The membrane-proximal tyrosine-based sorting signal of human immunodeficiency virus type 1 gp41 is required for optimal viral infectivity

The membrane-proximal tyrosine-based sorting motif in the cytoplasmic domain of the human immunodeficiency virus type 1 Env glycoprotein is important for endocytosis from the plasma membrane, basolateral targeting of viral budding in polarized epithelial cells, and polarized budding from a localized region of the lymphocyte plasma membrane. To study the role of the Env sorting motif (Y712XXL) in infectivity, the incorporation of Env into virions, and viral entry, we disrupted the motif with a tyrosine-to-alanine substitution. To investigate the relationship between the Env sorting motif and the enhancement of infectivity by Nef, the EnvY712A substitution was made in both Nef-positive and Nef-negative backgrounds. In spreading infections, including those using primary lymphocytes, the growth of the Y712A mutant was as impaired as Nef-negative virus, and the EnvY712A/Delta-Nef combination mutant was almost completely defective. In single-round infections using CD4-positive HeLa cells, the EnvY712A mutation impaired infectivity, and Nef retained the ability to enhance the infectivity in the context of EnvY712A. EnvY712 and Nef were required for the optimal infectivity of virions produced from either HEK293T or MT4 cells, but these sequences were required for the optimal incorporation of Env only when virions were produced from MT4 cells. Despite the wild-type levels of Env in viruses produced from 293T cells, the entry of the EnvY712A and Delta-Nef mutants into target cells was impaired. We conclude that the membrane-proximal tyrosine-based sorting motif of gp41 Env is, like Nef, important for optimal viral infectivity and, in the case of MT4 T cells, virion incorporation of Env. Nef does not require the Y712XXL motif to enhance viral infectivity. The finding that EnvY712 and Nef each affect the efficiency of viral entry independently of the Env content of virions suggests that both viral proteins are involved in trafficking events that influence morphogenesis to produce maximally fusogenic virus.

Regulation of human immunodeficiency virus type 1 Env-mediated membrane fusion by viral protease activity

We and others have presented evidence for a direct interaction between the matrix (MA) domain of the human immunodeficiency virus type 1 (HIV-1) Gag protein and the cytoplasmic tail of the transmembrane envelope (Env) glycoprotein gp41. In addition, it has been postulated that the MA domain of Gag undergoes a conformational change following Gag processing, and the cytoplasmic tail of gp41 has been shown to modulate Env-mediated membrane fusion activity. Together, these results raise the possibility that the interaction between the gp41 cytoplasmic tail and MA is regulated by protease (PR)-mediated Gag processing, perhaps affecting Env function. To examine whether Gag processing affects Env-mediated fusion, we compared the ability of wild-type (WT) HIV-1 Env and a mutant lacking the gp41 cytoplasmic tail to induce fusion in the context of an active (PR(+)) or inactive (PR(-)) viral PR. We observed that PR(-) virions bearing WT Env displayed defects in cell-cell fusion. Impaired fusion did not appear to be due to differences in the levels of virion-associated Env, in CD4-dependent binding to target cells, or in the formation of the CD4-induced gp41 six-helix bundle. Interestingly, truncation of the gp41 cytoplasmic tail reversed the fusion defect. These results suggest that interactions between unprocessed Gag and the gp41 cytoplasmic tail suppress fusion.

Inhibition of human immunodeficiency virus type 1 entry in cells expressing gp41-derived peptides

As the limitations of antiretroviral drug therapy, such as toxicity and resistance, become evident, interest in alternative therapeutic approaches for human immunodeficiency virus (HIV) infection is growing. We developed the first gene therapeutic strategy targeting entry of a broad range of HIV type 1 (HIV-1) variants. Infection was inhibited at the level of membrane fusion by retroviral expression of a membrane-anchored peptide derived from the second heptad repeat of the HIV-1 gp41 transmembrane glycoprotein. To achieve maximal expression and antiviral activity, the peptide itself, the scaffold for presentation of the peptide on the cell surface, and the retroviral vector backbone were optimized. This optimized construct effectively inhibited virus replication in cell lines and primary blood lymphocytes. The membrane-anchored C-peptide was also shown to bind to free gp41 N peptides, suggesting that membrane-anchored antiviral C peptides have a mode of action similar to that of free gp41 C peptides. Preclinical toxicity and efficacy studies of this antiviral vector have been completed, and clinical trials are in preparation.

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

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

Fast Folding of the HIV-1 and SIV gp41 Six-helix Bundles

Human (HIV-1) and simian (SIV) immunodeficiency virus fusion with the host cell is promoted by the receptor-triggered refolding of the gp41 envelope protein into a stable trimer-of-hairpins structure that brings viral and cellular membranes into close proximity. The core of this hairpin structure is a six-helix bundle in which an inner homotrimeric coiled coil is buttressed by three antiparallel outer HR2 helices. We have used stopped-flow circular dichroism spectroscopy to characterize the unfolding and refolding kinetics of the six-helix bundle using the HIV-1 and SIV N34(L6)C28 polypeptides. In each case, the time-course of ellipticity changes in refolding experiments is well described by a simple two-state model involving the native trimer and the unfolded monomers. The unfolding free energy of the HIV-1 and SIV trimers and their urea dependence calculated from kinetic data are in very good agreement with data measured directly by isothermal unfolding experiments. Thus, formation of the gp41 six-helix bundle structure involves no detectable population of stable, partly folded intermediates. Folding of HIV-1 N34(L6)C28 is five orders of magnitudes faster than folding of its SIV counterpart in aqueous buffer: k(on),(HIV-1)=1.3 x 10(15)M(-2)s(-1) versus k(on),(SIV)=1.1 x 10(10)M(-2)s(-1). The unfolding rates are similar: k(off),(HIV-1)=1.1 x 10(-5)s(-1) versus k(off),(SIV=)5.7 x 10(-4)s(-1). Kinetic m-values indicate that the transition state for folding of the HIV-1 protein is significantly more compact than the transition state of the SIV protein. Replacement of a single SIV threonine by isoleucine corresponding to position 573 in the HIV-1 sequence significantly stabilizes the protein and renders the folding rate close to that of the HIV-1 protein yet without making the transition state of the mutant as compact as that of the HIV-1 protein. Therefore, the overall reduction of surface exposure in the high-energy transition state seems not to account for different folding rates. While the available biological evidence suggests that refolding of the gp41 protein is slow, our study implies that structural elements outside the trimer-of-hairpins limit the rate of HIV-1 fusion kinetics.

Covalently linked human immunodeficiency virus type 1 gp120/gp41 is stably anchored in rhabdovirus particles and exposes critical neutralizing epitopes

Rabies virus (RV) vaccine strain-based vectors show significant promise as potential live-attenuated vaccines against human immunodeficiency virus type 1 (HIV-1). Here we describe a new RV construct that will also likely have applications as a live-attenuated or killed-particle immunogen. We have created a RV containing a chimeric HIV-1 Env protein, which contains introduced cysteine residues that give rise to an intermolecular disulfide bridge between gp120 and the ectodomain of gp41. This covalently linked gp140 (gp140 SOS) is fused in frame to the cytoplasmic domain of RV G glycoprotein and is efficiently incorporated into the RV virion. On the HIV-1 virion, the gp120 and gp41 moieties are noncovalently associated, which leads to extensive shedding of gp120 from virions and virus-infected cells. The ability to use HIV-1 particles as purified, inactivated immunogens has been confounded by the loss of gp120 during preparation. Additionally, monomeric gp120 and uncleaved gp160 molecules have been shown to be poor antigenic representations of virion-associated gp160. Because the gp120 and gp41 portions are covalently attached in the gp140 SOS molecule, the protein is maintained on the surface of the RV virion throughout purification. Surface immunostaining and fluorescence-activated cell sorting analysis with anti-envelope antibodies show that the gp140 SOS protein is stably expressed on the surface of infected cells and maintains CD4 binding capabilities. Furthermore, Western blot and immunoprecipitation experiments with infected-cell lysates and purified virions show that a panel of neutralizing anti-envelope antibodies efficiently recognize the gp140 SOS protein. The antigenic properties of this recombinant RV particle containing covalently attached Env, as well as the ability to present Env in a membrane-bound form, suggest that this approach could be a useful component of a HIV-1 vaccine strategy.

HIV-1 preferentially binds receptors copatched with cell-surface elastase

Human leukocyte elastase (HLE) interacts with HIV-1 glycoprotein (gp)41, suggesting a nonenzymatic receptor function for HLE in the context of HIV-1. HLE is found localized to the cell surface, but not granules in HIV permissive clones, and to granules, but not the cell surface of HIV nonpermissive clones. Inducing cell-surface HLE expression on HLE null, HIV nonpermissive clones permits HIV infectivity. HIV binding and infectivity diminish in proportion to HLE RNA subtraction. HIV binding and infectivity show dose dependence for the natural HLE ligand alpha1 proteinase inhibitor (alpha1antitrypsin, alpha1PI). Chemokines prevent, whereas alpha1PI promotes, copatching of HLE with the canonical HIV receptors. Recent demonstration that decreased viral RNA is significantly correlated with decreased circulating alpha1PI in HIV seropositive individuals is consistent with a model in which HLE and alpha1PI can serve as HIV coreceptor and cofactor, respectively, and potentially participate in the pathophysiology of HIV disease progression.

Localization of ligand-binding sites on human C1q globular head region using recombinant globular head fragments and single-chain antibodies

As a charge pattern recognition molecule, human C1q can bind a range of immunoglobulin and non-immunoglobulin ligands via its carboxy-terminal globular domain and activate the classical complement pathway. Each globular domain has a heterotrimeric organization, composed of the carboxy-terminal halves of one A (ghA), one B (ghB), and one C (ghC) chain. Recently, we have found that the recombinant forms of individual ghA, ghB and ghC bind differentially to IgG, IgM, gp41 peptide 601-613 of human immunodeficiency virus-1 (HIV-1), gp21 peptide 400-429 of human T cell lymphotrophic virus-I (HTLV-I), beta-amyloid peptide, and apoptotic cells, suggesting a modular organization of the globular domain. This paper examines the interaction of ghA, ghB and ghC with two known C1q ligands: Klebsiella pneumoniae porin OmpK36 and salivary agglutinin. In addition, we have used a panel of recombinant single-chain antibodies (scFv) specific for ghA, ghB and ghC in order to map sites on the heterotrimeric globular domain which are likely to interact with IgG1, IgG3, IgM, OmpK36, salivary agglutinin and gp41 loop peptide. The combined use of recombinant ghA, ghB, ghC and single-chain antibodies has revealed at least three ligand-binding sites on the globular domain of C1q: one is IgG- and OmpK36-specific, the second (IgM-binding site) is most likely overlapping with IgG/OmpK36 binding site, and the third (the gp41-binding site) seems to be located at the junction between the collagen and globular domains.

The hydrophobic internal region of bovine prion protein shares structural and functional properties with HIV type 1 fusion peptide

The conserved fusion peptide at the N-terminus of HIV-1 envelope glycoprotein gp41 is involved in the virus-cell fusion reaction and in the cytopathic effects promoted by expression in single cells. The conserved bovine prion protein 121KHVAGAAAAGAVVGGLGGYMLGSAMSR147 transmembrane region (BPrP(tm)) contains a sequence rich in Gly residues [i.e., 130GAVVGGLGGYMLGSAMSR147 (BPrP(mi))] that shows homology with HIV-1 fusion peptide. As in the case of the latter peptide, analysis of the BPrP(mi) interfacial hydrophobicity confirms that this stretch bears an intrinsic tendency to partitioning from the aqueous phase into membranes. Experimental analyses of BPrP(mi)-lipid interactions suggest several similarities between this sequence and HIV-1 fusion peptide. Infrared spectroscopy reveals a conformational plasticity of the membrane-associated prion sequence comparable to that displayed by the viral sequence. The adoption of a mainly alpha-helical structure correlates with the formation of lytic pores. This helical structure can be converted into a beta-sheet conformation by addition of calcium, a process that is accompanied by vesicle membrane fusion. In contrast, transmembrane BPrP(tm) associates with membranes in a nonlytic, mainly helical structure but also containing some random coil. Upon addition of calcium, the random coils disappear while the helical conformation remains. In the absence of membranes both prion and HIV-1 sequences form amyloid-type fibers. It is proposed that during the pathological processes induced by secreted PrPSc and its proteolytic fragments, conformational polymorphism displayed by membrane-inserted BPrP(mi) may play a role at perturbing the general architecture of the membrane lipid bilayer and inducing protein-protein aggregation at membrane surfaces. These findings suggest the existence of common mechanisms underlying cytotoxicity by PrP and HIV-1 gp41.

Functional evolution of the HIV-1 envelope glycoprotein 120 association site of glycoprotein 41

Protein-protein interaction surfaces can exhibit structural plasticity, a mechanism whereby an interface adapts to mutations as binding partners coevolve. The HIV-1 envelope glycoprotein gp120-gp41 complex, which is responsible for receptor attachment and membrane fusion, represents an extreme example of a coevolving complex as up to 35% amino acid sequence divergence has been observed in these proteins among HIV-1 isolates. In this study, the function of conserved gp120 contact residues, Leu593, Trp596, Gly597, Lys601, and Trp610 within the disulfide-bonded region of gp41, was examined in envelope glycoproteins derived from diverse HIV-1 isolates. We found that the gp120-gp41 association function of the disulfide-bonded region is conserved. However, the contribution of individual residues to gp41 folding and/or stability, gp120-gp41 association, membrane fusion function, and viral entry varied from isolate to isolate. In gp120-gp41 derived from the dual-tropic isolate, HIV-189.6, the importance of Trp596 for fusion function was dependent on the chemokine receptor utilized as a fusion cofactor. Thus, the engagement of alternative chemokine receptors may evoke distinct fusion-activation signals involving the site of gp120-gp41 association. An examination of chimeric glycoproteins revealed that the isolate-specific functional contributions of particular gp120-contact residues are influenced by the sequence of gp120 hypervariable regions 1, 2, and 3. These data indicate that the gp120-gp41 association site is structurally and functionally adaptable, perhaps to maintain a functional glycoprotein complex in a setting of host selective pressures driving the rapid coevolution of gp120 and gp41.

HIV-1 gp41: mediator of fusion and target for inhibition

The bipartite envelope glycoprotein (Env) of the human immunodeficiency virus type 1 (HIV-1) performs two essential functions for initiating virus infection. The gp120 surface subunit of Env binds cell receptors to attach virus to target cells and regulate viral entry. The gp41 transmembrane subunit fuses host-cell and viral membranes to deliver the viral core into the cell cytoplasm. The two subunits derive from a polyprotein precursor, gp160. Cleavage of gp160 in the biosynthetic pathway creates mature Env consisting of the noncovalently-associated gp120/gp41 that is primed for viral entry. While performing distinct operations in HIV entry, the activities of the gp120 and gp41 subunits must be highly coordinated in order to lead to successful infection. This review highlights structure-function relationships in Env, with a focus on the heptad-repeat regions in the ectodomain of gp41. The mechanism of Env-mediated membrane fusion and ways to interfere with this process using inhibitors and antibodies that target gp41 are discussed.

[Enfuvirtide, the first representative of a new class of drugs for the treatment of HIV infection : HIV fusion inhibitors]

In March 2003, enfuvirtide was approved in the USA and the European Union for the treatment of patients with HIV infection who have experienced failure or intolerable side effects of treatment with at least one representative of each antiretroviral drug class. Enfuvirtide has a new mode of action: it binds to the viral envelope glycoprotein 41 that is involved in the fusion of the virus to the membrane of the CD4 T cell. In two large phase III studies, 90 mg of enfuvirtide administered twice daily subcutaneously in addition to a background treatment of other antiretroviral drugs, had a significant favourable effect on both the plasma viral load (decrease) and the CD4 counts (increase) compared to the background treatment alone. Disadvantages of treatment with enfuvirtide are its subcutaneous administration (98% of the patients had local adverse reactions) and the high costs involved (1500 euro per patient per month).

Potential drug targets on the HIV-1 envelope glycoproteins, gp120 and gp41

HIV-1 entry is an attractive target for anti-HIV-1 therapy. However, there are no entry inhibitors approved for the clinical treatment of HIV-1 infection. This is likely to be changed in the near future since promising HIV-1 entry inhibitors, such as T20 and some chemokine receptor antagonists, are in the pipeline to join the repertoire of anti-HIV-1 therapeutics. This review will focus on what might be potential targets on the key components of the viral entry machinery, gp120 and gp41. These two molecules are the viral proteins responsible for HIV-1 entry. Binding to CD4 induces a series of structural changes in gp120 and allows it to interact with chemokine receptors. The receptor binding eventually triggers conformational changes in gp41, which result in the formation of a fusion active molecule to attack the cell membrane. The structural and functional motifs that operate this delicate fusion machinery could become the Achilles' heel of the virus.

Virus entry as a target for anti-HIV intervention

The replicative cycle of the human immunodeficiency virus (HIV) can be interrupted at several stages. Until recently only the viral reverse transcriptase and protease were the only enzymes targeted by antiretroviral agents. However, the first HIV entry inhibitor (T-20, Enfuvirtide, Fuseon) to be used in humans has been approved by the Food and Drug Administration (FDA). The HIV entry process is considered as an attractive target for chemotherapeutic intervention, as blocking HIV entry into its target cell leads to suppression of viral infectivity, replication and the cytotoxicity induced by virus-cell contacts. HIV-1 entry into target cells is a multistep process: virus attachment is initiated by the binding of trimeric envelope glycoprotein gp120 complexes on the virions to glycosylated T-cell surface receptor (CD4) and HIV GPCR coreceptors (CCR5 or CXCR4) leading to envelope glycoprotein gp41-dependent fusion-pore formation and membrane fusion. A number of compounds are being developed to specifically target each of these steps leading to virus entry and some compounds have reached early clinical development. Conversely, agents such as the CCR5 antagonist Tak-779 and the CXCR4 antagonist AMD3100 are not longer being thought as relevant anti-HIV agents but have given way to new analogues with improved properties. This review summarizes the current state of HIV entry inhibitors, their mechanisms of action and their therapeutic value against HIV infection and AIDS.

Nonequilibrium, multiple-timescale simulations of ligand-receptor interactions in structured protein systems

Predicting the long-time, nonequilibrium dynamics of receptor-ligand interactions for structured proteins in a host fluid is a formidable task, but of great importance to predicting and analyzing cell-signaling processes and small molecule drug efficacies. Such processes take place on timescales on the order of milliseconds to seconds, so "brute-force" real-time, molecular or atomic simulations to determine absolute ligand-binding rates to receptor targets and over a statistical ensemble of systems are not currently feasible. In the current study, we implement on real protein systems a previously developed 3-5 hybrid molecular dynamics/Brownian dynamics algorithm, which takes advantage of the underlying, disparate timescales involved and overcomes the limitations of brute-force approaches. The algorithm is based on a multiple timescale analysis of the total system Hamiltonian, including all atomic and molecular structure information for the system: water, ligand, and receptor. In general, the method can account for the complex hydrodynamic, translational-orientational diffusion aspects of ligand-docking dynamics as well as predict the actual or absolute rates of ligand binding. To test some of the underlying features of the method, simulations were conducted here for an artificially constructed spherical protein "made" from the real protein insulin. Excellent comparisons of simulation calculations of the so-called grand particle friction tensor to analytical values were obtained for this system when protein charge effects were neglected. When protein charges were included, we found anomalous results caused by the alteration of the spatial, microscopic structure of water proximal to the protein surface. Protein charge effects were found to be highly significant and consistent with the recent hypothesis of Hoppert and Mayer (Am Sci 1999;87:518-525) for charged macromolecules in water, which involves the formation of a "water dense region" proximal to the charged protein surface followed by a "dilute water region." We further studied the algorithm on a D-peptide/HIV capside protein system and demonstrated the algorithms utility to study the nonequilibrium docking dynamics in this contemporary problem. In general, protein charge effects, which alter water structural properties in an anomalous fashion proximal to the protein surface, were found to be much more important than the so-called hydrodynamic interaction effects between ligand and receptor. The diminished role of hydrodynamic interactions in protein systems allows for a much simpler overall dynamic algorithm for the nonequilibrium protein-docking process. Further studies are now underway to critically examine this simpler overall algorithm in analyzing the nonequilibrium protein-docking problem.

Role of the gp120 inner domain beta-sandwich in the interaction between the human immunodeficiency virus envelope glycoprotein subunits

The inner domain of the human immunodeficiency virus (HIV-1) gp120 glycoprotein has been proposed to mediate the noncovalent interaction with the gp41 transmembrane envelope glycoprotein. We used mutagenesis to investigate the functional importance of a conserved beta-sandwich located within the gp120 inner domain. Changes in aliphatic residues lining a hydrophobic groove on the surface of the beta-sandwich decreased the association of the gp120 and gp41 glycoproteins. Other changes in the base of the hydrophobic groove resulted in envelope glycoproteins that were structurally intact and able to bind receptors, but were inefficient in mediating either syncytium formation or virus entry. These results support a model in which the beta-sandwich in the gp120 inner domain contributes to gp120-gp41 contacts, thereby maintaining the integrity of the envelope glycoprotein complex and allowing adjustments in the gp120-gp41 interaction required for membrane fusion.

Envelope variants from women recently infected with clade A human immunodeficiency virus type 1 confer distinct phenotypes that are discerned by competition and neutralization experiments

Women infected with clade A human immunodeficiency virus type 1 harbor a virus population that is genetically diverse in the envelope gene, a fact that contrasts with the homogeneous virus population identified in newly infected men. It is not known whether viral genetic diversity at this early stage of infection is manifested as phenotypic diversity. This is a significant question because phenotypic diversity in the viral population that establishes infection in women may have important implications for pathogenesis and therapeutic intervention. Thus, in this study we compared the biological properties of three pairs of chimeric viruses that contained envelope genes representative of variant groups in each of three infected women-Q23, Q45, and Q47. Envelope chimeras were evaluated for replication in stimulated and resting peripheral blood mononuclear cells alone and in competition, for coreceptor use, and for neutralization sensitivity. All viruses utilized CCR5 exclusively and had a non-syncytium-inducing phenotype on MT-2 cells and in primary culture. There were no significant differences in replication parameters between paired variants in individual cultures. However, in competition experiments, one chimera of each variant pair always dominated. The dominant virus from Q23 and Q47, but not from Q45, infected a significantly higher number of CCR5- and CD4-expressing GHOST cells than the weaker chimeras. Significantly, chimeric viruses from Q47 and Q45 showed markedly different neutralization sensitivity to antibodies to CCR5 and gp120, respectively. These data indicate that distinct envelope genotypes identified in clade A-infected women near seroconversion confer unique phenotypes that affect viral fitness and that may be due, in part, to different requirements for relative configuration of CD4 and CCR5 on infected cells.

Modular organization of the carboxyl-terminal, globular head region of human C1q A, B, and C chains

The first step in the activation of the classical complement pathway, by immune complexes, involves the binding of the globular heads of C1q to the Fc regions of aggregated IgG or IgM. Located C-terminal to the collagen region, each globular head is composed of the C-terminal halves of one A (ghA), one B (ghB), and one C chain (ghC). To dissect their structural and functional autonomy, we have expressed ghA, ghB, and ghC in Escherichia coli as soluble proteins linked to maltose-binding protein (MBP). The affinity-purified fusion proteins (MBP-ghA, -ghB, and -ghC) bound differentially to heat-aggregated IgG and IgM, and also to three known C1q-binding peptides, derived from HIV-1, HTLV-I, and beta-amyloid. In the ELISAs, the MBP-ghA bound to heat-aggregated IgG and IgM as well as to the HIV-1 gp41 peptide; the MBP-ghB bound preferentially to IgG rather than IgM, in addition to binding beta-amyloid peptide, whereas the MBP-ghC showed a preference for IgM and the HTLV-I gp21 peptide. Both MBP-ghA and MBP-ghB also inhibited C1q-dependent hemolysis of IgG- and IgM-sensitized sheep erythrocytes. However, for IgM-coated erythrocytes, MBP-ghC was a better inhibitor of C1q than MBP-ghB. The recombinant forms of ghA, ghB, and ghC also bound specifically to apoptotic PBMCs. We conclude that the C1q globular head region is likely to have a modular organization, being composed of three structurally and functionally independent modules, which retains multivalency in the form of a heterotrimer. The heterotrimeric organization thus offers functional flexibility and versatility to the whole C1q molecule.

Are fusion peptides a good model to study viral cell fusion?

Fusion peptides are hydrophobic and conserved sequences located within glycoprotein ectodomains that protrude from the virion surface. Direct participation of fusion peptides in the viral membrane fusion phenomenon has been inferred from genetic analyses showing that even a single residue substitution or a deletion within these sequences may completely block the process. However, the specific fusion peptide activities associated to the multi-step fusion mechanism are not well defined. Based on the assumption that fusion peptides are transferred into target membranes, biophysical methodologies have been applied to study integration into model membranes of synthetic fragments representing functional and non-functional sequences. From these studies, it is inferred that, following insertion, functional sequences generate target membrane perturbations and adopt specific structural arrangements within. Further characterization of these artificial systems may help in understanding the molecular processes that bring initial bilayer destabilizations to the eventual opening of a fusion pore.

The HIV Env-mediated fusion reaction

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

Multiple interactions across the surface of the gp120 core structure determine the global neutralization resistance phenotype of human immunodeficiency virus type 1

Resistance to neutralization is an important characteristic of primary isolates of human immunodeficiency virus type 1 (HIV-1) that relates to the potential for successful vaccination to prevent infection and use of immunotherapeutics for treatment of established infection. In order to further elucidate mechanisms responsible for neutralization resistance, we studied the molecular mechanisms that determine the resistance of the primary virus isolate of the strain HIV-1 MN to neutralization by soluble CD4 (sCD4). As is the case for the global neutralization resistance phenotype, sCD4 resistance depended upon sequences in the amino-terminal heptad repeat region of gp41 (HR1), as well as on multiple functional interactions within the envelope complex. The functional interactions that determined the resistance included interactions between the variable loop 1 and 2 (V1/V2) region and sequences in or near the CD4 binding site (CD4bs) and with the V3 loop. Additionally, the V3 loop region was found to interact functionally with sequences in the outer domain of gp120, distant from the CD4bs and coreceptor-binding site, as well as with a residue thought to be located centrally in the coreceptor-binding site. These and previous results provide the basis for a model by which functional signals that determine the neutralization resistance, high-infectivity phenotype depend upon interactions occurring across the surface of the gp120 core structure and involving variable loop structures and gp41. This model should be useful in efforts to define epitopes that may be important for primary virus neutralization.

Direct evidence that C-peptide inhibitors of human immunodeficiency virus type 1 entry bind to the gp41 N-helical domain in receptor-activated viral envelope

While it has been established that peptides modeling the C-helical region of human immunodeficiency virus type 1 gp41 are potent in vivo inhibitors of virus replication, their mechanism of action has yet to be determined. It has been proposed, but never directly demonstrated, that these peptides block virus entry by interacting with gp41 to disrupt the formation or function of a six-helix bundle structure. Using a six-helix bundle-specific monoclonal antibody with isolate-restricted Env reactivity, we provide the first direct evidence that, in receptor-activated viral Env, C-peptide entry inhibitors bind to the gp41 N-helical coiled-coil to form a peptide/protein hybrid structure and, in doing so, disrupt native six-helix bundle formation.

HIV-1 and its transmembrane protein gp41 bind to different Candida species modulating adhesion

Oral candidiasis in HIV-1-infected individuals is widely believed to be triggered by the acquired T-lymphocyte immunodeficiency. Recently, binding of the HIV-1 envelope protein gp160 and its subunit gp41, and also of the whole virus itself, to Candida albicans has been shown. The present study shows that, in addition to C. albicans, HIV-1 gp41 also binds to yeast and hyphal forms of Candida dubliniensis, a species which is closely related to C. albicans, and to Candida tropicalis but not to Candida krusei, Candida glabrata or Saccharomyces cerevisiae. The previous finding that gp41 binding to C. albicans augments fungal virulence in vitro is supported by the observation that the yeast showed an enhanced adhesion to HIV-infected H9 cells in comparison to uninfected cells. In line with these results soluble gp41 itself reduced binding of C. albicans to both endothelial and epithelial cell lines, confirming a dominant role of the gp41 binding moiety on the surface of Candida for adhesion. Surface-associated secreted aspartic proteinases (Saps) play an important role in candidial adhesion, but are not likely to be involved in the interaction as gp41 binding to the C. albicans parental wild-type strain was comparable to that of three different isogenic Sap deletion mutants. Furthermore, gp41 binding to the yeast killer toxin-susceptible C. albicans strain 10S was not inhibitable by an anti-YKT receptor antibody. In conclusion, HIV-1 interacts with different clinically important Candida spp., and may thereby affect the outcome of the respective fungal infection.