyzed by specialized protein machinery on the viral surface. These viral fusion proteins undergo a series of dramatic structural changes during membrane fusion where they engage, remodel, and ultimately fuse with the host membrane. The structural and dynamic nature of these conformational changes and their impact on the membranes have long-eluded characterization. Recent advances in structural and biophysical methodologies have enabled researchers to directly observe viral fusion proteins as they carry out their functions during membrane fusion. Here we review the structure and function of type I viral fusion proteins and mechanisms of protein-mediated membrane fusion. We highlight how recent technological advances and new biophysical approaches are providing unprecedented new insight into the membrane fusion reaction.

Conserved Residue Asn-145 in the C-Terminal Heptad Repeat Region of HIV-1 gp41 is Critical for Viral Fusion and Regulates the Antiviral Activity of Fusion Inhibitors

Entry of HIV-1 into target cells is mediated by its envelope (Env) glycoprotein composed of the receptor binding subunit gp120 and the fusion protein gp41. Refolding of the gp41 N- and C-terminal heptad repeats (NHR and CHR) into a six-helix bundle (6-HB) conformation drives the viral and cellular membranes in close apposition and generates huge amounts of energy to overcome the kinetic barrier leading to membrane fusion. In this study, we focused on characterizing the structural and functional properties of a single Asn-145 residue, which locates at the middle CHR site of gp41 and is extremely conserved among all the HIV-1, HIV-2, and simian immunodeficiency virus (SIV) isolates. By mutational analysis, we found that Asn-145 plays critical roles for Env-mediated cell-cell fusion and HIV-1 entry. As determined by circular dichroism (CD) spectroscopy and isothermal titration calorimetry (ITC), the substitution of Asn-145 with alanine (N145A) severely impaired the interactions between the NHR and CHR helices. Asn-145 was also verified to be important for the antiviral activity of CHR-derived peptide fusion inhibitors and served as a turn-point for the inhibitory potency. Intriguingly, Asn-145 could regulate the functionality of the M-T hook structure at the N-terminus of the inhibitors and displayed comparable activities with the C-terminal IDL anchor. Crystallographic studies further demonstrated the importance of Asn-145-mediated interhelical and intrahelical interactions in the 6-HB structure. Combined, the present results have provided valuable information for the structure-function relationship of HIV-1 gp41 and the structure-activity relationship of gp41-dependent fusion inhibitors.

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.

Antigenic characterization of the human immunodeficiency virus (HIV-1) envelope glycoprotein precursor incorporated into nanodiscs

The entry of human immunodeficiency virus (HIV-1) into host cells is mediated by the viral envelope glycoproteins (Envs), which are derived by the proteolytic cleavage of a trimeric gp160 Env precursor. The mature Env trimer is a major target for entry inhibitors and vaccine-induced neutralizing antibodies. Env interstrain variability, conformational flexibility and heavy glycosylation contribute to evasion of the host immune response, and create challenges for structural characterization and vaccine development. Here we investigate variables associated with reconstitution of the HIV-1 Env precursor into nanodiscs, nanoscale lipid bilayer discs enclosed by membrane scaffolding proteins. We identified detergents, as well as lipids similar in composition to the viral lipidome, that allowed efficient formation of Env-nanodiscs (Env-NDs). Env-NDs were created with the full-length Env precursor and with an Env precursor with the majority of the cytoplasmic tail intact. The self-association of Env-NDs was decreased by glutaraldehyde crosslinking. The Env-NDs exhibited an antigenic profile expected for the HIV-1 Env precursor. Env-NDs were recognized by broadly neutralizing antibodies. Of note, neutralizing antibody epitopes in the gp41 membrane-proximal external region and in the gp120:gp41 interface were well exposed on Env-NDs compared with Env expressed on cell surfaces. Most Env epitopes recognized by non-neutralizing antibodies were masked on the Env-NDs. This antigenic profile was stable for several days, exhibiting a considerably longer half-life than that of Env solubilized in detergents. Negative selection with weak neutralizing antibodies could be used to improve the antigenic profile of the Env-NDs. Finally, we show that lipid adjuvants can be incorporated into Env-NDs. These results indicate that Env-NDs represent a potentially useful platform for investigating the structural, functional and antigenic properties of the HIV-1 Env trimer in a membrane context.

Experimental Estimation of the Effects of All Amino-Acid Mutations to HIV's Envelope Protein on Viral Replication in Cell Culture

HIV is notorious for its capacity to evade immunity and anti-viral drugs through rapid sequence evolution. Knowledge of the functional effects of mutations to HIV is critical for understanding this evolution. HIV's most rapidly evolving protein is its envelope (Env). Here we use deep mutational scanning to experimentally estimate the effects of all amino-acid mutations to Env on viral replication in cell culture. Most mutations are under purifying selection in our experiments, although a few sites experience strong selection for mutations that enhance HIV's replication in cell culture. We compare our experimental measurements of each site's preference for each amino acid to the actual frequencies of these amino acids in naturally occurring HIV sequences. Our measured amino-acid preferences correlate with amino-acid frequencies in natural sequences for most sites. However, our measured preferences are less concordant with natural amino-acid frequencies at surface-exposed sites that are subject to pressures absent from our experiments such as antibody selection. Our data enable us to quantify the inherent mutational tolerance of each site in Env. We show that the epitopes of broadly neutralizing antibodies have a significantly reduced inherent capacity to tolerate mutations, rigorously validating a pervasive idea in the field. Overall, our results help disentangle the role of inherent functional constraints and external selection pressures in shaping Env's evolution.

Identification and characterization of a subpocket on the N-trimer of HIV-1 Gp41: implication for viral entry and drug target

OBJECTIVE

Crystallographic studies of HIV-1 gp41 demonstrate a stable six-helix bundle (6-HB) folded by trimeric N and C-terminal heptad repeats (NHR and CHR), and a deep hydrophobic pocket (pocket-1) on the NHR helices (N-trimer); however, previous crystal structures of 6-HB core were determined by peptide fragments missing the downstream sequence of pocket-1; thus, the structural features of this site could not be observed.

DESIGN

We recently determined several 6-HB structures containing the pocket-1 and its downstream site. Here, we focused to investigate the structural features of N-trimer previously uncharacterized.

METHODS

Biophysical, biochemical and functional approaches were combined to characterize the downstream residues of pocket-1.

RESULTS

A subpocket (designated pocket-2) was visualized on the C-terminal portion of N-trimer, which is formed by a cluster of seven residues, including Leu587, Lys588 and Glu584 on one NHR helix and Tyr586, Val583, Ala582 and Arg579 of another NHR helix. Mutagenesis studies demonstrated that the pocket-2 residues play essential roles for HIV-1 Env-mediated cell entry and critically determine the antiviral activity of NHR-derived peptide fusion inhibitor T21. Further, the pocket-2 mutations dramatically impaired the thermostability and conformation of 6-HB structure and reduced the binding affinity of CHR-derived inhibitor HP23 that specifically targets the deep pocket-1.

CONCLUSION

These data have provided important information for the structure-function relationship of HIV-1 gp41 and for the development of antiviral entry inhibitors.

Effects of the I559P gp41 change on the conformation and function of the human immunodeficiency virus (HIV-1) membrane envelope glycoprotein trimer

The mature human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer is produced by proteolytic cleavage of a precursor and consists of three gp120 exterior and three gp41 transmembrane subunits. The metastable Env complex is induced to undergo conformational changes required for virus entry by the binding of gp120 to the receptors, CD4 and CCR5/CXCR4. An isoleucine-to-proline change (I559P) in the gp41 ectodomain has been used to stabilize soluble forms of HIV-1 Env trimers for structural characterization and for use as immunogens. In the native membrane-anchored HIV-1_BG505 Env, the I559P change modestly decreased proteolytic maturation, increased the non-covalent association of gp120 with the Env trimer, and resulted in an Env conformation distinctly different from that of the wild-type HIV-1_BG505 Env. Compared with the wild-type Env, the I559P Env was recognized inefficiently by polyclonal sera from HIV-1-infected individuals, by several gp41-directed antibodies, by some antibodies against the CD4-binding site of gp120, and by antibodies that preferentially recognize the CD4-bound Env. Some of the gp120-associated antigenic differences between the wild-type HIV-1_BG505 Env and the I559P mutant were compensated by the SOS disulfide bond between gp120 and gp41, which has been used to stabilize cleaved soluble Env trimers. Nonetheless, regardless of the presence of the SOS changes, Envs with proline 559 were recognized less efficiently than Envs with isoleucine 559 by the VRC01 neutralizing antibody, which binds the CD4-binding site of gp120, and the PGT151 neutralizing antibody, which binds a hybrid gp120-gp41 epitope. The I559P change completely eliminated the ability of the HIV-1_BG505 Env to mediate cell-cell fusion and virus entry, and abolished the capacity of the SOS Env to support virus infection in the presence of a reducing agent. These results suggest that differences exist between the quaternary structures of functional Env spikes and I559P Envs.

C peptides as entry inhibitors for gene therapy

Peptides derived from the C-terminal heptad repeat 2 region of the HIV-1 gp41 envelope glycoprotein, so-called C peptides, are very potent HIV-1 fusion inhibitors. Antiviral genes encoding either membrane-anchored (ma) or secreted (iSAVE) C peptides have been engineered and allow direct in vivo production of the therapeutic peptides by genetically modified host cells. Membrane-anchored C peptides expressed in the HIV-1 target cells by T-cell or hematopoietic stem cell gene therapy efficiently prevent virus entry into the modified cells. Such gene-protection confers a selective survival advantage and allows accumulation of the genetically modified cells. Membrane-anchored C peptides have been successfully tested in a nonhuman primate model of AIDS and were found to be safe in a phase I clinical trial in AIDS patients transplanted with autologous gene-modified T-cells. Secreted C peptides have the crucial advantage of not only protecting genetically modified cells from HIV-1 infection, but also neighboring cells, thus suppressing virus replication even if only a small fraction of cells is genetically modified. Accordingly, various cell types can be considered as potential in vivo producer cells for iSAVE-based gene therapeutics, which could even be modified by direct in vivo gene delivery in future. In conclusion, C peptide gene therapeutics may provide a strong benefit to AIDS patients and could present an effective alternative to current antiretroviral drug regimens.

Molecular dynamics studies of the inhibitor C34 binding to the wild-type and mutant HIV-1 gp41: inhibitory and drug resistant mechanism

Mutations on NHR (N-terminal heptad repeat) associated with resistance to fusion inhibitor were observed. In addition, mutations on CHR (C-terminal heptad repeat) accompanied NHR mutations of gp41 are noted in many cases, like N43D/S138A double mutation. In this work, we explored the drug resistant mechanism of N43D mutation and the role of S138A second mutation in drug resistance. The binding modes of the wild type gp41 and the two mutants, N43D and N43D/S138A, with the HIV-1 fusion inhibitor C34, a 34-residue peptide mimicking CHR of gp41, were carried out by using molecular dynamics simulations. Based on the MD simulations, N43D mutation affects not only the stability of C34 binding, but also the binding energy of the inhibitor C34. Because N43D mutation may also affect the stable conformation of 6-HB, we introduced S138A second mutation into CHR of gp41 and determined the impact of this mutation. Through the comparative analysis of MD results of the N43D mutant and the N43D/S138A mutant, we found that CHR with S138A mutation shown more favorable affinity to NHR. Compelling differences in structures have been observed for these two mutants, particularly in the binding modes and in the hydrophobic interactions of the CHR (C34) located near the hydrophobic groove of the NHR. Because the conformational stability of 6-HB is important to HIV-1 infection, we suggested a hypothetical mechanism for the drug resistance: N43D single mutation not only impact the binding of inhibitor, but also affect the affinity between NHR and CHR of gp41, thus may reduce the rate of membrane fusion; compensatory mutation S138A would induce greater hydrophobic interactions between NHR and CHR, and render the CHR more compatible to NHR than inhibitors.

Hydrophobic mutations in buried polar residues enhance HIV-1 gp41 N-terminal heptad repeat-C-terminal heptad repeat interactions and C-peptides' anti-HIV activity

OBJECTIVE

To investigate the effect of mutations in a highly conserved buried polar area on the function of HIV-1 gp41.

DESIGN

During HIV-1 entry, a six helical bundle (6-HB) formation between the C-terminal and N-terminal heptad repeat (CHR and NHR) of gp41 provides energy for virus cell membrane fusion. In 6-HB, residues at a and d (a-d) positions of CHR directly interact with NHR and are buried. They are considered critical residues for 6-HB stability and for anti-HIV-1 activity of CHR-derived peptides (C-peptides). Most of a-d residues in CHR are hydrophobic, as buried hydrophobic residues facilitate protein stability. However, HIV-1 gp41 CHR contains a highly conserved polar area with four successive buried a-d polar residues: S649/Q652/N656/E659. We mutated these buried polar residues to hydrophobic residues, either Leu or Ile, and studied its effect on the gp41 NHR-CHR interactions and anti-HIV activities of the C-peptides.

METHODS

We measured the C-peptide mutants' ability to form 6-HB with NHR, thermal stability of the 6-HBs and C-peptides' inhibitory activity against both T20-sensitive and resistant HIV-1 strains.

RESULTS

All the mutated C-peptides retained their ability to form stable 6-HB with NHR and strongly inhibited HIV-1 replication. Strikingly, S649L and E659I mutations endow C-peptide with a significantly enhanced activity against T20-resistant HIV-1 strains.

CONCLUSION

The highly conserved buried a-d polar residues in HIV-1 gp41 CHR can be mutated as a means of developing new fusion inhibitors against drug-resistant HIV-1 strains. The concept can also be utilized to design fusion inhibitors against other viruses with similar mechanisms.

Molecular dynamics modeling the synthetic and biological polymers interactions pre-studied via docking: anchors modified polyanions interference with the HIV-1 fusion mediator

In previous works we reported the design, synthesis and in vitro evaluations of synthetic anionic polymers modified by alicyclic pendant groups (hydrophobic anchors), as a novel class of inhibitors of the human immunodeficiency virus type 1 (HIV-1) entry into human cells. Recently, these synthetic polymers interactions with key mediator of HIV-1 entry-fusion, the tri-helix core of the first heptad repeat regions [HR1]3 of viral envelope protein gp41, were pre-studied via docking in terms of newly formulated algorithm for stepwise approximation from fragments of polymeric backbone and side-group models toward real polymeric chains. In the present article the docking results were verified under molecular dynamics (MD) modeling. In contrast with limited capabilities of the docking, the MD allowed of using much more large models of the polymeric ligands, considering flexibility of both ligand and target simultaneously. Among the synthesized polymers the dinorbornen anchors containing alternating copolymers of maleic acid were selected as the most representative ligands (possessing the top anti-HIV activity in vitro in correlation with the highest binding energy in the docking). To verify the probability of binding of the polymers with the [HR1]3 in the sites defined via docking, various starting positions of polymer chains were tried. The MD simulations confirmed the main docking-predicted priority for binding sites, and possibilities for axial and belting modes of the ligands-target interactions. Some newly MD-discovered aspects of the ligand's backbone and anchor units dynamic cooperation in binding the viral target clarify mechanisms of the synthetic polymers anti-HIV activity and drug resistance prevention.

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.

Binding ofN-substituted pyrrole derivatives to HIV-1 gp41

Developing small molecule inhibitors of human immunodeficiency virus type 1 (HIV-1) fusion has attracted significant interest. Recently, Jiang have reported several natural and synthetic N -substituted pyrrole derivatives targeting gp41 that are experimentally shown to inhibit cell–cell fusion in the low micromolar range. In order to help gain insight on the binding mechanism, we carried out computational study to help identify possible binding modes and to characterize structures of binding complexes. Detailed gp41-molecule binding interactions and free energies of binding are obtained through molecular dynamics (MD) simulation and MM-PBSA calculation. Specific molecular interactions in the gp41-inhibitor complexes are identified. Current computational study complements the corresponding experimental investigation and provides theoretical understanding on the binding mechanism which is helpful for further refinement of small molecule inhibitors of gp41.

COMPUTATIONAL STUDY OF HIV-1 gp41 NHR TRIMER: INHIBITION MECHANISMS OF N-SUBSTITUTED PYRROLE DERIVATIVES AND FRAGMENT-BASED VIRTUAL SCREENING

Fusion of HIV-1 viral and host cellular membranes is an important step for HIV infection. The HIV-1 envelope glycoprotein mediating the membrane fusion consists of subunits gp120 and gp41 whereas gp120 recognizes the cell-surface receptors and gp41 promotes viral-cell membrane fusion. The trimeric helical complex composed of heterodimer of N-terminal and C-terminal extraviral segments has been used for the gp41 function study, and the trimeric N-terminal teptad repeat (NHR) is considered as an antiviral drug target for developing HIV-1 membrane fusion inhibitors. By using computational solvent probe mapping, we have explored druggable sites on the trimeric NHR peptides, and identified residues K574 and R579 as the hot spots for inhibitor designing. We further demonstrated that although NB-2 and NB-64 are all N-substituted Pyrrole derivatives and have very similar chemical structures, it is possible that diverse inhibitory mechanisms targeting different negative electrostatic residues (K574 and R579) exist. Results from fragment-based virtual screening identified series of potential lead compounds which could be used for further design of fusion inhibitors.

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.

A novel view of modelling interactions between synthetic and biological polymers via docking

Multipoint interactions between synthetic and natural polymers provide a promising platform for many topical applications, including therapeutic blockage of virus-specific targets. Docking may become a useful tool for modelling of such interactions. However, the rigid docking cannot be correctly applied to synthetic polymers with flexible chains. The application of flexible docking to these polymers as whole macromolecule ligands is also limited by too many possible conformations. We propose to solve this problem via stepwise flexible docking. Step 1 is docking of separate polymer components: (1) backbone units (BU), multi-repeated along the chain, and (2) side groups (SG) consisting of functionally active elements (SG(F)) and bridges (SG(B)) linking SG(F) with BU. At this step, probable binding sites locations and binding energies for the components are scored. Step 2 is docking of component-integrating models: [BU](m), SG = SG(F)-SG(B), BU-SG, BU-BU(SG)-BU, BU(SG)-[BU](m)-BU(SG), and [BU(var)(SG(var))](m). Every modelling level yields new information, including how the linkage of various components influences on the ligand-target contacts positioning, orientation, and binding energy in step-by-step approximation to polymeric ligand motifs. Step 3 extrapolates the docking results to real-scale macromolecules. This approach has been demonstrated by studying the interactions between hetero-SG modified anionic polymers and the N-heptad repeat region tri-helix core of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp41, the key mediator of HIV-1 fusion during virus entry. The docking results are compared to real polymeric compounds, acting as HIV-1 entry inhibitors in vitro. This study clarifies the optimal macromolecular design for the viral fusion inhibition and drug resistance prevention.

The conserved residue Arg46 in the N-terminal heptad repeat domain of HIV-1 gp41 is critical for viral fusion and entry

During the process of HIV-1 fusion with the target cell, the N-terminal heptad repeat (NHR) of gp41 interacts with the C-terminal heptad repeat (CHR) to form fusogenic six-helix bundle (6-HB) core. We previously identified a crucial residue for 6-HB formation and virus entry - Lys63 (K63) in the C-terminal region of NHR (aa 54–70), which forms a hydrophobic cavity. It can form an important salt bridge with Asp121 (D121) in gp41 CHR. Here, we found another important conserved residue for virus fusion and entry, Arg46 (R46), in the N-terminal region of NHR (aa 35–53), which forms a hydrogen bond with a polar residue, Asn43 (N43), in NHR, as a part of the hydrogen-bond network. R46 can also form a salt bridge with a negatively charged residue, Glu137 (E137), in gp41 CHR. Substitution of R46 with the hydrophobic residue Ala (R46A) or the negatively charged residue Glu (R46E) resulted in disruption of the hydrogen bond network, breakage of the salt bridge and reduction of 6-HB’s stability, leading to impairment of viral fusion and decreased inhibition of N36, an NHR peptide. Similarly, CHR peptide C34 with substitution of E137 for Ala (E137A) or Arg (E137R) also exhibited reduced inhibitory activity against HIV-1 infection and HIV-1-mediated cell-to-cell fusion. These results suggest that the positively charged residue R46 and its hydrogen bond network, together with the salt bridge between R46 and E137, are important for viral fusion and entry and may therefore serve as a target for designing novel HIV fusion/entry inhibitors.

The M-T hook structure is critical for design of HIV-1 fusion inhibitors

CP621-652 is a potent HIV-1 fusion inhibitor peptide derived from the C-terminal heptad repeat of gp41. We recently identified that its N-terminal residues Met-626 and Thr-627 adopt a unique hook-like structure (termed M-T hook) thus stabilizing the interaction of the inhibitor with the deep pocket on the N-terminal heptad repeat. In this study, we further demonstrated that the M-T hook structure is a key determinant of CP621-652 in terms of its thermostability and anti-HIV activity. To directly define the structure and function of the M-T hook, we generated the peptide MT-C34 by incorporating Met-626 and Thr-627 into the N terminus of the C-terminal heptad repeat-derived peptide C34. The high resolution crystal structure (1.9 Å) of MT-C34 complexed by an N-terminal heptad repeat-derived peptide reveals that the M-T hook conformation is well preserved at the N-terminal extreme of the inhibitor. Strikingly, addition of two hook residues could dramatically enhance the binding affinity and thermostability of 6-helix bundle core. Compared with C34, MT-C34 exhibited significantly increased activity to inhibit HIV-1 envelope-mediated cell fusion (6.6-fold), virus entry (4.5-fold), and replication (6-fold). Mechanistically, MT-C34 had a 10.5-fold higher increase than C34 in blocking 6-helix bundle formation. We further showed that MT-C34 possessed higher potency against T20 (Enfuvirtide, Fuzeon)-resistant HIV-1 variants. Therefore, this study provides convincing data for our proposed concept that the M-T hook structure is critical for designing HIV-1 fusion inhibitors.

Discovery of critical residues for viral entry and inhibition through structural Insight of HIV-1 fusion inhibitor CP621-652

The core structure of HIV-1 gp41 is a stable six-helix bundle (6-HB) folded by its trimeric N- and C-terminal heptad repeats (NHR and CHR). We previously identified that the (621)QIWNNMT(627) motif located at the upstream region of gp41 CHR plays critical roles for the stabilization of the 6-HB core and peptide CP621-652 containing this motif is a potent HIV-1 fusion inhibitor, however, the molecular determinants underlying the stability and anti-HIV activity remained elusive. In this study, we determined the high-resolution crystal structure of CP621-652 complexed by T21. We find that the (621)QIWNNMT(627) motif does not maintain the α-helical conformation. Instead, residues Met(626) and Thr(627) form a unique hook-like structure (denoted as M-T hook), in which Thr(627) redirects the peptide chain to position Met(626) above the left side of the hydrophobic pocket on the NHR trimer. The side chain of Met(626) caps the hydrophobic pocket, stabilizing the interaction between the pocket and the pocket-binding domain. Our mutagenesis studies demonstrate that mutations of the M-T hook residues could completely abolish HIV-1 Env-mediated cell fusion and virus entry, and significantly destabilize the interaction of NHR and CHR peptides and reduce the anti-HIV activity of CP621-652. Our results identify an unusual structural feature that stabilizes the six-helix bundle, providing novel insights into the mechanisms of HIV-1 fusion and inhibition.

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.

Increasing hydrophobicity of residues in an anti-HIV-1 Env peptide synergistically improves potency

T-20/Fuzeon/Enfuvirtide (ENF), a peptide inhibitor of HIV-1 infection, targets the grooves created by heptad repeat 2 (HR2) of Env's coiled-coil, but mutants resistant to ENF emerge. In this study, ENF-resistant mutants--V38A, N43D, N43D/S138A, Q40H/L45M--were combined with modified inhibitory peptides to identify what we believe to be novel ways to improve peptide efficacy. V38A did not substantially reduce infectivity, but was relatively resistant to inhibitory peptides. N43D was more resistant to inhibitory peptides than wild-type, but infectivity was reduced. The additional mutation S138A (N43D/S138A) increased infectivity and further reduced peptide inhibitory potency. It is concluded that S138A increased binding of HR2/ENF into grooves and that S138A compensated for electrostatic repulsion between N43D and HR2. The six-helix bundle structure indicated that E148A should increase hydrophobic interactions between the coiled-coil and peptide. Importantly, the modifications S138A and E148A in the same peptide retained potency against ENF-escape mutants. The double mutant's increase in potency was greater than the increases from the sum of S138A and E148A individually, showing that these two altered residues synergistically contributed to peptide binding. Isothermal titration calorimetry established that hydrophobic substitutions at positions S138 and E148 improved potency of inhibitory peptides against escape mutants by increasing enthalpic release of energy upon peptide binding.

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.

Alanine scanning mutagenesis of HIV-1 gp41 heptad repeat 1: insight into the gp120-gp41 interaction

On the basis of mutagenesis, biochemical, and structural studies, heptad repeat 1 of HIV gp41 (HR1) has been shown to play numerous critical roles in HIV entry, including interacting with gp120 in prefusion states and interacting with gp41 heptad repeat 2 (HR2) in the fusion state. Moreover, HR1 is the site of therapeutic intervention by enfuviritide, a peptide analogue of HR2. In this study, the functional importance of each amino acid residue in gp41 HR1 has been systematically examined by alanine scanning mutagenesis, with subsequent characterization of the mutagenic effects on folding (as measured by incorporation into virions), association with gp120, and membrane fusion. The mutational effects on entry can be grouped into three classes: (1) wild type (defined as >40% of wild-type entry), (2) impaired (defined as 5-40% of wild-type entry), and (3) nonfunctional (defined as <5% of wild-type entry). Interestingly, the majority of HR1 mutations (77%) exhibit impaired or nonfunctional entry. Surprisingly, effects of mutations on folding, association, or fusion are not correlated to heptad position; however, folding defects are most often found in the N-terminal region of HR1. Moreover, disruption of the gp41-gp120 interaction is correlated to the C-terminal region of HR1, suggesting that this region interacts most closely with gp120. In summary, the sensitivity of gp41 HR1 to alanine substitutions suggests that even subtle changes in the local environment may severely affect envelope function, thereby strengthening the notion that HR1 is an attractive site for therapeutic intervention.

Impact of the enfuvirtide resistance mutation N43D and the associated baseline polymorphism E137K on peptide sensitivity and six-helix bundle structure

Enfuvirtide (ENF), the first human immunodeficiency virus type 1 (HIV-1) fusion inhibitor approved for clinical use, acts by binding to gp41 heptad repeat 1 (HR1) and preventing its interaction with the viral HR2 region. Treatment-emergent resistance to ENF has been mapped to residues within HR1, and these mutations decrease its susceptibility to ENF and may reduce viral fitness and pathogenesis, although the mechanism for these effects is not clear. N43D, a common ENF resistance mutation, was found in in vitro assays to cause a 5-50-fold in antiviral activity. We introduced this mutation into peptide models and determined the impact of this mutation by circular dichroism and X-ray crystallography. We find that the mutation results in a decrease in the thermal stability of the six-helix bundle and causes a significant change in the HR1-HR2 interface, including a loss of HR2 helicity. These data form a mechanistic basis for the decrease in ENF sensitivity and six-helix bundle stability. The E137K polymorphism, generally present at baseline in patients who develop N43D, partially compensates for the loss of stability, and we show that these residues likely form an ion pair. These data form a framework for understanding the impact of resistance mutations on viral fitness and pathogenesis and provide a pathway for the development of novel fusion inhibitor peptides.

The six-helix bundle of human immunodeficiency virus Env controls pore formation and enlargement and is initiated at residues proximal to the hairpin turn

Residues that create the grooves of the human immunodeficiency virus type 1 (HIV-1) Env triple-stranded coiled coil (HR1) and the residues that pack into the grooves (HR2) to complete the formation of the six-helix bundle (6HB) were mutated. The extent and kinetics of fusion as well as pore enlargement were measured for each mutant. Mutations near the hairpin turns of each monomer of the 6HB were more important than those far from the turn, for both HR1 and HR2. This result is consistent with the idea that binding of HR2 to the HR1 grooves is initiated near the hairpin turn of each monomer. Mutations at the distal portions also reduced fusion, albeit to a smaller extent. An intermediate of fusion (temperature-arrested state [TAS]) was formed, and the consequences of mutation were compared; a mutant that exhibited less fusion also showed slower kinetics from TAS. This suggests that formation of the bundle is a rate-limiting step downstream of the intermediate state. The rate of enlargement of a fusion pore also correlated with the extent and kinetics of fusion. The rate of pore enlargement was severely reduced by mutation. This supports our prior conclusion that formation of the 6HB occurs after pore creation and strongly suggests that the free energy released by bundle formation is directly used to promote pore growth.

Detailed mechanistic insights into HIV-1 sensitivity to three generations of fusion inhibitors

Peptides based on the second heptad repeat (HR2) of viral class I fusion proteins are effective inhibitors of virus entry. One such fusion inhibitor has been approved for treatment of human immunodeficiency virus-1 (T20, enfuvirtide). Resistance to T20 usually maps to the peptide binding site in HR1. To better understand fusion inhibitor potency and resistance, we combined virological, computational, and biophysical experiments with comprehensive mutational analyses and tested resistance to T20 and second and third generation inhibitors (T1249 and T2635). We found that most amino acid substitutions caused resistance to the first generation peptide T20. Only charged amino acids caused resistance to T1249, and none caused resistance to T2635. Depending on the drug, we can distinguish four mechanisms of drug resistance: reduced contact, steric obstruction, electrostatic repulsion, and electrostatic attraction. Implications for the design of novel antiviral peptide inhibitors are discussed.

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.

Common principles and intermediates of viral protein-mediated fusion: the HIV-1 paradigm

Enveloped viruses encode specialized fusion proteins which promote the merger of viral and cell membranes, permitting the cytosolic release of the viral cores. Understanding the molecular details of this process is essential for antiviral strategies. Recent structural studies revealed a stunning diversity of viral fusion proteins in their native state. In spite of this diversity, the post-fusion structures of these proteins share a common trimeric hairpin motif in which the amino- and carboxy-terminal hydrophobic domains are positioned at the same end of a rod-shaped molecule. The converging hairpin motif, along with biochemical and functional data, implies that disparate viral proteins promote membrane merger via a universal "cast-and-fold" mechanism. According to this model, fusion proteins first anchor themselves to the target membrane through their hydrophobic segments and then fold back, bringing the viral and cellular membranes together and forcing their merger. However, the pathways of protein refolding and the mechanism by which this refolding is coupled to membrane rearrangements are still not understood. The availability of specific inhibitors targeting distinct steps of HIV-1 entry permitted the identification of key conformational states of its envelope glycoprotein en route to fusion. These studies provided functional evidence for the direct engagement of the target membrane by HIV-1 envelope glycoprotein prior to fusion and revealed the role of partially folded pre-hairpin conformations in promoting the pore formation.

Potent HIV fusion inhibitors against Enfuvirtide-resistant HIV-1 strains

T20 (generic name: Enfuvirtide, brand name: Fuzeon) is the only FDA-approved HIV fusion inhibitor that is being used for treatment of HIV/AIDS patients who have failed to respond to current antiretroviral drugs. However, it rapidly induces drug resistance in vitro and in vivo. On the basis of the structural and functional information of anti-HIV peptides from a previous study, we designed an HIV fusion inhibitor named CP32M, a 32-mer synthetic peptide that is highly effective in inhibiting infection by a wide range of primary HIV-1 isolates from multiple genotypes with R5- or dual-tropic (R5X4) phenotype, including a group O virus (BCF02) that is resistant to T20 and C34 (another anti-HIV peptide). Strikingly, CP32M is exceptionally potent (at low picomolar level) against infection by a panel of HIV-1 mutants highly resistant to T20 and C34. These findings suggest that CP32M can be further developed as an antiviral therapeutic against multidrug resistant HIV-1.

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.

Design of an engineered N-terminal HIV-1 gp41 trimer with enhanced stability and potency

HIV fusion is mediated by a conformational transition in which the C-terminal region (HR2) of gp41 interacts with the N-terminal region (HR1) to form a six-helix bundle. Peptides derived from the HR1 form a well-characterized, trimeric coiled-coil bundle in the presence of HR2 peptides, but there is little structural information on the isolated HR1 trimer. Using protein design, we have designed synthetic HR1 peptides that form soluble, thermostable HR1 trimers. In vitro binding of HR2 peptides to the engineered trimer suggests that the design strategy has not significantly impacted the ability to form the six-helix bundle. The peptides have enhanced antiviral activity compared to wild type, with up to 30-fold greater potency against certain viral isolates. In vitro passaging was used to generate HR1-resistant virus and the observed resistance mutations map to the HR2 region of gp41, demonstrating that the peptides block the fusion process by binding to the viral HR2 domain. Interestingly, the activity of the HR2 fusion inhibitor, enfuvirtide (ENF), against these resistant viruses is maintained or improved up to fivefold. The 1.5 A crystal structure of one of these designs has been determined, and we show that the isolated HR1 is very similar to the conformation of the HR1 in the six-helix bundle. These results provide an initial model of the pre-fusogenic state, are attractive starting points for identifying novel fusion inhibitors, and offer new opportunities for developing HIV therapeutics based on HR1 peptides.

Identification of a critical motif for the human immunodeficiency virus type 1 (HIV-1) gp41 core structure: implications for designing novel anti-HIV fusion inhibitors

Human immunodeficiency virus type 1 (HIV-1) entry into the host cell involves a cascade of events and currently represents one of most attractive targets in the search for new antiviral drugs. The fusion-active gp41 core structure is a stable six-helix bundle (6-HB) folded by its trimeric N-terminal heptad repeat (NHR) and C-terminal heptad repeat (CHR). Peptides derived from the CHR region of HIV-1 gp41 are potent fusion inhibitors that target the NHR to block viral and cellular membrane fusion in a dominant negative fashion. However, all CHR peptides reported to date are derived primarily from residues 628 to 673 of gp41; little attention has been paid to the upstream sequence of the pocket binding domain (PBD) in the CHR. Here, we have identified a motif ((621)QIWNNMT(627)) located at the upstream region of the gp41 CHR, immediately adjacent to the PBD ((628)WMEWEREI(635)). Biophysical characterization demonstrated that this motif is critical for the stabilization of the gp41 6-HB core. The peptide CP621-652, containing the (621)QIWNNMT(627) motif, was able to interact with T21, a counterpart peptide derived from the NHR, to form a typical 6-HB structure with a high thermostability (thermal unfolding transition [T(m)] value of 82 degrees C). In contrast, the 6-HB formed by the peptides N36 and C34, which has been considered to be a core structure of the fusion-active gp41, had a T(m) of 64 degrees C. Different from T-20 (brand name Fuseon), which is the first and only HIV-1 fusion inhibitor approved for clinical use, CP621-652 could efficiently block 6-HB formation in a dose-dependent manner. Significantly, CP621-652 had potent inhibitory activity against HIV-1-mediated cell-cell fusion and infection, especially against T-20- and C34-resistant virus. Therefore, our works provide important information for understanding the core structure of the fusion-active gp41 and for designing novel anti-HIV peptides.

Conserved residue Lys574 in the cavity of HIV-1 Gp41 coiled-coil domain is critical for six-helix bundle stability and virus entry

The fusion-active HIV-1 gp41 core structure is a stable six-helix bundle (6-HB) formed by its N- and C-terminal heptad-repeat sequences (NHR and CHR). A highly conserved, deep hydrophobic cavity on the surface of the N-helical trimer is important for stability of the 6-HB and serves as an ideal target for developing anti-human immunodeficiency virus (HIV) fusion inhibitors. We have recently identified several small molecule HIV-1 fusion inhibitors that bind to the gp41 cavity through hydrophobic and ionic interactions and block the gp41 6-HB formation. Molecular docking analysis reveals that these small molecules fit inside the hydrophobic cavity and interact with positively charged residue Lys574 to form a conserved salt bridge. In this study, the functionality of Lys574 has been finely characterized by mutational analysis and biophysical approaches. We found that substitutions of Lys574 with non-conserved residues (K574D, K574E, and K574V) could completely abolish virus infectivity. With a set of wild-type and mutant N36 peptides derived from the NHR sequence as a model, we demonstrated that non-conservative Lys574 substitutions severely impaired the stability and conformation of 6-HBs as detected by circular dichroism spectroscopy, native polyacrylamide gel electrophoresis, and enzyme-linked immunosorbent assay. The binding affinity of N36 mutants bearing non-conservative Lys574 substitutions to the peptide C34 derived from the CHR sequence dramatically decreased as measured by isothermal titration calorimetry. These substitutions also significantly reduced the potency of N-peptides to inhibit HIV-1 infection. Collectively, these data suggest that conserved Lys574 plays a critical role in 6-HB formation and HIV-1 infectivity, and may serve as an important target for designing anti-HIV drugs.

Human immunodeficiency virus type 1 variants resistant to first- and second-version fusion inhibitors and cytopathic in ex vivo human lymphoid tissue

Human immunodeficiency virus type 1 (HIV-1) fusion inhibitors blocking viral entry by binding the gp41 heptad repeat 1 (HR1) region offer great promise for antiretroviral therapy, and the first of these inhibitors, T20 (Fuzeon; enfuvirtide), is successfully used in the clinic. It has been reported previously that changes in the 3-amino-acid GIV motif at positions 36 to 38 of gp41 HR1 mediate resistance to T20 but usually not to second-version fusion inhibitors, such as T1249, which target an overlapping but distinct region in HR1 including a conserved hydrophobic pocket (HP). Based on the common lack of cross-resistance and the difficulty of selecting T1249-resistant HIV-1 variants, it has been suggested that the determinants of resistance to first- and second-version fusion inhibitors may be different. To further assess HIV-1 resistance to fusion inhibitors and to analyze where changes in HR1 are tolerated, we randomized 16 codons in the HR1 region, including those making contact with HR2 codons and/or encoding residues in the GIV motif and the HP. We found that changes only at positions 37I, 38V, and 40Q near the N terminus of HR1 were tolerated. The propagation of randomly gp41-mutated HIV-1 variants in the presence of T1249 allowed the effective selection of highly resistant forms, all containing changes in the IV residues. Overall, the extent of T1249 resistance was inversely correlated to viral fitness and cytopathicity. Notably, one HIV-1 mutant showing approximately 10-fold-reduced susceptibility to T1249 inhibition replicated with wild type-like kinetics and caused substantial CD4+-T-cell depletion in ex vivo-infected human lymphoid tissue in the presence and absence of an inhibitor. Taken together, our results show that the GIV motif also plays a key role in resistance to second-version fusion inhibitors and suggest that some resistant HIV-1 variants may be pathogenic in vivo.

Analysis of the neutralization breadth of the anti-V3 antibody F425-B4e8 and re-assessment of its epitope fine specificity by scanning mutagenesis

The identification of cross-neutralizing antibodies to HIV-1 is important for designing antigens aimed at eliciting similar antibodies upon immunization. The monoclonal antibody (mAb) F425-B4e8 had been suggested previously to bind an epitope at the base of V3 and shown to neutralize two primary HIV isolates. Here, we have assessed the neutralization breadth of mAb F425-B4e8 using a 40-member panel of primary HIV-1 and determined the epitope specificity of the mAb. The antibody was able to neutralize 8 clade B viruses (n=16), 1 clade C virus (n=11), and 2 clade D viruses (n=6), thus placing it among the more broadly neutralizing anti-V3 antibodies described so far. Contrary to an initial report, results from our scanning mutagenesis of the V3 region suggest that mAb F425-B4e8 interacts primarily with the crown/tip of V3, notably Ile(309), Arg(315), and Phe(317). Despite the somewhat limited neutralization breadth of mAb F425-B4e8, the results presented here, along with analyses from other cross-neutralizing anti-V3 mAbs, may facilitate the template-based design of antigens that target V3 and permit neutralization of HIV-1 strains in which the V3 region is accessible to antibodies.

Role of envelope processing and gp41 membrane spanning domain in the formation of human immunodeficiency virus type 1 (HIV-1) fusion-competent envelope glycoprotein complex

Human immunodeficiency virus type 1 (HIV-1) entry into target cells is directed by the envelope (Env) glycoproteins, which are present on the surface of HIV-1 virion or infected cells in the form of trimers consisting of gp120/gp41 complexes. The surface subunit, gp120, initiates the entry process by interacting sequentially with the CD4 receptor and a co-receptor, thereby inducing a conformational change that allows the transmembrane (TM) gp41 subunit to mediate fusion between viral and target cell membranes. Cleavage of Env into its gp120 and gp41 components is necessary for activation of its fusogenic activity. Here, the gp41 TM glycoprotein was altered by either deleting an isoleucine residue (DeltaI642) in a critical region of its ectodomain or by substituting its membrane spanning domain (MSD) by that of the influenza hemagglutinin (HA) glycoprotein (TM-HA) to examine the contribution of these regions to Env functions. Characterization of these mutant forms of gp41 revealed that they both affected the infectivity of pseudotyped virions, however, through distinct defects in Env functions. While deletion of Ile 642 drastically altered processing of Env, replacement of gp41 MSD by that of HA led to a marked fusion defect even though the TM-HA Env was efficiently processed and incorporated into viral particles. Interestingly, both DeltaI642 and TM-HA Env were found to act as trans dominant-negative mutant of viral infectivity, presumably via their ability to form hetero-oligomers with wild type Env. Together, these results support a previously proposed model whereby all three gp120-gp41 monomers must be cleaved for the Env homo-trimer to function and suggest that the gp41 MSD plays a critical role in the formation of fusion-competent Env trimers.

Characterization of HIV-1 envelope gp41 genetic diversity and functional domains following perinatal transmission

Background

HIV-1 envelope gp41 is a transmembrane protein that promotes fusion of the virus with the plasma membrane of the host cells required for virus entry. In addition, gp41 is an important target for the immune response and development of antiviral and vaccine strategies, especially when targeting the highly variable envelope gp120 has not met with resounding success. Mutations in gp41 may affect HIV-1 entry, replication, pathogenesis, and transmission. We, therefore, characterized the molecular properties of gp41, including genetic diversity, functional motifs, and evolutionary dynamics from five mother-infant pairs following perinatal transmission.

Results

The gp41 open reading frame (ORF) was maintained with a frequency of 84.17% in five mother-infant pairs' sequences following perinatal transmission. There was a low degree of viral heterogeneity and estimates of genetic diversity in gp41 sequences. Both mother and infant gp41 sequences were under positive selection pressure, as determined by ratios of non-synonymous to synonymous substitutions. Phylogenetic analysis of 157 mother-infant gp41 sequences revealed distinct clusters for each mother-infant pair, suggesting that the epidemiologically linked mother-infant pairs were evolutionarily closer to each other as compared with epidemiologically unlinked sequences. The functional domains of gp41, including fusion peptide, heptad repeats, glycosylation sites and lentiviral lytic peptides were mostly conserved in gp41 sequences analyzed in this study. The CTL recognition epitopes and motifs recognized by fusion inhibitors were also conserved in the five mother-infant pairs.

Conclusion

The maintenance of an intact envelope gp41 ORF with conserved functional domains and a low degree of genetic variability as well as positive selection pressure for adaptive evolution following perinatal transmission is consistent with an indispensable role of envelope gp41 in HIV-1 replication and pathogenesis.

Two domains that control prefusion stability and transport competence of the measles virus fusion protein

Most viral glycoproteins mediating membrane fusion adopt a metastable native conformation and undergo major conformational changes during fusion. We previously described a panel of compounds that specifically prevent fusion induced by measles virus (MV), most likely by interfering with conformational rearrangements of the MV fusion (F) protein. To further elucidate the basis of inhibition and better understand the mechanism of MV glycoprotein-mediated fusion, we generated and characterized resistant MV variants. Spontaneous mutations conferring drug resistance were confirmed in transient assays and in the context of recombinant virions and were in all cases located in the fusion protein. Several mutations emerged independently at F position 462, which is located in the C-terminal heptad repeat (HR-B) domain. In peptide competition assays, all HR-B mutants at residue 462 revealed reduced affinity for binding to the HR-A core complex compared to unmodified HR-B. Combining mutations at residue 462 with mutations in the distal F head region, which we had previously identified as mediating drug resistance, causes intracellular retention of the mutant proteins. The transport competence and activity of the mutants can be restored, however, by incubation at reduced temperature or in the presence of the inhibitory compounds, indicating that the F escape mutants have a reduced conformational stability and that the inhibitors stabilize a transport-competent conformation of the F trimer. The data support the conclusion that residues located in the head domain of the F trimer and the HR-B region contribute jointly to controlling F conformational stability.

The complex antigenicity of a small external region of the C-terminal tail of the HIV-1 gp41 envelope protein: a lesson in epitope analysis

The newly discovered external tail loop within the C-terminal tail of the gp41 transmembrane subunit of the HIV-1 envelope protein comprises approximately 40 residues, and within this are 18-residues ((734)PDRPEGIEEEGGERDRDR(751)) that include three antibody-reactive regions. The antigenicity is complex, and changes according to the biological context of the gp41. It is thus of interest both to the HIV specialist and protein immunologists. The antibody-reactive region, centred on the sequence ERDRD, encompasses three distinct epitopes which are expressed in different combinations on infected cells, wt virions, prefusion virion-cell complexes, and a neutralising antibody escape mutant virion. In addition ERDRD-specific antibodies have one or more antiviral activities, and variously neutralise the infectivity of free virions, neutralise virions already attached to the target cell, reduce the production of infectious progeny, and inhibit the ability of infected cells to fuse with non-infected cells. Antibodies to PDRPEG and IEEE have no apparent antiviral activity even though the footprints of the IEEE- and ERDRD-specific antibodies overlap. This review marshals the available experimental data with the aim of understanding the significance of the gp41 tail loop to the HIV-1 life cycle, and its relevance to potential anti-viral measures. There are lessons here, too, that are relevant to the comprehension of the antigenicity of short protein segments in general.

Efficient trapping of HIV-1 envelope protein by hetero-oligomerization with an N-helix chimera

BACKGROUND

The N-heptad repeat region of the HIV-1 Transmembrane Envelope protein is a trimerization domain that forms part of a "six helix bundle" crucial to Envelope-mediated membrane fusion. N-heptad repeat peptides have been used as extracellular reagents to inhibit virus fusion.

RESULTS

When expressed intracellularly with wild-type HIV-1 Envelope protein, the N-heptad repeat domain efficiently hetero-oligomerized with Envelope and trapped it in the endoplasmic reticulum or early Golgi, as indicated by lack of transport to the cell surface, absent proteolytic processing, and aberrant glycosylation.

CONCLUSION

Post-translational processing of HIV Envelope is very sensitive to an agent that binds to the N-heptad repeat during synthesis, suggesting that it might be possible to modify drugs that bind to this region to have transport-blocking properties.