Environment-Sensitive Fluorescence of 7-Nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-Labeled Ligands for Serotonin Receptors.

Serotonin is a neurotransmitter that plays a crucial role in the regulation of several behavioral and cognitive functions by binding to a number of different serotonin receptors present on the cell surface. We report here the synthesis and characterization of several novel fluorescent analogs of serotonin in which the fluorescent NBD (7-nitrobenz-2-oxa-1,3-diazol-4-yl) group is covalently attached to serotonin. The fluorescent ligands compete with the serotonin1A receptor specific radiolabeled agonist for binding to the receptor. Interestingly, these fluorescent ligands display a high environmental sensitivity of their fluorescence. Importantly, the human serotonin1A receptor stably expressed in CHO-K1 cells could be specifically labeled with one of the fluorescent ligands with minimal nonspecific labeling. Interestingly, we show by spectral imaging that the NBD-labeled ligand exhibits a red edge excitation shift (REES) of 29 nm when bound to the receptor, implying that it is localized in a restricted microenvironment. Taken together, our results show that NBD-labeled serotonin analogs offer an attractive fluorescent approach for elucidating the molecular environment of the serotonin binding site in serotonin receptors. In view of the multiple roles played by the serotonergic systems in the central and peripheral nervous systems, these fluorescent ligands would be useful in future studies involving serotonin receptors.

Ceramide kinase regulates phospholipase C and phosphatidylinositol 4, 5, bisphosphate in phototransduction.

Phosphoinositide-specific phospholipase C (PLC) is a central effector for many biological responses regulated by G-protein-coupled receptors including Drosophila phototransduction where light sensitive channels are activated downstream of NORPA, a PLCbeta homolog. Here we show that the sphingolipid biosynthetic enzyme, ceramide kinase, is a novel regulator of PLC signaling and photoreceptor homeostasis. A mutation in ceramide kinase specifically leads to proteolysis of NORPA, consequent loss of PLC activity, and failure in light signal transduction. The mutant photoreceptors also undergo activity-dependent degeneration. Furthermore, we show that a significant increase in ceramide, resulting from lack of ceramide kinase, perturbs the membrane microenvironment of phosphatidylinositol 4, 5, bisphosphate (PIP(2)), altering its distribution. Fluorescence image correlation spectroscopic studies on model membranes suggest that an increase in ceramide decreases clustering of PIP(2) and its partitioning into ordered membrane domains. Thus ceramide kinase-mediated maintenance of ceramide level is important for the local regulation of PIP(2) and PLC during phototransduction.

Role of tryptophan residues in gramicidin channel organization and function.

The linear peptide gramicidin forms prototypical ion channels specific for monovalent cations and has been used extensively to study the organization, dynamics, and function of membrane-spanning channels. The tryptophan residues in gramicidin channels are crucial for maintaining the structure and function of the channel. We explored the structural basis for the reduction in channel conductance in the case of single-tryptophan analogs of gramicidin with three Trp --> hydrophobic substitutions using a combination of fluorescence approaches, which include red edge excitation shift and membrane penetration depth analysis, size-exclusion chromatography, and circular dichroism spectroscopy. We show here that the gramicidin analogs containing single-tryptophan residues adopt a mixture of nonchannel and channel conformations, as evident from analysis of membrane penetration depth, size-exclusion chromatography, and backbone circular dichroism data. These results are potentially useful in analyzing the effect of tryptophan substitution on the functioning of other ion channels and membrane proteins.

Cell-nonautonomous function of ceramidase in photoreceptor homeostasis.

Neutral ceramidase, a key enzyme of sphingolipid metabolism, hydrolyzes ceramide to sphingosine. These sphingolipids are critical structural components of cell membranes and act as second messengers in diverse signal transduction cascades. Here, we have isolated and characterized functional null mutants of Drosophila ceramidase. We show that secreted ceramidase functions in a cell-nonautonomous manner to maintain photoreceptor homeostasis. In the absence of ceramidase, photoreceptors degenerate in a light-dependent manner, are defective in normal endocytic turnover of rhodopsin, and do not respond to light stimulus. Consistent with a cell-nonautonomous function, overexpression of ceramidase in tissues distant from photoreceptors suppresses photoreceptor degeneration in an arrestin mutant and facilitates membrane turnover in a rhodopsin null mutant. Furthermore, our results show that secreted ceramidase is internalized and localizes to endosomes. Our findings establish a role for a secreted sphingolipid enzyme in the regulation of photoreceptor structure and function.

Restricted lateral mobility of plasma membrane CD4 impairs HIV-1 envelope glycoprotein mediated fusion.

We investigated the effect of receptor mobility on HIV-1 envelope glycoprotein (Env)-triggered fusion using B16 mouse melanoma cells that are engineered to express CD4 and CXCR4 or CCR5. These engineered cells are resistant to fusion mediated CD4-dependent HIV-1 envelope glycoprotein. Receptor mobility was measured by fluorescence recovery after photobleaching (FRAP) using either fluorescently-labeled antibodies or transient expression of GFP-tagged receptors in the cells. No significant differences between B16 and NIH3T3 (fusion-permissive) cells were seen in lateral mobility of CCR5 or lipid probes. By contrast CD4 mobility in B16 cells was about seven-fold reduced compared to its mobility in fusion-permissive NIH3T3 cells. However, a CD4 mutant (RA5) that localizes to non-raft membrane microdomains exhibited a three-fold increased mobility in B16 cells as compared with WT-CD4. Interestingly, the B16 cells expressing the RA5 mutant (but not the wild type CD4) and coreceptors supported HIV-1 Env-mediated fusion. Our data demonstrate that the lateral mobility of CD4 is an important determinant of HIV-1 fusion/entry.

Light-induced recruitment of INAD-signaling complexes to detergent-resistant lipid rafts in Drosophila photoreceptors.

Here, we reveal a novel feature of the dynamic organization of signaling components in Drosophila photoreceptors. We show that the multi-PDZ protein INAD and its target proteins undergo light-induced recruitment to detergent-resistant membrane (DRM) rafts. Reduction of ergosterol, considered to be a key component of lipid rafts in Drosophila, resulted in a loss of INAD-signaling complexes associated with DRM fractions. Genetic analysis demonstrated that translocation of INAD-signaling complexes to DRM rafts requires activation of the entire phototransduction cascade, while constitutive activation of the light-activated channels resulted in recruitment of complexes to DRM rafts in the dark. Mutations affecting INAD and TRP showed that PDZ4 and PDZ5 domains of INAD, as well as the INAD-TRP interaction, are required for translocation of components to DRM rafts. Finally, selective recruitment of phosphorylated, and therefore activatable, eye-PKC to DRM rafts suggests that DRM domains are likely to function in signaling, rather than trafficking.

Ceramide transfer protein function is essential for normal oxidative stress response and lifespan.

Ceramide transfer protein (CERT) transfers ceramide from the endoplasmic reticulum to the Golgi complex, a process critical in synthesis and maintenance of normal levels of sphingolipids in mammalian cells. However, how its function is integrated into development and physiology of the animal is less clear. Here, we report the in vivo consequences of loss of functional CERT protein. We generated Drosophila melanogaster mutant flies lacking a functional CERT (Dcert) protein using chemical mutagenesis and a Western blot-based genetic screen. The mutant flies die early between days 10 and 30, whereas controls lived between 75 and 90 days. They display >70% decrease in ceramide phosphoethanolamine (the sphingomyelin analog in Drosophila) and ceramide. These changes resulted in increased plasma membrane fluidity that renders them susceptible to reactive oxygen species and results in enhanced oxidative damage to cellular proteins. Consequently, the flies showed reduced thermal tolerance that was exacerbated with aging and metabolic compromise such as decreasing ATP and increasing glucose levels, reminiscent of premature aging. Our studies demonstrate that maintenance of physiological levels of ceramide phosphoethanolamine by CERT in vivo is required to prevent oxidative damages to cellular components that are critical for viability and normal lifespan of the animal.

Sphingomyelinase restricts the lateral diffusion of CD4 and inhibits human immunodeficiency virus fusion.

Previously, we reported that treatment of cells with sphingomyelinase inhibits human immunodeficiency virus type 1 (HIV-1) entry. Here, we determined by measuring fluorescence recovery after photobleaching that the lateral diffusion of CD4 decreased 4-fold following sphingomyelinase treatment, while the effective diffusion rate of CCR5 remained unchanged. Notably, sphingomyelinase treatment of cells did not influence gp120 binding, HIV-1 attachment, or fluid-phase and receptor-mediated endocytosis. Furthermore, sphingomyelinase treatment did not affect the membrane disposition of the HIV receptor proteins CD4, CXCR4, and CCR5, as determined by Triton X-100 extraction. Restriction of CD4 diffusion by antibody cross-linking also inhibited HIV infection. We therefore interpret the decrease in CD4 lateral mobility following sphingomyelinase treatment in terms of clustering of CD4 molecules. Examination of fusion intermediates indicated that sphingomyelinase treatment inhibited HIV at a step in the fusion process after CD4 engagement. Maximal inhibition of fusion was observed following short coculture times and with target cells that express low levels of CD4. As HIV entry into cells requires the sequential engagement of viral envelope protein with CD4 and coreceptor, we propose that sphingomyelinase inhibits HIV infection by inducing CD4 clustering that prevents coreceptor engagement and HIV fusion.

The role of cholesterol and sphingolipids in chemokine receptor function and HIV-1 envelope glycoprotein-mediated fusion.

BACKGROUND: HIV-1 entry into cells is a multifaceted process involving target cell CD4 and the chemokine receptors, CXCR4 or CCR5. The lipid composition of the host cell plays a significant role in the HIV fusion process as it orchestrates the appropriate disposition of CD4 and co-receptors required for HIV-1 envelope glycoprotein (Env)-mediated fusion. The cell membrane is primarily composed of sphingolipids and cholesterol. The effects of lipid modulation on CD4 disposition in the membrane and their role in HIV-1 entry have extensively been studied. To focus on the role of lipid composition on chemokine receptor function, we have by-passed the CD4 requirement for HIV-1 Env-mediated fusion by using a CD4-independent strain of HIV-1 Env. RESULTS: Cell fusion mediated by a CD4-independent strain of HIV-1 Env was monitored by observing dye transfer between Env-expressing cells and NIH3T3 cells bearing CXCR4 or CCR5 in the presence or absence of CD4. Chemokine receptor signaling was assessed by monitoring changes in intracellular [Ca2+] mobilization induced by CCR5 or CXCR4 ligand. To modulate target membrane cholesterol or sphingolipids we used Methyl-beta-cyclodextrin (MbetaCD) or 1-phenyl-2-hexadecanoylamino-3-morpholino-1-propanol (PPMP), respectively. Treatment of the target cells with these agents did not change the levels of CD4 or CXCR4, but reduced levels of CCR5 on the cell surface. Chemokine receptor signalling was inhibited by cholesterol removal but not by treatment with PPMP. HIV-1 Env mediated fusion was inhibited by >50% by cholesterol removal. Overall, PPMP treatment appeared to slow down the rates of CD4-independent HIV-1 Env-mediated Fusion. However, in the case of CXCR4-dependent fusion, the differences between untreated and PPMP-treated cells did not appear to be significant. CONCLUSION: Although modulation of cholesterol and sphingolipids has similar effects on CD4-dependent HIV-1 Env-mediated fusion, sphingolipid modulation had little effect on CD4-independent HIV-1 Env-mediated fusion. Chemokine receptor function remained intact following treatment of cells with PPMP. Therefore such treatment may be considered a more suitable agent to inhibit CD4 dependent HIV-1 infection.

Theta-defensins prevent HIV-1 Env-mediated fusion by binding gp41 and blocking 6-helix bundle formation.

Retrocyclin-1, a -defensin, protects target cells from human immunodeficiency virus, type 1 (HIV-1) by preventing viral entry. To delineate its mechanism, we conducted fusion assays between susceptible target cells and effector cells that expressed HIV-1 Env. Retrocyclin-1 (4 microm) completely blocked fusion mediated by HIV-1 Envs that used CXCR4 or CCR5 but had little effect on cell fusion mediated by HIV-2 and simian immunodeficiency virus Envs. Retrocyclin-1 inhibited HIV-1 Env-mediated fusion without impairing the lateral mobility of CD4, and it inhibited the fusion of CD4-deficient cells with cells bearing CD4-independent HIV-1 Env. Thus, it could act without cross-linking membrane proteins or inhibiting gp120-CD4 interactions. Retrocyclin-1 acted late in the HIV-1 Env fusion cascade but prior to 6-helix bundle formation. Surface plasmon resonance experiments revealed that retrocyclin bound the ectodomain of gp41 with high affinity in a glycan-independent manner and that it bound selectively to the gp41 C-terminal heptad repeat. Native-PAGE, enzyme-linked immunosorbent assay, and CD spectroscopic analyses all revealed that retrocyclin-1 prevented 6-helix bundle formation. This mode of action, although novel for an innate effector molecule, resembles the mechanism of peptidic entry inhibitors based on portions of the gp41 sequence.

Sphingolipids: modulators of HIV-1 infection and pathogenesis.

HIV-1 infects host cells by sequential interactions of its fusion protein (gp120-gp41) with receptors CD4, CXCR4 and/or CCR5 followed by fusion of viral and host membranes. Studies indicate that additional factors such as receptor density and composition of viral and cellular lipids can dramatically modulate the fusion reaction. Lipid rafts, which primarily consist of sphingolipids and cholesterol, have been implicated for infectious route of HIV-1 entry. Plasma membrane Glycosphingolipids (GSLs) have been proposed to support HIV-1 infection in multiple ways: (a) as alternate receptor(s) for CD4-independent entry in neuronal and other cell types, (b) viral transmission, and (c) gp120-gp41-mediated membrane fusion. However, the exact mechanism(s) by which GSLs support fusion is still elusive. This article will focus on the contribution of target membrane sphingolipids and their metabolites in modulating viral entry. We will discuss the current working hypotheses underlying the mechanisms by which these lipids promote and/or block HIV-1 entry. Recent approaches in the design and development of novel glycosyl derivatives, as anti-HIV agents will be summarized.

Effect of structural transition of the host assembly on dynamics of an ion channel peptide: a fluorescence approach.

Structural transition can be induced in charged micelles by increasing the ionic strength of the medium. We have monitored the organization and dynamics of the functionally important tryptophan residues of gramicidin in spherical and rod-shaped sodium dodecyl sulfate micelles utilizing a combination of wavelength-selective fluorescence and related fluorescence approaches. Our results show that tryptophans in gramicidin, present in the single-stranded beta(6.3) conformation, experience slow solvent relaxation giving rise to red edge excitation shift in spherical and rod-shaped micelles. In addition, changes in fluorescence polarization with increasing excitation or emission wavelength reinforce that the gramicidin tryptophans are localized in motionally restricted regions of these micelles. Fluorescence quenching experiments using acrylamide as a quencher of tryptophan fluorescence show that there is reduced water penetration in rod-shaped micelles. Taken together, we show that gramicidin conformation and dynamics is sensitive to the salt-induced structural transition in charged micelles. In addition, these results demonstrate that deformation of the host assembly could modulate protein conformation and dynamics.

Ceramide, a target for antiretroviral therapy.

Studies of ceramide metabolism and function in a wide range of biological processes have revealed a role for this lipid in regulating key cellular responses. Our research on the role of sphingolipids in HIV entry has led to the hypothesis that modulation of ceramide levels in target cells affects their susceptibility to HIV infection by rearranging HIV receptors. Cellular ceramide levels were modulated by application of pharmacological agents such as N-(4-hydroxyphenyl)retinamide (4-HPR, fenretinide), by treatment with sphingomyelinase (Smase), or by exogenous addition of long-chain ceramide, and determined after metabolic incorporation of [3H]sphingosine. Infectivity assays were performed by using a HeLa-derived indicator cell line, TZM-bl, CD4+ lymphocytes, and monocytes. We observed a dose-dependent inhibition by 4-HPR of infection of TZM-bl cells by a broad range of HIV-1 isolates at low micromolar concentrations with an IC50 of <1 microM for most isolates tested. Nearly complete inhibition was seen at 5 microM, a dose that enhanced ceramide levels by 50-100%, yet was nontoxic to the cells. Treating cells with other pharmacological agents that enhanced ceramide levels, with Smase, or exogenous addition of long-chain ceramide also resulted in inhibition of HIV-1 infection. Enhancing ceramide levels in CD4+ lymphocytes and in monocyte-derived macrophages with 4-HPR or Smase significantly reduced infectivity without toxicity. The minimal toxicity of normal cells exposed to 4-HPR should make the drug exceedingly suitable as an anti-HIV therapeutic.

Monitoring gramicidin conformations in membranes: a fluorescence approach.

We have monitored the membrane-bound channel and nonchannel conformations of gramicidin utilizing red-edge excitation shift (REES), and related fluorescence parameters. In particular, we have used fluorescence lifetime, polarization, quenching, chemical modification, and membrane penetration depth analysis in addition to REES measurements to distinguish these two conformations. Our results show that REES of gramicidin tryptophans can be effectively used to distinguish conformations of membrane-bound gramicidin. The interfacially localized tryptophans in the channel conformation display REES of 7 nm whereas the tryptophans in the nonchannel conformation exhibit REES of 2 nm which highlights the difference in their average environments in terms of localization in the membrane. This is supported by tryptophan penetration depth measurements using the parallax method and fluorescence lifetime and polarization measurements. Further differences in the average tryptophan microenvironments in the two conformations are brought out by fluorescence quenching experiments using acrylamide and chemical modification of the tryptophans by N-bromosuccinimide. In summary, we report novel fluorescence-based approaches to monitor conformations of this important ion channel peptide. Our results offer vital information on the organization and dynamics of the functionally important tryptophan residues in gramicidin.

The stability of the intact envelope glycoproteins is a major determinant of sensitivity of HIV/SIV to peptidic fusion inhibitors.

C-peptides derived from the HIV envelope glycoprotein transmembrane subunit gp41 C-terminal heptad repeat (C-HR) region are potent HIV fusion inhibitors. These peptides interact with the gp41 N-terminal heptad repeat (N-HR) region and block the gp41 six-helix bundle formation that is required for fusion. However, the parameters that govern this inhibition have yet to be elucidated. We address this issue by comparing the ability of C34, derived from HIV-1, HIV-2 and SIV gp41, to inhibit HIV-1, HIV-2 and SIV envelope-mediated fusion and the ability of these peptides to form stable six-helix bundles with N36 peptides derived from gp41 of these three viruses. The ability to form six-helix bundles was examined by circular dichroism spectroscopy, and HIV/SIV Env-mediated membrane fusion was monitored by a dye transfer assay. HIV-1 N36 formed stable helix bundles with HIV-1, HIV-2 and SIV C34, which all inhibited HIV-1 Env-mediated fusion at IC(50)<10nM. The three C34 peptides were poor inhibitors of HIV-2 and SIV fusion (IC(50)>100nM), although HIV-2 and SIV N36 formed stable helix bundles with SIV C34. Priming experiments with sCD4 indicate that, in contrast to HIV-1, HIV-2 and SIV Env do not expose their N-HR region to SIV C34 following CD4 binding, but rapidly proceed to co-receptor engagement and six-helix bundle formation resulting in fusion. Our results suggest that several factors, including six-helix bundle stability and the ability of CD4 to destabilize the envelope glycoprotein, serve as determinants of sensitivity to entry inhibitors.

Elevated expression of GM3 in receptor-bearing targets confers resistance to human immunodeficiency virus type 1 fusion.

GM3, a major ganglioside of T lymphocytes, promotes human immunodeficiency virus type 1 (HIV-1) entry via interactions with HIV-1 receptors and the viral envelope glycoprotein (Env). Increased GM3 levels in T lymphocytes and the appearance of anti-GM3 antibodies in AIDS patients have been reported earlier. In this study, we investigated the effect of GM3 regulation on HIV-1 entry by utilizing a mouse cell line (B16F10), which expresses exceptionally high levels of GM3. Strikingly, B16 cells bearing CD4, CXCR4, and/or CCR5 were highly resistant to CD4-dependent HIV-1 Env-mediated membrane fusion. In contrast, these targets supported membrane fusion mediated by CD4-requiring HIV-2, SIV, and CD4-independent HIV-1 Envs. Coreceptor function was not impaired by GM3 overexpression as indicated by Ca(2+) fluxes mediated by the CXCR4 ligand SDF-1alpha and the CCR5 ligand MIP-1beta. Reduction in GM3 levels of B16 target cells resulted in a significant recovery of CD4-dependent HIV-1 Env-mediated fusion. We propose that GM3 in the plasma membrane blocks HIV-1 Env-mediated fusion by interfering with the lateral association of HIV-1 receptors. Our findings offer a novel mechanism of interplay between membrane lipids and receptors by which host cells may escape viral infections.

The role of glycosphingolipids in HIV signaling, entry and pathogenesis.

Although HIV uses CD4 and coreceptors (CCR5 and CXCR4) for productive infection of T cells, glycosphingolipids (GSL) may play ancillary roles in lymphoid and non-lymphoid cells. Interactions of the HIV Envelope Glycoprotein (Env) with GSL may help HIV in various steps of its pathogenesis. Physical-chemical aspects of the interactions between HIV Env and GSL leading to CD4-dependent entry into lymphocytes, the role of GSL in HIV transcytosis, and CD4-independent entry into non-lymphoid cells are reviewed. An overview of signaling properties of HIV receptors is provided with some speculation on how GSL may play a role in these events by virtue of being in membrane rafts. Finally, we summarize how interactions between HIV and coreceptors leading to signaling and/or fusion can be analyzed by the use of various tyrosine kinase and cytoskeletal inhibitors.

An inhibitor of glycosphingolipid metabolism blocks HIV-1 infection of primary T-cells.

OBJECTIVE: HIV-1 uses CD4 and chemokine receptors to enter cells. However, other target membrane components may also be involved. This study examines the role of glycosphingolipids (GSL) in HIV-1 entry into primary lymphocytes and its modulation by an inhibitor of GSL biosynthesis. METHODS: CD4 lymphocytes purified from normal or the p-group subtype individuals that were defective in Gb3 synthesis were treated with a GSL biosynthesis inhibitor, 1-phenyl-2-hexadecanoylamino-3-morpholino-1-propanol (PPMP). The PPMP-treated cells were tested for HIV-1 replication by measuring p24 antigen production for 7-14 days post-infection and for susceptibility to HIV-1 Env-mediated fusion monitored by a fluorescent dye transfer assay. The effects of PPMP treatment on HIV-1 binding to CD4 lymphocytes were also examined by measuring HIV-1 p24. RESULTS: CD4 lymphocytes from p donors that are devoid of Gb3, but have elevated levels of GM3 were highly susceptible to HIV-1 fusion/entry. Pre-treatment of primary human CD4 lymphocytes from normal or p-sub-group type with PPMP, significantly reduced HIV-1 replication with no change in CD4 and CXCR4 levels. Inhibition of HIV-1 infection was due to the block in HIV-1 Env-mediated plasma membrane fusion. Binding of HIV-1 to CD4 lymphocytes was not affected by PPMP treatment. CONCLUSION: Manipulation of glycosphingolipid metabolic pathways may alter susceptibility of CD4 lymphocytes to HIV-1 entry.

Functional expression of CD4, CXCR4, and CCR5 in glycosphingolipid-deficient mouse melanoma GM95 cells and susceptibility to HIV-1 envelope glycoprotein-triggered membrane fusion.

We had previously reported that glycosphingolipids (GSL) support human immunodeficiency virus type 1 (HIV-1) entry. In this study, we further examined this issue by expressing HIV-1 receptors in GSL-deficient GM95 cells. GM95 cells expressing low levels of CD4 and CXCR4 or CCR5 did not support HIV-1 Env-mediated fusion. However, higher expression of these receptors rendered GM95 cells highly susceptible to fusion with cells expressing appropriate HIV-1 envelope glycoproteins (HIV-1 Envs). The GM95 cells exhibited a different fusion phenotype when compared with GSL(+) NIH3T3 cells bearing similar receptor levels. Fusion of GM95 targets expressing higher levels of CD4 and coreceptors occurred at 25 degrees C and was sensitive to cholesterol depletion or disruption of the cytoskeleton. In contrast, the fusion threshold of NIH3T3CD4X4/R5 targets was at >/=28 degrees C as previously reported and was insensitive to cholesterol depletion or cytoskeletal network disruption. On the basis of these observations, we propose that target membrane GSLs support HIV-1 Env-mediated fusion at low density of receptors by stabilizing receptor pools in natural targets.

Role of the fusion peptide and membrane-proximal domain in HIV-1 envelope glycoprotein-mediated membrane fusion.

The N-terminal fusion peptide and the interfacial sequence preceding the transmembrane anchor of HIV-1 gp41 are required for viral fusion. Studies with synthetic peptides indicated that these regions function by destabilizing membranes, which is regarded as a crucial step in the membrane fusion reaction. However, it is not clear whether membrane destabilization is induced by these sequences in the intact gp41. We address this question by examining fusion and destabilization of membranes expressing HIV-1(IIIB) wild-type Env and two mutant Envs. (1) A Glu residue at position 2 of the gp41 fusion peptide is substituted for Val (V2E) to produce one mutant. (2) Residues 665-682 in the membrane-proximal domain are deleted to form the other. The process of membrane destabilization was monitored by the influx of Sytox, an impermeant fluorescent dye, into the Env-expressing cells following the interaction with CD4-CXCR4 complexes, and fusion was monitored by observing dye transfer between Env-expressing cells and appropriate target cells. We also monitored the conformational changes in the Envs following their interactions with CD4 and CXCR4 by immunofluorescence using an anti-gp41 mAb that reacts with the six-helix bundle. In contrast to the wild type, both Env mutants did not mediate cell fusion. The V2E Env did not mediate membrane destabilization. However, the Env with an unmodified fusion peptide but with a deletion of residues 665-682 in the membrane-proximal domain did mediate membrane destabilization. The wild type and both mutant Envs undergo conformational changes detected by the anti-gp41 six-helix bundle mAbs. Our results suggest that in intact HIV-1 Env the membrane-proximal domain is not required for membrane perturbations, but rather enables the bending of gp41 that is required for viral and target membranes to come together. Moreover, the observation that the Delta665-683 Env self-inserts its fusion peptide but does not cause fusion suggests that self-insertion of the fusion peptide is not sufficient for HIV-1 Env-mediated fusion.