Photoinduced reactivity of the HIV-1 envelope glycoprotein with a membrane-embedded probe reveals insertion of portions of the HIV-1 Gp41 cytoplasmic tail into the viral membrane

HIV-1 envelope facilitates the development of protease inhibitor resistance through acquiring mutations associated with viral entry and immune escape

Introduction

There is increasing evidence supporting a role for HIV-1 envelope in the development of Protease Inhibitor drug resistance, and a recent report from our group suggested that Env mutations co-evolve with Gag-Protease mutations in the pathway to Lopinavir resistance. In this study, we investigated the effect of co-evolving Env mutations on virus function and structure.

Methods

Co-receptor usage and n-linked glycosylation were investigated using Geno2Pheno as well as tools available at the Los Alamos sequence database. Molecular dynamics simulations were performed using Amber 18 and analyzed using Cpptraj, and molecular interactions were calculated using the Ring server.

Results

The results showed that under Protease Inhibitor drug selection pressure, the envelope gene modulates viral entry by protecting the virus from antibody recognition through the increased length and number of N-glycosylation sites observed in V1/V2 and to some extent V5. Furthermore, gp120 mutations appear to modulate viral entry through a switch to the CXCR4 coreceptor, induced by higher charge in the V3 region and specific mutations at the coreceptor binding sites. In gp41, S534A formed a hydrogen bond with L602 found in the disulfide loop region between the Heptad Repeat 1 and Heptad Repeat 2 domains and could negatively affect the association of gp120-gp41 during viral entry. Lastly, P724Q/S formed both intermolecular and intramolecular interactions with residues within the Kennedy loop, a known epitope.

Discussion

In conclusion, the results suggest that mutations in envelope during Protease Inhibitor treatment failure are related to immune escape and that S534A mutants could preferentially use the cell-to-cell route of infection.

Role of Protein-Lipid Interactions in Viral Entry

The viral entry consists of several sequential events that ensure the attachment of the virus to the host cell and the introduction of its genetic material for the continuation of the replication cycle. Both cellular and viral lipids have gained a wider focus in recent years in the field of viral entry, as they are found to play key roles in different steps of the process. The specific role is summarized that lipids and lipid membrane nanostructures play in viral attachment, fusion, and immune evasion and how they can be targeted with antiviral therapies. Finally, some of the limitations of techniques commonly used for protein-lipid interactions studies are discussed, and new emerging tools are reviewed that can be applied to this field.

Cholesterol in the Viral Membrane is a Molecular Switch Governing HIV-1 Env Clustering

HIV-1 entry requires the redistribution of envelope glycoproteins (Env) into a cluster and the presence of cholesterol (chol) in the viral membrane. However, the molecular mechanisms underlying the specific role of chol in infectivity and the driving force behind Env clustering remain unknown. Here, gp41 is demonstrated to directly interact with chol in the viral membrane via residues 751-854 in the cytoplasmic tail (CT751-854). Super-resolution stimulated emission depletion (STED) nanoscopy analysis of Env distribution further demonstrates that both truncation of gp41 CT751-854 and depletion of chol leads to dispersion of Env clusters in the viral membrane and inhibition of virus entry. This work reveals a direct interaction of gp41 CT with chol and indicates that this interaction is an important orchestrator of Env clustering.

The Interplay between HIV-1 Gag Binding to the Plasma Membrane and Env Incorporation

Advancement in drug therapies and patient care have drastically improved the mortality rates of HIV-1 infected individuals. Many of these therapies were developed or improved upon by using structure-based techniques, which underscore the importance of understanding essential mechanisms in the replication cycle of HIV-1 at the structural level. One such process which remains poorly understood is the incorporation of the envelope glycoprotein (Env) into budding virus particles. Assembly of HIV particles is initiated by targeting of the Gag polyproteins to the inner leaflet of the plasma membrane (PM), a process mediated by the N-terminally myristoylated matrix (MA) domain and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). There is strong evidence that formation of the Gag lattice on the PM is a prerequisite for the incorporation of Env into budding particles. It is also suggested that Env incorporation is mediated by an interaction between its cytoplasmic tail (gp41CT) and the MA domain of Gag. In this review, we highlight the latest developments and current efforts to understand the interplay between gp41CT, MA, and the membrane during assembly. Elucidation of the molecular determinants of Gag-Env-membrane interactions may help in the development of new antiviral therapeutic agents that inhibit particle assembly, Env incorporation and ultimately virus production.

The lipid membrane of HIV-1 stabilizes the viral envelope glycoproteins and modulates their sensitivity to antibody neutralization

The envelope glycoproteins (Envs) of HIV-1 are embedded in the cholesterol-rich lipid membrane of the virus. Chemical depletion of cholesterol from HIV-1 particles inactivates their infectivity. We observed that diverse HIV-1 strains exhibit a range of sensitivities to such treatment. Differences in sensitivity to cholesterol depletion could not be explained by variation in Env components known to interact with cholesterol, including the cholesterol-recognition motif and cytoplasmic tail of gp41. Using antibody-binding assays, measurements of virus infectivity, and analyses of lipid membrane order, we found that depletion of cholesterol from HIV-1 particles decreases the conformational stability of Env. It enhances exposure of partially cryptic epitopes on the trimer and increases sensitivity to structure-perturbing treatments such as antibodies and cold denaturation. Substitutions in the cholesterol-interacting motif of gp41 induced similar effects as depletion of cholesterol. Surface-acting agents, which are incorporated into the virus lipid membrane, caused similar effects as disruption of the Env-cholesterol interaction. Furthermore, substitutions in gp120 that increased structural stability of Env (i.e. induced a "closed" conformation of the trimer) increased virus resistance to cholesterol depletion and to the surface-acting agents. Collectively, these results indicate a critical contribution of the viral membrane to the stability of the Env trimer and to neutralization resistance against antibodies. Our findings suggest that the potency of poorly neutralizing antibodies, which are commonly elicited in vaccinated individuals, may be markedly enhanced by altering the lipid composition of the viral membrane.

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.

Biochemical evidence of a role for matrix trimerization in HIV-1 envelope glycoprotein incorporation

The matrix (MA) domain of HIV Gag has important functions in directing the trafficking of Gag to sites of assembly and mediating the incorporation of the envelope glycoprotein (Env) into assembling particles. HIV-1 MA has been shown to form trimers in vitro; however, neither the presence nor the role of MA trimers has been documented in HIV-1 virions. We developed a cross-linking strategy to reveal MA trimers in virions of replication-competent HIV-1. By mutagenesis of trimer interface residues, we demonstrated a correlation between loss of MA trimerization and loss of Env incorporation. Additionally, we found that truncating the long cytoplasmic tail of Env restores incorporation of Env into MA trimer-defective particles, thus rescuing infectivity. We therefore propose a model whereby MA trimerization is required to form a lattice capable of accommodating the long cytoplasmic tail of HIV-1 Env; in the absence of MA trimerization, Env is sterically excluded from the assembling particle. These findings establish MA trimerization as an obligatory step in the assembly of infectious HIV-1 virions. As such, the MA trimer interface may represent a novel drug target for the development of antiretrovirals.

The cytoplasmic tail of retroviral envelope glycoproteins

Retroviruses comprise a large, diverse group that infects a broad range of host organisms. Pathogenicity varies widely; the human immunodeficiency virus is the causative agent of acquired immunodeficiency syndrome, one of the world's leading infectious causes of death, while many nonhuman retroviruses cause cancer in the host. Retroviruses have been studied intensively, and great strides have been made in understanding aspects of retroviral biology. While the principal functions of the viral structural proteins are well understood, there remain many incompletely characterized domains. One of these is the cytoplasmic tail (CT) of the envelope glycoprotein. Several functions of the CT are highly conserved, whereas other properties are unique to a specific retrovirus. For example, the lentiviruses encode envelope glycoproteins with particularly large cytoplasmic domains. The functions of the long lentiviral envelope CT are still being deciphered. The reported functions of retroviral envelope CTs are discussed in this chapter.

Second generation inactivated eastern equine encephalitis virus vaccine candidates protect mice against a lethal aerosol challenge

Currently, there are no FDA-licensed vaccines or therapeutics for eastern equine encephalitis virus (EEEV) for human use. We recently developed several methods to inactivate CVEV1219, a chimeric live-attenuated eastern equine encephalitis virus (EEEV). Dosage and schedule studies were conducted to evaluate the immunogenicity and protective efficacy of three potential second-generation inactivated EEEV (iEEEV) vaccine candidates in mice: formalin-inactivated CVEV1219 (fCVEV1219), INA-inactivated CVEV1219 (iCVEV1219) and gamma-irradiated CVEV1219 (gCVEV1219). Both fCVEV1219 and gCVEV1219 provided partial to complete protection against an aerosol challenge when administered by different routes and schedules at various doses, while iCVEV1219 was unable to provide substantial protection against an aerosol challenge by any route, dose, or schedule tested. When evaluating antibody responses, neutralizing antibody, not virus specific IgG or IgA, was the best correlate of protection. The results of these studies suggest that both fCVEV1219 and gCVEV1219 should be evaluated further and considered for advancement as potential second-generation inactivated vaccine candidates for EEEV.

The role of matrix in HIV-1 envelope glycoprotein incorporation

Incorporation of the viral envelope (Env) glycoprotein is a critical requirement for the production of infectious HIV-1 particles. It has long been appreciated that the matrix (MA) domain of the Gag polyprotein and the cytoplasmic tail of Env are central players in the process of Env incorporation, but the precise mechanisms have been elusive. Several recent developments have thrown light on the contributions of both proteins, prompting a re-evaluation of the role of MA during Env incorporation. The two domains appear to play distinct but complementary roles, with the cytoplasmic tail of Env responsible for directing Env to the site of assembly and the matrix domain accommodating the cytoplasmic tail of Env in the Gag lattice.

Understanding the process of envelope glycoprotein incorporation into virions in simian and feline immunodeficiency viruses

The lentiviral envelope glycoproteins (Env) mediate virus entry by interacting with specific receptors present at the cell surface, thereby determining viral tropism and pathogenesis. Therefore, Env incorporation into the virions formed by assembly of the viral Gag polyprotein at the plasma membrane of the infected cells is a key step in the replication cycle of lentiviruses. Besides being useful models of human immunodeficiency virus (HIV) infections in humans and valuable tools for developing AIDS therapies and vaccines, simian and feline immunodeficiency viruses (SIV and FIV, respectively) are relevant animal retroviruses; the study of which provides important information on how lentiviral replication strategies have evolved. In this review, we discuss the molecular mechanisms underlying the incorporation of the SIV and FIV Env glycoproteins into viral particles.

Detailed topology mapping reveals substantial exposure of the "cytoplasmic" C-terminal tail (CTT) sequences in HIV-1 Env proteins at the cell surface

Substantial controversy surrounds the membrane topology of the HIV-1 gp41 C-terminal tail (CTT). While few studies have been designed to directly address the topology of the CTT, results from envelope (Env) protein trafficking studies suggest that the CTT sequence is cytoplasmically localized, as interactions with intracellular binding partners are required for proper Env targeting. However, previous studies from our lab demonstrate the exposure of a short CTT sequence, the Kennedy epitope, at the plasma membrane of intact Env-expressing cells, the exposure of which is not observed on viral particles. To address the topology of the entire CTT sequence, we serially replaced CTT sequences with a VSV-G epitope tag sequence and examined reactivity of cell- and virion-surface Env to an anti-VSV-G monoclonal antibody. Our results demonstrate that the majority of the CTT sequence is accessible to antibody binding on the surface of Env expressing cells, and that the CTT-exposed Env constitutes 20–50% of the cell-surface Env. Cell surface CTT exposure was also apparent in virus-infected cells. Passive transfer of Env through cell culture media to Env negative (non-transfected) cells was not responsible for the apparent cell surface CTT exposure. In contrast to the cell surface results, CTT-exposed Env was not detected on infectious pseudoviral particles containing VSV-G-substituted Env. Finally, a monoclonal antibody directed to the Kennedy epitope neutralized virus in a temperature-dependent manner in a post-attachment neutralization assay. Collectively, these results suggest that the membrane topology of the HIV gp41 CTT is more complex than the widely accepted intracytoplasmic model.

The frantic play of the concealed HIV envelope cytoplasmic tail

Lentiviruses have unusually long envelope (Env) cytoplasmic tails, longer than those of other retroviruses. Whereas the Env ectodomain has received much attention, the gp41 cytoplasmic tail (gp41-CT) is one of the least studied parts of the virus. It displays relatively high conservation compared to the rest of Env. It has been long established that the gp41-CT interacts with the Gag precursor protein to ensure Env incorporation into the virion. The gp41-CT contains distinct motifs and domains that mediate both intensive Env intracellular trafficking and interactions with numerous cellular and viral proteins, optimizing viral infectivity. Although they are not fully understood, a multiplicity of interactions between the gp41-CT and cellular factors have been described over the last decade; these interactions illustrate how Env expression and incorporation into virions is a finely tuned process that has evolved to best exploit the host system with minimized genetic information. This review addresses the structure and topology of the gp41-CT of lentiviruses (mainly HIV and SIV), their domains and believed functions. It also considers the cellular and viral proteins that have been described to interact with the gp41-CT, with a particular focus on subtype-related polymorphisms.

The tale of the long tail: the cytoplasmic domain of HIV-1 gp41

Envelope glycoproteins (Env) of lentiviruses typically possess unusually long cytoplasmic domains, often 150 amino acids or longer. It is becoming increasingly clear that these sequences contribute a diverse array of functional activities to the life cycle of their viruses. The cytoplasmic domain of gp41 (gp41CD) is required for replication of human immunodeficiency virus type 1 (HIV-1) in most but not all cell types, whereas it is largely dispensable for replication of simian immunodeficiency virus (SIV). Functionally, gp41CD has been shown to regulate rapid clathrin-mediated endocytosis of Env. The resultant low levels of Env expression at the cell surface likely serve as an immune avoidance mechanism to limit accessibility to the humoral immune response. Intracellular trafficking of Env is also regulated by gp41CD through interactions with a variety of cellular proteins. Furthermore, gp41CD has been implicated in the incorporation of Env into virions through an interaction with the virally encoded matrix protein. Most recently, the gp41CDs of HIV-1 and SIV were shown to activate the key cellular-transcription factor NF-κB via the serine/threonine kinase TAK1. Less well understood are the cytotoxicity- and apoptosis-inducing activities of gp41CD as well as potential roles in modulating the actin cytoskeleton and overcoming host cell restrictions. In this review, we summarize what is currently known about the cytoplasmic domains of HIV-1 and SIV and attempt to integrate the wealth of information in terms of defined functional activities.

HIV entry and envelope glycoprotein-mediated fusion

HIV entry involves binding of the trimeric viral envelope glycoprotein (Env) gp120/gp41 to cell surface receptors, which triggers conformational changes in Env that drive the membrane fusion reaction. The conformational landscape that the lipids and Env navigate en route to fusion has been examined by biophysical measurements on the microscale, whereas electron tomography, x-rays, and NMR have provided insights into the process on the nanoscale and atomic scale. However, the coupling between the lipid and protein pathways that give rise to fusion has not been resolved. Here, we discuss the known and unknown about the overall HIV Env-mediated fusion process.

C-terminal tail of human immunodeficiency virus gp41: functionally rich and structurally enigmatic

The human immunodeficiency virus (HIV) and acquired immunodeficiency syndrome (AIDS) pandemic is amongst the most important current worldwide public health threats. While much research has been focused on AIDS vaccines that target the surface viral envelope (Env) protein, including gp120 and the gp41 ectodomain, the C-terminal tail (CTT) of gp41 has received relatively little attention. Despite early studies highlighting the immunogenicity of a particular CTT sequence, the CTT has been classically portrayed as a type I membrane protein limited to functioning in Env trafficking and virion incorporation. Recent studies demonstrate, however, that the Env CTT has other important functions. The CTT has been shown to additionally modulate Env ectodomain structure on the cell and virion surface, affect Env reactivity and viral sensitivity to conformation-dependent neutralizing antibodies, and alter cell–cell and virus–cell fusogenicity of Env. This review provides an overview of the Env structure and function with a particular emphasis on the CTT and recent studies that highlight its functionally rich nature.

Targeting HIV-1 gp41-induced fusion and pathogenesis for anti-viral therapy

HIV gp41 is a metastable protein whose native conformation is maintained in the form of a heterodimer with gp120. The non-covalently associated gp41/gp120 complex forms a trimer on the virus surface. As gp120 engages with HIV's receptor, CD4, and coreceptor, CXCR4 or CCR5, gp41 undergoes several conformational changes resulting in fusion between the viral and cellular membranes. Several lipophilic and amphiphilic domains have been shown to be critical in that process. While the obvious function of gp41 in viral entry is well-established its role in cellular membrane fusion and the link with pathogenesis are only now beginning to appear. Recent targeting of gp41 via fusion inhibitors has revealed an important role of this protein not only in viral entry but also in bystander apoptosis and HIV pathogenesis. Studies by our group and others have shown that the phenomenon of gp41-mediated hemifusion initiates apoptosis in bystander cells and correlates with virus pathogenesis. More interestingly, recent clinical evidence suggests that gp41 mutants arising after Enfuvirtide therapy are associated with CD4 cell increase and immunological benefits. This has in turn been correlated to a decrease in bystander apoptosis in our in vitro as well as in vivo assays. Although a great deal of work has been done to unravel HIV-1 gp41-mediated fusion mechanisms, the factors that regulate gp41-mediated fusion versus hemifusion and the mechanism by which hemifusion initiates bystander apoptosis are not fully understood. Further insight into these issues will open new avenues for drug development making gp41 a critical anti-HIV target both for neutralization and virus attenuation.

Highly conserved structural properties of the C-terminal tail of HIV-1 gp41 protein despite substantial sequence variation among diverse clades: implications for functions in viral replication

Although the HIV-1 Env gp120 and gp41 ectodomain have been extensively characterized in terms of structure and function, similar characterizations of the C-terminal tail (CTT) of HIV gp41 remain relatively limited and contradictory. The current study was designed to examine in detail CTT sequence conservation relative to gp120 and the gp41 ectodomain and to examine the conservation of predicted physicochemical and structural properties across a number of divergent HIV clades and groups. Results demonstrate that CTT sequences display intermediate levels of sequence evolution and diversity in comparison to the more diverse gp120 and the more conserved gp41 ectodomain. Despite the relatively high level of CTT sequence variation, the physicochemical properties of the lentivirus lytic peptide domains (LLPs) within the CTT are evidently highly conserved across clades/groups. Additionally, predictions using PEP-FOLD indicate a high level of structural similarity in the LLP regions that was confirmed by circular dichroism measurements of secondary structure of LLP peptides from clades B, C, and group O. Results demonstrate that LLP peptides adopt helical structure in the presence of SDS or trifluoroethanol but are predominantly unstructured in aqueous buffer. Thus, these data for the first time demonstrate strong conservations of characteristic CTT physicochemical and structural properties despite substantial sequence diversity, apparently indicating a delicate balance between evolutionary pressures and the conservation of CTT structure and associated functional roles in virus replication.

Characterization of the effects of aryl-azido compounds and UVA irradiation on the viral proteins and infectivity of human immunodeficiency virus type 1

Hydrophobic UV-activatable compounds have been shown to partition into the hydrophobic region of biological membranes to selectively label transmembrane proteins, and to inactivate enveloped viruses. Here, we analyze various UV-activatable azido- and iodo-based hydrophobic compounds for their ability to inactivate a model-enveloped virus, human immunodeficiency virus (HIV-1 MN). Treatment of HIV-1 with 1,5-diazidonapthalene (DAN), 1-iodo, 5-azidonaphthalene (INA), 1-azidonaphthalene (AzNAP) or 4,4'-diazidobiphenyl (DABIPH) followed by UVA irradiation for 2 min resulted in complete viral inactivation, whereas treatment using analogous non-azido-containing controls had no effect. Incorporation of an azido moiety within these hydrophobic compounds to promote photoinduced covalent reactions with proteins was found to be the primary mechanism of viral inactivation for this class of compounds. Prolonged UVA irradiation of the virus in the presence of these azido compounds resulted in further modifications of viral proteins, due to the generation of reactive oxygen species, leading to aggregation as visualized via Western blot analysis, providing additional viral modifications that may inhibit viral infectivity. Furthermore, inactivation using these compounds resulted in the preservation of surface antigenic structures (recognized by neutralizing antibodies b12, 2g12 and 4e10), which is favorable for the creation of vaccines from these inactivated virus preparations.

Topology of the C-terminal tail of HIV-1 gp41: differential exposure of the Kennedy epitope on cell and viral membranes

The C-terminal tail (CTT) of the HIV-1 gp41 envelope (Env) protein is increasingly recognized as an important determinant of Env structure and functional properties, including fusogenicity and antigenicity. While the CTT has been commonly referred to as the “intracytoplasmic domain” based on the assumption of an exclusive localization inside the membrane lipid bilayer, early antigenicity studies and recent biochemical analyses have produced a credible case for surface exposure of specific CTT sequences, including the classical “Kennedy epitope” (KE) of gp41, leading to an alternative model of gp41 topology with multiple membrane-spanning domains. The current study was designed to test these conflicting models of CTT topology by characterizing the exposure of native CTT sequences and substituted VSV-G epitope tags in cell- and virion-associated Env to reference monoclonal antibodies (MAbs). Surface staining and FACS analysis of intact, Env-expressing cells demonstrated that the KE is accessible to binding by MAbs directed to both an inserted VSV-G epitope tag and the native KE sequence. Importantly, the VSV-G tag was only reactive when inserted into the KE; no reactivity was observed in cells expressing Env with the VSV-G tag inserted into the LLP2 domain. In contrast to cell-surface expressed Env, no binding of KE-directed MAbs was observed to Env on the surface of intact virions using either immune precipitation or surface plasmon resonance spectroscopy. These data indicate apparently distinct CTT topologies for virion- and cell-associated Env species and add to the case for a reconsideration of CTT topology that is more complex than currently envisioned.

Membrane topology analysis of HIV-1 envelope glycoprotein gp41

BACKGROUND

The gp41 subunit of the HIV-1 envelope glycoprotein (Env) has been widely regarded as a type I transmembrane protein with a single membrane-spanning domain (MSD). An alternative topology model suggested multiple MSDs. The major discrepancy between the two models is that the cytoplasmic Kennedy sequence in the single MSD model is assigned as the extracellular loop accessible to neutralizing antibodies in the other model. We examined the membrane topology of the gp41 subunit in both prokaryotic and mammalian systems. We attached topological markers to the C-termini of serially truncated gp41. In the prokaryotic system, we utilized a green fluorescent protein (GFP) that is only active in the cytoplasm. The tag protein (HaloTag) and a membrane-impermeable ligand specific to HaloTag was used in the mammalian system.

RESULTS

In the absence of membrane fusion, both the prokaryotic and mammalian systems (293FT cells) supported the single MSD model. In the presence of membrane fusion in mammalian cells (293CD4 cells), the data obtained seem to support the multiple MSD model. However, the region predicted to be a potential MSD is the highly hydrophilic Kennedy sequence and is least likely to become a MSD based on several algorithms. Further analysis revealed the induction of membrane permeability during membrane fusion, allowing the membrane-impermeable ligand and antibodies to cross the membrane. Therefore, we cannot completely rule out the possible artifacts. Addition of membrane fusion inhibitors or alterations of the MSD sequence decreased the induction of membrane permeability.

CONCLUSIONS

It is likely that a single MSD model for HIV-1 gp41 holds true even in the presence of membrane fusion. The degree of the augmentation of membrane permeability we observed was dependent on the membrane fusion and sequence of the MSD.

Safety and protective efficacy of INA-inactivated Venezuelan equine encephalitis virus: implication in vaccine development

We have previously shown that a hydrophobic alkylating compound, 1,5-iodonaphthyl-azide (INA) can efficiently inactivate the virulent strain of Venezuelan equine encephalitis virus (VEEV), V3000 in vitro. In this study, we have evaluated the safety of INA-inactivated V3000 and V3526 and the protective efficacy of INA-inactivated V3000. INA-inactivated V3000 and V3526 did not cause disease in suckling mice. RNA isolated from the INA-inactivated V3000 and V3526 was also not infectious. Immunization of adult mice with INA-inactivated V3000 induced an anti-VEEV antibody response and protected mice from virulent VEEV challenge. The protective efficacy of INA-inactivated V3000 increased with the use of adjuvants. Results suggest that inactivation of enveloped viruses by INA may occur by two independent mechanisms and the INA-inactivated VEEV elicit a protective antibody response in mice.

In-solution virus capture assay helps deconstruct heterogeneous antibody recognition of human immunodeficiency virus type 1

Human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) on whole virions is heterogeneous, so molecular analysis of Env with monoclonal antibodies (MAbs) is challenging. Virus capture assays (VCAs) involving immobilized MAbs are typically used, but these assays suffer from immobilization artifacts and do not provide binding constants. Furthermore, we show here that certain HIV-1 neutralizing MAbs, including 2G12, 4E10, 2F5, Z13e1, and D5, will capture virion particles completely devoid of Env. We modified the VCA such that MAbs and virions are incubated in solution, and unbound MAbs are removed prior to the capture step. This modification nearly eliminated evidence of Env-independent binding by MAbs to virions and allowed determination of apparent affinity constants in solution. Three important qualitative observations were further revealed. First, neutralizing MAbs 2F5, 4E10, and Z13e1 against the membrane-proximal external region (MPER) of HIV-1 gp41 were found to capture virions efficiently only if a significant amount of uncleaved gp160 or synthetic MPER peptide was present. Second, we show how non-native forms of Env vary by Env genotype and that Env from HIV-1(JR-FL) is more homogeneously trimeric than that from HIV-1(JR-CSF). Third, we determined that Env containing all or parts of gp41, including uncleaved gp160, binds spontaneously to free virions. This exogenous Env is an indiscriminate molecular "bridge" between Env-specific Ab and virions and can affect VCA analyses, particularly using pseudotyped virions. Heterogeneity in Env from endogenous and exogenous sources might also subvert humoral immunity to HIV-1, so in-solution VCAs may help to dissect this heterogeneity for vaccine design purposes.

The fusion peptide domain is the primary membrane-inserted region and enhances membrane interaction of the ectodomain of HIV-1 gp41

To execute the membrane fusion function, it is necessary for the fusion protein of the virus to penetrate into the hydrophobic milieu of membrane bilayer. Hence identification of the region(s) of the ectodomain of viral fusion proteins involved in the membrane insertion and their interaction with the rest of the fusion protein in the membrane would be important for the mechanistic study of membrane fusion. To this end, we examined membrane activity of the fusion peptide, and the ectodomain protein with or without the fusion peptide domain of HIV-1 gp41 by several biophysical measurements. The results revealed that the ectodomain protein containing the fusion peptide domain had higher membrane-perturbing activity and deeper membrane insertion, while the construct lacking the fusion peptide domain had much lower membrane activity. Strikingly, the N-terminal heptad repeat region was found to be induced deeper into the membrane by the fusion peptide, consistent with the role of the latter in the membrane penetration. We concluded that the fusion peptide is the only stretch of gp41 ectodomain that embeds deeply in the membrane interior in the prefusion stage. The function of fusion peptide in terms of membrane interaction and the implications of its interplay with other domains of gp41 on the membrane fusion cascade were discussed.

The cytoplasmic domain of human immunodeficiency virus type 1 transmembrane protein gp41 harbors lipid raft association determinants

The molecular basis for localization of the human immunodeficiency virus type 1 envelope glycoprotein (Env) in detergent-resistant membranes (DRMs), also called lipid rafts, still remains unclear. The C-terminal cytoplasmic tail of gp41 contains three membrane-interacting, amphipathic alpha-helical sequences, termed lentivirus lytic peptide 2 (LLP-2), LLP-3, and LLP-1, in that order. Here we identify determinants in the cytoplasmic tail which are crucial for Env's association with Triton X-100-resistant rafts. Truncations of LLP-1 greatly reduced Env localization in lipid rafts, and the property of Gag-independent gp41 localization in rafts was conserved among different strains. Analyses of mutants containing single deletions or substitutions in LLP-1 showed that the alpha-helical structure of the LLP-1 hydrophobic face has a more-critical role in Env-raft associations than that of the hydrophilic face. With the exception of a Pro substitution for Val-833, all Pro substitution and charge-inverting mutants showed wild-type virus-like one-cycle viral infectivity, replication kinetics, and Env incorporation into the virus. The intracellular localization and cell surface expression of mutants not localized in lipid rafts, such as the TM844, TM813, 829P, and 843P mutants, were apparently normal compared to those of wild-type Env. Cytoplasmic subdomain targeting analyses revealed that the sequence spanning LLP-3 and LLP-1 could target a cytoplasmic reporter protein to DRMs. Mutations of LLP-1 that affected Env association with lipid rafts also disrupted the DRM-targeting ability of the LLP-3/LLP-1 sequence. Our results clearly demonstrate that LLP motifs located in the C-terminal cytoplasmic tail of gp41 harbor Triton X-100-resistant raft association determinants.

Membrane topology of gp41 and amyloid precursor protein: interfering transmembrane interactions as potential targets for HIV and Alzheimer treatment

The amyloid precursor protein (APP), that plays a critical role in the development of senile plaques in Alzheimer disease (AD), and the gp41 envelope protein of the human immunodeficiency virus (HIV), the causative agent of the acquired immunodeficiency syndrome (AIDS), are single-spanning type-1 transmembrane (TM) glycoproteins with the ability to form homo-oligomers. In this review we describe similarities, both in structural terms and sequence determinants of their TM and juxtamembrane regions. The TM domains are essential not only for anchoring the proteins in membranes but also have functional roles. Both TM segments contain GxxxG motifs that drive TM associations within the lipid bilayer. They also each possess similar sequence motifs, positioned at the membrane interface preceding their TM domains. These domains are known as cholesterol recognition/interaction amino acid consensus (CRAC) motif in gp41 and CRAC-like motif in APP. Moreover, in the cytoplasmic domain of both proteins other α-helical membranotropic regions with functional implications have been identified. Recent drug developments targeting both diseases are reviewed and the potential use of TM interaction modulators as therapeutic targets is discussed.

Mode of interaction between the HIV-1-neutralizing monoclonal antibody 2F5 and its epitope

OBJECTIVE

To determine the mechanism of interaction between the HIV-1 gp41-specific broadly neutralizing monoclonal antibody (mAb) 2F5, its epitope in the membrane proximal external region and a domain located in the fusion peptide proximal region in the N-terminal region of gp41. Knowledge of these interactions would be useful for the design of antigens used to induce 2F5-like antibodies.

METHODS

The binding and avidity of the mAb 2F5 were analyzed using enzyme-linked immunosorbent assays, epitope mapping and surface plasmon resonance analysis. Inhibition of virus neutralization by 2F5 was analyzed using peptides corresponding to the gp41 sequence.

RESULTS

Using transmembrane envelope proteins of gammaretroviruses, we had previously induced neutralizing antibodies that recognize two epitopes, one located in the N-terminal part of the transmembrane protein (designated E1) and the other in the C-terminal membrane proximal external region (E2). The E2 epitope corresponds to the mAb 2F5/4E10 epitope in the gp41 of HIV and we have now identified a corresponding E1 domain in gp41. Although 2F5 did not bind directly to E1, the presence of E1 peptides increased the binding of 2F5 to peptides carrying its epitope. Neutralization of HIV-1 by 2F5 was inhibited more effectively by both gp41-derived peptides E1 and E2 together than with the peptide E2 alone.

CONCLUSION

The interaction between the E1 and E2 domains of gp41 increased the efficacy of mAb 2F5 binding to its E2 epitope. Such an interaction may occur after gp41 folding into a six-helix bundle. Antigens containing both domains might be used to induce broadly neutralizing 2F5-like antibodies.

Photo-activation of the hydrophobic probe iodonaphthylazide in cells alters membrane protein function leading to cell death

Background

Photo-activation of the hydrophobic membrane probe 1, 5 iodonaphthylazide (INA) by irradiation with UV light (310–380 nm) results in the covalent modification of transmembrane anchors of membrane proteins. This unique selectivity of INA towards the transmembrane anchor has been exploited to specifically label proteins inserted in membranes. Previously, we have demonstrated that photo-activation of INA in enveloped viruses resulted in the inhibition of viral membrane protein-induced membrane fusion and viral entry into cells. In this study we show that photo-activation of INA in various cell lines, including those over-expressing the multi-drug resistance transporters MRP1 or Pgp, leads to cell death. We analyzed mechanisms of cell killing by INA-UV treatment. The effects of INA-UV treatment on signaling via various cell surface receptors, on the activity of the multi-drug resistance transporter MRP1 and on membrane protein lateral mobility were also investigated.

Results

INA treatment of various cell lines followed by irradiation with UV light (310–380 nm) resulted in loss of cell viability in a dose dependent manner. The mechanism of cell death appeared to be apoptosis as indicated by phosphatidylserine exposure, mitochondrial depolarization and DNA fragmentation. Inhibition by pan-caspase inhibitors and cleavage of caspase specific substrates indicated that at low concentrations of INA apoptosis was caspase dependent. The INA-UV treatment showed similar cell killing efficacy in cells over-expressing MRP1 function as control cells. Efflux of an MRP1 substrate was blocked by INA-UV treatment of the MRP1-overexpressing cells. Although INA-UV treatment resulted in inhibition of calcium mobilization triggered by chemokine receptor signaling, Akt phosphorylation triggered by IGF1 receptor signaling was enhanced. Furthermore, fluorescence recovery after photobleaching experiments indicated that INA-UV treatment resulted in reduced lateral mobility of a seven transmembrane G protein-coupled receptor.

Conclusion

INA is a photo-activable agent that induces apoptosis in various cancer cell lines. It reacts with membrane proteins to alter the normal physiological function resulting in apoptosis. This activity of INA maybe exploited for use as an anti-cancer agent.

Role of HIV Gp41 mediated fusion/hemifusion in bystander apoptosis

Mechanisms of HIV-mediated CD4+ T cell loss leading to immunodeficiency are amongst the most extensively studied yet unanswered questions in HIV biology. The level of CD4+ T cell depletion in HIV infected patients far exceeds the number of infected T cells, suggesting an indirect mechanism of HIV pathogenesis termed bystander cell death. Evidence is accumulating that the HIV envelope glycoprotein (Env) is a major determinant of HIV pathogenesis and plays a critical role in bystander cell death. The complex structure and function of HIV Env makes the determination of the mechanism of Env mediated apoptosis more complex than previously thought. This review will examine the complex relationship between HIV Env phenotype, coreceptor expression and immune activation in determining HIV pathogenesis. We review data here corresponding to the role of HIV Env hemifusion activity in HIV pathogenesis and how it interplays with other AIDS associated factors such as chemokine receptor expression and immune activation.