Histone deacetylase 6 regulates human immunodeficiency virus type 1 infection

Efficient human immunodeficiency virus (HIV)-1 infection depends on multiple interactions between the viral gp41/gp120 envelope (Env) proteins and cell surface receptors. However, cytoskeleton-associated proteins that modify membrane dynamics may also regulate the formation of the HIV-mediated fusion pore and hence viral infection. Because the effects of HDAC6-tubulin deacetylase on cortical alpha-tubulin regulate cell migration and immune synapse organization, we explored the possible role of HDAC6 in HIV-1-envelope-mediated cell fusion and infection. The binding of the gp120 protein to CD4+-permissive cells increased the level of acetylated alpha-tubulin in a CD4-dependent manner. Furthermore, overexpression of active HDAC6 inhibited the acetylation of alpha-tubulin, and remarkably, prevented HIV-1 envelope-dependent cell fusion and infection without affecting the expression and codistribution of HIV-1 receptors. In contrast, knockdown of HDAC6 expression or inhibition of its tubulin deacetylase activity strongly enhanced HIV-1 infection and syncytia formation. These results demonstrate that HDAC6 plays a significant role in regulating HIV-1 infection and Env-mediated syncytia formation.     

Endocytic entry of HIV-1

Enveloped viruses enter target cells by membrane fusion or endocytosis. In the latter case, fusion of the viral envelope is induced by the acidic pH of the endocytic vesicle [1]. As with most other retroviruses, entry of the human immunodeficiency virus (HIV) is thought to be exclusively by pH-independent membrane fusion after interaction of its envelope with CD4 and a chemokine co-receptor on the target cell [2,3]. Expression of CD4 on the virus-producing cell impairs the release and infectivity of HIV-1(NL4-3) particles [4-6]. In sharp contrast, we found that the infectivity of another HIV isolate, HIV-1SF2, was enhanced by expression of CD4 on the producer cells, which correlated with significantly increased amounts of viral proteins in the vesicular fraction of target cells. Endocytic inhibitors decreased infectivity of HIV-1SF2 but enhanced that of HIV-1 NL4-3. Expression of CD4 in the producer cell did not remove gp41 from HIV-1SF2 virions. With these cells, the formation of syncytia could be induced by acidic medium. Thus, HIV-1SF2 can enter the cytoplasm by an endocytic route after activation of gp41 by the acidic pH of endocytic vesicles. Endocytic entry might expand the range of cells that HIV could infect and should be considered in antiviral strategies against AIDS.     

A nonneutralizing anti-HIV-1 antibody turns into a neutralizing antibody when expressed on the surface of HIV-1-susceptible cells: a new way to fight HIV

During HIV-1 infection or vaccination, HIV-1 envelope spikes elicit Ab responses. Neutralizing Abs block viral entry by recognizing epitopes on spikes critical for their interaction with receptor, coreceptors or fusion. In contrast, nonneutralizing Abs fail to do so because they recognize epitopes either buried or exposed but not critical for viral entry. Previously, we produced a high-affinity human mAb against the cluster II determinant of gp41. This Ab or its recombinant Fab and single-chain Fv have been repeatedly shown to bind to HIV-1 gp160 or gp41, but fail to block viral entry. We report that, surprisingly, expression of this nonneutralizing anti-HIV-1 gp41 single-chain Fv on the surface of human CD4 T cells markedly inhibits HIV-1 replication and cell-cell fusion. The inhibition targets the HIV-1 envelope at the level of viral entry, regardless of HIV-1 tropism. Although this bona fide nonneutralizing Ab does not neutralize HIV-1 entry when produced as a soluble protein, it acts as a neutralizing Ab when expressed on the cell surface. Expressing Abs on the surface of HIV-1-susceptible cells can be a new way to fight HIV-1.     

Regions of the CD4 molecule not involved in virus binding or syncytia formation are required for HIV-1 infection of lymphocytes.

Cell surface-expressed CD4 binds to the envelope glycoprotein of HIV-1 and mediates syncytia formation through interacting with membrane expressed HIV-1 gp120. Further possible roles of the CD4 molecule in the process of cell infection by HIV-1 remain poorly understood. In our study we describe two mAb that recognize the V3/V4 domain of the CD4 molecule. Although these mAb do not inhibit gp120-CD4 binding or HIV-1-induced syncytia formation, they inhibit HIV-1 infection of human PBL. These findings suggest that discrete, definable domains of the CD4 molecule may be involved in interactions after HIV-1 envelope binding that lead to virus entry into the cell.     

Target cell-specific determinants of membrane fusion within the human immunodeficiency virus type 1 gp120 third variable region and gp41 amino terminus.

The entry of human immunodeficiency virus type 1 (HIV-1) into target cells involves binding to the viral receptor (CD4) and membrane fusion events, the latter influenced by target cell factors other than CD4. The third variable (V3) region of the HIV-1 gp120 exterior envelope glycoprotein and the amino terminus of the HIV-1 gp41 transmembrane envelope glycoprotein have been shown to be important for the membrane fusion process. Here we demonstrate that some HIV-1 envelope glycoproteins containing an altered V3 region or gp41 amino terminus exhibit qualitatively different abilities to mediate syncytium formation and virus entry when different target cells are used. These results demonstrate that the structure of these HIV-1 envelope glycoprotein regions determines the efficiency of membrane fusion in a target cell-specific manner and support a model in which the gp41 amino terminus interacts directly or indirectly with the target cell during virus entry.     

Modulation of human immunodeficiency virus type 1 infectivity through incorporation of tetraspanin proteins

Accumulating evidence indicates that human immunodeficiency virus type 1 (HIV-1) acquires various cellular membrane proteins in the lipid bilayer of the viral envelope membrane. Although some virion-incorporated cellular membrane proteins are known to potently affect HIV-1 infectivity, the virological functions of most virion-incorporated membrane proteins remain unclear. Among these host proteins, we found that CD63 was eliminated from the plasma membranes of HIV-1-producing T cells after activation, followed by a decrease in the amount of virion-incorporated CD63, and in contrast, an increase in the infectivity of the released virions. On the other hand, we found that CD63 at the cell surface was preferentially embedded on the membrane of released virions in an HIV-1 envelope protein (Env)-independent manner and that virion-incorporated CD63 had the potential to inhibit HIV-1 Env-mediated infection in a strain-specific manner at the postattachment entry step(s). In addition, these behaviors were commonly observed in other tetraspanin proteins, such as CD9, CD81, CD82, and CD231. However, L6 protein, whose topology is similar to that of tetraspanins but which does not belong to the tetraspanin superfamily, did not have the potential to prevent HIV-1 infection, despite its successful incorporation into the released particles. Taken together, these results suggest that tetraspanin proteins have the unique potential to modulate HIV-1 infectivity through incorporation into released HIV-1 particles, and our findings may provide a clue to undiscovered aspects of HIV-1 entry.     

Determinants of human immunodeficiency virus type 1 entry in the CDR2 loop of the CD4 glycoprotein.

Various roles for the viral receptor, CD4, have been proposed in facilitating human immunodeficiency virus type 1 (HIV-1) entry, including virion binding to the target cell and the induction of conformational changes in the viral envelope glycoproteins required for the membrane fusion reaction. Here, we compare the structural requirements in the CDR2-like loop of CD4 domain 1, the major contact site of the gp120 envelope glycoprotein, for gp120 binding and virus entry. For every CD4 mutant examined, the level of cell surface expression and the gp120 binding affinity were sufficient to explain the relative ability to function as a viral receptor. The decrease in relative infectibility associated with decreased gp120 binding affinity was more pronounced at lower cell surface CD4 concentrations. These results imply that both receptor density and affinity determine the efficiency of HIV-1 entry and that specific structures in the CD4 residues examined are probably not required for HIV-1 entry functions other than gp120 binding.     

Lack of the trans-receptor mechanism of HIV-1 infection: CD4- and coreceptor-independent incorporation of HIV-1-resistant cells into syncytia induced by HIV-1

Human immunodeficiency virus type 1 (HIV-1) infects cells through an interaction of HIV-1 envelope protein with CD4 and an appropriate coreceptor on target cells. This interaction often leads to cell fusion, and formation of syncytia. HIV-1-resistant cells expressing either CD4 or a coreceptor are often surrounding HIV-1-susceptible cells, expressing both CD4 and a compatible coreceptor, in vivo. It is therefore worthwhile to investigate whether these HIV-1-resistant cells could cooperate in HIV-1 infection or cell fusion leading to their incorporation into syncytia. When CD4-positive, coreceptor-negative cells were co-cultured with CD4-negative, coreceptor-positive cells and exposed to HIV-1, HIV-1 infection was not established, indicating that CD4 and the coreceptor expressed on different cell surfaces could not cooperate in HIV-1 entry. However, when HIV-1-resistant cells expressing CD4 or a coreceptor or lacking both were mixed with HIV-1-susceptible cells and inoculated with HIV-1, all these HIV-1-resistant cells were similarly incorporated into syncytia induced by HIV-1, indicating a CD4- and coreceptor-independent incorporation of HIV-1-resistant cells into syncytia. This incorporation was impaired by the transfection of these cells with siRNAs for adhesion molecules. Our study demonstrates that HIV-1-resistant cells can be incorporated into syncytia induced by HIV-1 and this incorporation may partially be mediated through adhesion molecules.     

Lack of correlation between soluble CD4-induced shedding of the human immunodeficiency virus type 1 exterior envelope glycoprotein and subsequent membrane fusion events.

The noncovalent association of the gp120 and gp41 envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) is disrupted by soluble CD4 binding, resulting in shedding of the gp120 exterior envelope glycoprotein. This observation has led to the speculation that interaction of gp120 with the CD4 receptor triggers shedding of the exterior envelope glycoprotein, allowing exposure of gp41 domains necessary for membrane fusion steps involved in virus entry or syncytium formation. To test this hypothesis, a set of HIV-1 envelope glycoprotein mutants were used to examine the relationship of soluble CD4-induced shedding of the gp120 glycoprotein to envelope glycoprotein function in syncytium formation and virus entry. All mutants with a threefold or greater reduction in CD4-binding ability exhibited marked decreases in gp120 shedding in response to soluble CD4, even though several of these mutants exhibited significant levels of envelope glycoprotein function. Conversely, most fusion-defective mutants with wild-type gp120-CD4 binding affinity, including those with changes in the V3 loop, efficiently shed gp120 following soluble CD4 binding. Thus, soluble CD4-induced shedding of gp120 is not a generally useful marker for conformational changes in the HIV-1 envelope glycoproteins necessary for the virus entry or syncytium formation processes. Some gp120 mutants, despite being expressed on the cell surface and capable of efficiently binding soluble CD4, exhibited decreased gp120 shedding. These mutants were still sensitive to neutralization by soluble CD4, indicating that, for envelope glycoproteins exhibiting high affinity for soluble CD4, competitive inhibition may be more important than gp120 shedding for the antiviral effect.     

Lineage-Specific Differences between Human and Simian Immunodeficiency Virus Regulation of gp120 Trimer Association and CD4 Binding

Metastable conformations of the gp120 and gp41 envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) must be maintained in the unliganded state of the envelope glycoprotein trimer. Binding of gp120 to the primary receptor, CD4, triggers the transition to an open conformation of the trimer, promoting interaction with the CCR5 chemokine receptor and ultimately leading to gp41-mediated virus-cell membrane fusion and entry. Topological layers in the gp120 inner domain contribute to gp120-trimer association in the unliganded state and to CD4 binding. Here we describe similarities and differences between HIV-1 and SIVmac gp120. In both viruses, the gp120 N/C termini and the inner domain β-sandwich and layer 2 support the noncovalent association of gp120 with the envelope glycoprotein trimer. Layer 1 of the SIVmac gp120 inner domain contributes more to trimer association than the corresponding region of HIV-1 gp120. On the other hand, layer 1 plays an important role in stabilizing the CD4-bound conformation of HIV-1 but not SIVmac gp120 and thus contributes to HIV-1 binding to CD4. In SIVmac, CD4 binding is instead enhanced by tryptophan 375, which fills the Phe 43 cavity of gp120. Activation of SIVmac by soluble CD4 is dependent on tryptophan 375 and on layer 1 residues that determine a tight association of gp120 with the trimer. Distinct biological requirements for CD4 usage have resulted in lineage-specific differences in the HIV-1 and SIV gp120 structures that modulate trimer association and CD4 binding.     

CD4-Induced Conformational Changes in the Human Immunodeficiency Virus Type 1 gp120 Glycoprotein: Consequences for Virus Entry and Neutralization

Human immunodeficiency virus type 1 (HIV-1) entry into target cells involves sequential binding of the gp120 exterior envelope glycoprotein to CD4 and to specific chemokine receptors. Soluble CD4 (sCD4) is thought to mimic membrane-anchored CD4, and its binding alters the conformation of the HIV-1 envelope glycoproteins. Two cross-competing monoclonal antibodies, 17b and CG10, that recognize CD4-inducible gp120 epitopes and that block gp120-chemokine receptor binding were used to investigate the nature and functional significance of gp120 conformational changes initiated by CD4 binding. Envelope glycoproteins derived from both T-cell line-adapted and primary HIV-1 isolates exhibited increased binding of the 17b antibody in the presence of sCD4. CD4-induced exposure of the 17b epitope on the oligomeric envelope glycoprotein complex occurred over a wide range of temperatures and involved movement of the gp120 V1/V2 variable loops. Amino acid changes that reduced the efficiency of 17b epitope exposure following CD4 binding invariably compromised the ability of the HIV-1 envelope glycoproteins to form syncytia or to support virus entry. Comparison of the CD4 dependence and neutralization efficiencies of the 17b and CG10 antibodies suggested that the epitopes for these antibodies are minimally accessible following attachment of gp120 to cell surface CD4. These results underscore the functional importance of these CD4-induced changes in gp120 conformation and illustrate viral strategies for sequestering chemokine receptor-binding regions from the humoral immune response.     

Effects of changes in gp120-CD4 binding affinity on human immunodeficiency virus type 1 envelope glycoprotein function and soluble CD4 sensitivity.

Mutant gp120 glycoproteins exhibiting a range of affinities for CD4 were tested for ability to form syncytia and to complement an env-defective provirus for replication. Surprisingly, gp120 mutants that efficiently induced syncytia and/or complemented virus replication were identified that exhibited marked (up to 50-fold) reductions in CD4-binding ability. Temperature-dependent changes in gp120, which result in a seven- to ninefold increase in affinity for CD4, were shown not to be necessary for subsequent membrane fusion or virus entry events. Mutant glycoproteins demonstrating even relatively small decreases in CD4-binding ability exhibited reduced sensitivity to soluble CD4. The considerable range of CD4-binding affinities tolerated by replication-competent HIV-1 variants has important implications for antiviral strategies directed at the gp120-CD4 interaction.     

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.     

Human immunodeficiency virus type 1 Vpu protein induces degradation of chimeric envelope glycoproteins bearing the cytoplasmic and anchor domains of CD4: role of the cytoplasmic domain in Vpu-induced degradation in the endoplasmic reticulum.

The human immunodeficiency virus type 1 (HIV-1) Vpu protein is a transmembrane phosphoprotein which induces rapid degradation of CD4 in the endoplasmic reticulum (ER). To identify sequences in CD4 for Vpu-induced degradation, we generated four chimeric envelope glycoproteins having the ectodomain of HIV-1 gp160, the anchor domain of CD4, and 38, 25, 24, and 18 amino acids (aa) of the CD4 cytoplasmic domain. Using the vaccinia virus-T7 RNA polymerase expression system, we analyzed the expression of chimeric proteins in the presence and absence of Vpu. In singly transfected cells, the chimeric envelope glycoproteins having 38, 24, and 18 aa of the CD4 cytoplasmic domain were endoproteolytically cleaved and biologically active in the fusion of HeLa CD4+ cells. However, one of the chimeras having 25 aa of the CD4 cytoplasmic tail was retained in the ER using the transmembrane ER retention signal and was defective in membrane fusion. Furthermore, biochemical analyses of the coexpressing cells revealed that the Vpu protein induced degradation of the envelope glycoproteins having 38, 25, and 24 aa of the CD4 cytoplasmic tail and degradation occurred in the ER. Consequently, the fusion-competent glycoproteins did not induce the formation of syncytia in HeLa CD4+ cells expressing Vpu. However, the HIV-1 gp160 and chimeric envelope glycoprotein having the membrane-proximal 18 aa of the CD4 cytoplasmic tail were stable and fusion competent in cells expressing Vpu. In addition, we examined the stability of CD4 molecules in the presence of Vpu. Coexpression analyses revealed that the Vpu protein induced degradation of CD4 whereas mutant CD4 having the membrane-proximal 18 aa of the cytoplasmic domain was relatively stable in the presence of Vpu. Taken together, these studies have elucidated that the Vpu protein requires sequences or sequence determinants in the cytoplasmic domain of CD4 to induce degradation of the glycoproteins in the cell.     

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).     

Hepatitis C virus entry and the tetraspanin CD81

CD81, a member of the tetraspanin integral membrane protein family, has been identified as an essential receptor for HCV (hepatitis C virus). The present review highlights recent published data on the role that CD81 plays in HCV entry, including the importance of actin-dependent lateral diffusion of CD81 within the cell membrane, CD81 endocytosis and the CD81-Claudin-1 receptor complex in HCV internalization. Additional functions for CD81 in the viral life cycle and the role of HCV-CD81 interactions in HCV-induced B-cell and CNS (central nervous system) abnormalities are discussed.     

Subunit stoichiometry of human immunodeficiency virus type 1 envelope glycoprotein trimers during virus entry into host cells

The envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) function as a homotrimer of gp120/gp41 heterodimers to support virus entry. During the process of virus entry, an individual HIV-1 envelope glycoprotein trimer binds the cellular receptors CD4 and CCR5/CXCR4 and mediates the fusion of the viral and the target cellular membranes. By studying the function of heterotrimers between wild-type and nonfunctional mutant envelope glycoproteins, we found that two wild-type subunits within an envelope glycoprotein trimer are required to support virus entry. Complementation between HIV-1 envelope glycoprotein mutants defective in different functions to allow virus entry was not evident. These results assist our understanding of the mechanisms whereby the HIV-1 envelope glycoproteins mediate virus entry and membrane fusion and guide attempts to inhibit these processes.     

Fusion proteins of HIV-1 envelope glycoprotein gp120 with CD4-induced antibodies showed enhanced binding to CD4 and CD4 binding site antibodies

Development of successful AIDS vaccine immunogens continues to be a major challenge. One of the mechanisms by which HIV-1 evades antibody-mediated neutralizing responses is the remarkable conformational flexibility of its envelope glycoprotein (Env) gp120. Some recombinant gp120s do not preserve their conformations on gp140s and functional viral spikes, and exhibit decreased recognition by CD4 and neutralizing antibodies. CD4 binding induces conformational changes in gp120 leading to exposure of the coreceptor-binding site (CoRbs). In this study, we test our hypothesis that CD4-induced (CD4i) antibodies, which target the CoRbs, could also induce conformational changes in gp120 leading to better exposed conserved neutralizing antibody epitopes including the CD4-binding site (CD4bs). We found that a mixture of CD4i antibodies with gp120 only weakly enhanced CD4 binding. However, such interactions in single-chain fusion proteins resulted in gp120 conformations which bound to CD4 and CD4bs antibodies better than the original or mutagenically stabilized gp120s. Moreover, the two molecules in the fusion proteins synergized with each other in neutralizing HIV-1. Therefore, fusion proteins of gp120 with CD4i antibodies could have potential as components of HIV-1 vaccines and inhibitors of HIV-1 entry, and could be used as reagents to explore the conformational flexibility of gp120 and mechanisms of entry and immune evasion.     

Early activation events render T cells susceptible to HIV-1-induced syncytia formation. Role of protein kinase C.

In human immunodeficiency virus-1 (HIV-1)-infected cell cultures, cell-to-cell fusion and the formation of multinucleated giant cells (syncytia) are induced as a consequence of interactions between the viral envelope glycoprotein on infected cells and cell surface CD4 molecules on uninfected cells. Although activated CD4+ T cells rapidly form syncytia when cultured with HIV-1 envelope glycoprotein expressing (env+) cells, freshly isolated, unstimulated CD4+ T cells do so more slowly. In these studies, we sought to explore the role of T cell activation in rendering CD4+ T cells susceptible to HIV-1-mediated syncytia formation. Our results indicate that within 2 h of exposure to immunologic stimuli, CD4+ T cells acquire the ability to form syncytia with HIV-1 env+ cells. Both cholera toxin, an inhibitor of protein kinase C (PKC) through its effects on inositol triphosphate and diacylglycerol production, and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride, a noncompetitive inhibitor (with respect to ATP) of PKC, prevented unstimulated but not previously stimulated CD4+ T cells from forming syncytia with HIV-1 env+ cells. 1-Oleoyl-2-acetyl glycerol, an analog of the PKC activator, diacylglycerol, enhanced syncytia formation whereas ionomycin, a calcium ionophore, had no effect. These results suggest that activation of PKC is essential for previously unstimulated CD4+ T cells to become fusogenic.     

Calcium ions are required for cell fusion mediated by the CD4-human immunodeficiency virus type 1 envelope glycoprotein interaction.

Calcium ions are required for fusion of a wide variety of artificial and biological membranes. To examine the role of calcium ions for cell fusion mediated by interactions between CD4 and the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (gp120-gp41), we used two experimental systems: (i) cells expressing gp120-gp41 and its receptor CD4, both encoded by recombinant vaccinia viruses, and (ii) chronically infected cells producing low levels of HIV-1. Fusion was measured by counting the number of syncytia and by monitoring the redistribution of fluorescence dyes by video microscopy. Syncytia did not form in solutions without calcium ions. Addition of calcium ions partially restored the formation of syncytia. EDTA and EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] blocked syncytium formation in culture media containing calcium ions. Membrane fusion as monitored by fluorescence dye redistribution also required calcium ions. Cell fusion increased with an increase in calcium ion concentration from 100 microM to 10 mM but was not affected by magnesium ions in the concentration range from 0 to 30 mM. Fibrinogen and fibronectin did not promote fusion in the absence or presence of Ca2+. Binding of soluble CD4 to gp120-gp41-expressing cells was not affected by Ca2+ and Mg2+. We conclude that Ca2+ is involved in postbinding steps in cell fusion mediated by the CD4-HIV-1 envelope glycoprotein interaction.