An amphipathic peptide from the C-terminal region of the human immunodeficiency virus envelope glycoprotein causes pore formation in membranes

Protective effect of glutathione in HIV-1 lytic peptide 1-induced cell death in human neuronal cells

To elucidate the pathogenic mechanisms involved in neurodegeneration in AIDS patients with cognitive deficits, we have examined the toxic effect of the lentivirus lytic peptide 1 (LLP-1) corresponding to the carboxyl terminus of HIV-1 transmembrane glycoprotein gp41 on human neuronal and glial cell lines. LLP-1 induced a significant lactate dehydrogenase (LDH, a marker of cell death) release from these cells in a concentration- and time-dependent manner, while the noncytolytic LLP-1 analog 2 had little effect. Application of LLP-1 to SH-SY5Y, a well-characterized human neuronal cell line, caused the decline of intracellular glutathione (GSH) content that appeared to occur before a significant LDH release. Furthermore, LLP-1 elicited a significant loss of mitochondrial function as measured by mitochondrial transmembrane potential (MTP). Among the reducing agents and antioxidants tested, GSH and a GSH prodrug N-acetylcysteine (NAC) provided protection against LLP-1-induced neuronal cell death, evidently by restoring the intracellular GSH levels and blocking the disruption of mitochondrial integrity. Thus, gp41-derived LLP-1 may be a potential neurotoxic agent capable of causing the intracellular GSH depletion and disturbing the mitochondrial function, possibly contributing to the neurodegenerative cascade as seen in HIV-1-associated dementia. Our data indicate that restoring both GSH concentration and mitochondrial function may hold promise as possible therapeutic strategies for slowing disease progression of dementia in AIDS patients.

Cellular membrane-binding ability of the C-terminal cytoplasmic domain of human immunodeficiency virus type 1 envelope transmembrane protein gp41

The amphipathic alpha-helices located in the cytoplasmic tail of the envelope (Env) transmembrane glycoprotein gp41 of human immunodeficiency virus type 1 have been implicated in membrane association and cytopathicity. Deletion of the last 12 amino acids in the C terminus of this domain severely impairs infectivity. However, the nature of the involvement of the cytoplasmic tail in Env-membrane interactions in cells and the molecular basis for the defect in infectivity of this mutant virus are still poorly understood. In this study we examined the interaction of the cytoplasmic tail with membranes in living mammalian cells by expressing a recombinant cytoplasmic tail fragment and an Escherichia coli beta-galactosidase/cytoplasmic tail fusion protein, both of them lacking gp120, the gp41 ectodomain, and the transmembrane region. We found through cell fractionation, in vivo membrane flotation, and confocal immunofluorescence studies that the cytoplasmic tail contained determinants to be routed to a perinuclear membrane region in cells. Further mapping showed that each of the three lentivirus lytic peptide (LLP-1, LLP-2, and LLP-3) sequences conferred this cellular membrane-targeting ability. Deletion of the last 12 amino acids from the C terminus abolished the ability of the LLP-1 motif to bind to membranes. High salt extraction, in vitro transcription and translation, and posttranslational membrane binding analyses indicated that the beta-galactosidase/LLP fusion proteins were inserted into membranes via the LLP sequences. Subcellular fractionation and confocal microscopy studies revealed that each of the LLP motifs, acting in a position-independent manner, targeted non-endoplasmic reticulum (ER)-associated beta-galactosidase and enhanced green fluorescence protein to the ER. Our study provides a basis for the involvement of the gp41 cytoplasmic tail during Env maturation and also supports the notion that the membrane apposition of the C-terminal cytoplasmic tail plays a crucial role in virus-host interaction.

Voltage-induced nonconductive pre-pores and metastable single pores in unmodified planar lipid bilayer

Electric fields promote pore formation in both biological and model membranes. We clamped unmodified planar bilayers at 150-550 mV to monitor transient single pores for a long period of time. We observed fast transitions between different conductance levels reflecting opening and closing of metastable lipid pores. Although mean lifetime of the pores was 3 +/- 0.8 ms (250 mV), some pores remained open for up to approximately 1 s. The mean amplitude of conductance fluctuations (approximately 500 pS) was independent of voltage and close for bilayers of different area (40,000 and 10 microm(2)), indicating the local nature of the conductive defects. The distribution of pore conductance was rather broad (dispersion of approximately 250 pS). Based on the conductance value and its dependence of the ion size, the radius of the average pore was estimated as approximately 1 nm. Short bursts of conductance spikes (opening and closing of pores) were often separated by periods of background conductance. Within the same burst the conductance between spikes was indistinguishable from the background. The mean time interval between spikes in the burst was much smaller than that between adjacent bursts. These data indicate that opening and closing of lipidic pores proceed through some electrically invisible (silent) pre-pores. Similar pre-pore defects and metastable conductive pores might be involved in remodeling of cell membranes in different biologically relevant processes.

Modification of virus infectivity by cytoplasmic tail of HIV-1 TM protein

Envelope glycoprotein incorporation is an essential process in formation of infectious particles of human immunodeficiency virus. Accumulated data have indicated that the cytoplasmic tail of Env gp41 is required for efficient incorporation. By analyzing mutant viruses with truncated cytoplasmic tails, we found that the domain was required in a cell-type-dependent manner for maintaining virus infectivity. Although the viruses with truncated cytoplasmic tails produced from HeLa, A3.01 and SupT1 cells showed a greatly reduced infectivity, those from SW480 and MT-4 cells retained a significant infectivity. To clarify the different effect of the cytoplasmic tail mutation on virus infectivity, we performed biochemical studies on the virions produced from HeLa and SW480 cells. Although the truncation of cytoplasmic tail appeared to reduce the Env incorporation in both cell lines, it caused a significant incorporation of Env precursor with HeLa cells. The results suggested that the cytoplasmic tail regulated selective incorporation of processed Env into virions in a cell-type-dependent manner.

Mode of action of an antiviral peptide from HIV-1. Inhibition at a post-lipid mixing stage

DP178, a synthetic peptide corresponding to a segment of the transmembrane envelope glycoprotein (gp41) of human immunodeficiency virus, type 1 (HIV-1), is a potent inhibitor of viral infection and virus-mediated cell-cell fusion. Nevertheless, DP178 does not contain gp41 coiled-coil cavity binding residues postulated to be essential for inhibiting HIV-1 entry. We find that DP178 inhibits phospholipid redistribution mediated by the HIV-1 envelope glycoprotein at a concentration 8 times greater than that of solute redistribution (the IC(50) values are 43 and 335 nm, respectively). In contrast, C34, a synthetic peptide which overlaps with DP178 but contains the cavity binding residues, did not show this phenomenon (11 and 25 nm, respectively). The ability of DP178 to inhibit membrane fusion at a post-lipid mixing stage correlates with its ability to bind and oligomerize on the surface of membranes. Furthermore, our results are consistent with a model in which DP178 inhibits the formation of gp41 viral hairpin structure at low affinity, whereas C34 inhibits its formation at high affinity: the failure to form the viral hairpin prevents both lipid and solute from redistributing between cells. However, our data also suggest an additional membrane-bound inhibitory site for DP178 in the ectodomain of gp41 within a region immediately adjacent to the membrane-spanning domain. By binding to this higher affinity site, DP178 inhibits the recruitment of several gp41-membrane complexes, thus inhibiting fusion pore formation.

Mutational analysis of conserved domains within the cytoplasmic tail of gp41 from human immunodeficiency virus type 1: effects on glycoprotein incorporation and infectivity

The transmembrane (TM) glycoprotein gp41 of human immunodeficiency virus type 1 possesses an unusually long ( approximately 150 amino acids) and highly conserved cytoplasmic region. Previous studies in which this cytoplasmic tail had been deleted partially or entirely have suggested that it is important for virus infectivity and incorporation of the gp120-gp41 glycoprotein complex into virions. To determine which regions of the conserved C-terminal domains are important for glycoprotein incorporation and infectivity, several small deletions and amino acid substitutions which modify highly conserved motifs were constructed in the infectious proviral background of NL4.3. The effects of these mutations on infectivity and glycoprotein incorporation into virions produced from transfected 293-T cells and infected H9 and CEMx174 cells were determined. With the exception of a mutation deleting amino acids QGL, all of the constructs resulted in decreased infectivity of the progeny virus both in a single-round infectivity assay and in a multiple-infection assay in H9 and CEMx174 cells. For most mutations, the decreased infectivity was correlated with a decreased incorporation of glycoprotein into virions. Substitution of the arginines (residues 839 and 846) with glutamates also reduced infectivity, but without a noticeable decrease in the amount of glycoprotein incorporated into virus produced from infected T cells. These results demonstrate that minor alterations in the conserved C-terminal region of the gp41 cytoplasmic tail can result in reductions in infectivity that correlate for most but not all constructs with a decrease in glycoprotein incorporation. Observed cell-dependent differences suggest the involvement of cellular factors in regulating glycoprotein incorporation and infectivity.

The effect of lipid composition and physical state of phospholipid monolayer on the binding and incorporation of a basic amphipathic peptide from the C-terminal region of the HIV envelope protein gp41

The interaction of a peptide identical to the carboxy terminal region of the envelope glycoprotein gp41(828) of HIV with negatively charged phospholipids in a monolayer was studied by a Wilhelmy film balance. No significant interaction of the peptide with a monolayer composed of pure neutral but a strong affinity to negatively charged phospholipids could be observed. In mixed phospholipid monolayers the binding of the gp41(828) is primarily limited by the amount of acidic phospholipids. The physical state of the monolayer is another important parameter for binding. Clustering of negatively charged phospholipids and the surface pressure are crucial. Ca(2+) ions strongly interfere with the peptide-lipid interaction up to complete abolishment. The effects observed are dependent on the nature of the acidic lipid. Phosphatidylglycerol was found to be more sensitive than phosphatidylserine. The significance of the results for processes like virus assembly and budding will be discussed.

Multimerization potential of the cytoplasmic domain of the human immunodeficiency virus type 1 transmembrane glycoprotein gp41

We previously demonstrated that an envelope mutant of human immunodeficiency virus type 1 lacking the entire cytoplasmic domain interferes in trans with the production of infectious virus by inclusion of the mutant envelope into the wild-type envelope complex. We also showed that the envelope incorporation into virions is not affected when the wild-type envelope is coexpressed with the mutant envelope. These results suggest that an oligomeric structure of the cytoplasmic domain is functionally required for viral infectivity. To understand whether the cytoplasmic domain of human immunodeficiency virus type 1 transmembrane protein gp41 has the potential to self-assemble as an oligomer, in the present study we fused the coding sequence of the entire cytoplasmic domain at 3' to the Escherichia coli malE gene, which encodes a monomeric maltose-binding protein. The expressed fusion protein was examined by chemical cross-linking, sucrose gradient centrifugation, and gel filtration. The results showed that the cytoplasmic domain of gp41 assembles into a high-ordered structural complex. The intersubunit interaction of the cytoplasmic domain was also confirmed by a mammalian two-hybrid system that detects protein-protein interactions in eucaryotic cells. A cytoplasmic domain fragment expressed in eucaryotic cells was pulled down by glutathione-Sepharose 4B beads via its association with another cytoplasmic domain fragment fused to the C terminus of the glutathione S-transferase moiety. We also found that sequences encompassing the lentiviral lytic peptide-1 and lentiviral lytic peptide-2, which are located within residues 828-856 and 770-795, respectively, play a critical role in cytoplasmic domain self-assembly. Taken together, the results from the present study indicate that the cytoplasmic domain of gp41 by itself is sufficient to assemble into a multimeric structure. This finding supports the hypothesis that a multimeric form of the gp41 cytoplasmic domain plays a crucial role in virus infectivity.

The long cytoplasmic tail of gp41 is required in a cell type-dependent manner for HIV-1 envelope glycoprotein incorporation into virions

Lentiviruses, including HIV-1, have transmembrane envelope (Env) glycoproteins with cytoplasmic tails that are quite long compared with those of other retroviruses. However, mainly because of the lack of biochemical studies performed in cell types that are targets for HIV-1 infection, no clear consensus exists regarding the function of the long lentiviral Env cytoplasmic tail in virus replication. In this report, we characterize the biological and biochemical properties of an HIV-1 mutant lacking the gp41 cytoplasmic tail. We find that the gp41 cytoplasmic tail is necessary for the efficient establishment of a productive, spreading infection in the majority of T cell lines tested, peripheral blood mononuclear cells, and monocyte-derived macrophages. Biochemical studies using a high-level, transient HIV-1 expression system based on pseudotyping with the vesicular stomatitis virus glycoprotein demonstrate that in HeLa and MT-4 cells, mutant Env incorporation into virions is reduced only 3-fold relative to wild type. In contrast, gp120 levels in virions produced from a number of other T cell lines and primary macrophages are reduced more than 10-fold by the gp41 truncation. The Env incorporation defect imposed by the cytoplasmic tail truncation is not the result of increased shedding of gp120 from virions or reduced cell-surface Env expression. These results demonstrate that in the majority of T cell lines, and in primary cell types that serve as natural targets for HIV-1 infection in vivo, the gp41 cytoplasmic tail is essential for efficient Env incorporation into virions.

Concentration-dependent differential induction of necrosis or apoptosis by HIV-1 lytic peptide 1

The mechanism by which human immunodeficiency virus type 1 induces depletion of CD4+ T-lymphocytes remains controversial, but may involve cytotoxic viral proteins. Synthetic peptides (lentivirus lytic peptide type 1) corresponding to the carboxyl terminus of the human immunodeficiency virus type 1 transmembrane glycoprotein induce cytopathology at concentrations of 100 nM and above. At these concentrations lentivirus lytic peptide type 1 disrupts mitochondrial integrity of CD4+ T-lymphoblastoid cells and induces other changes characteristic of necrosis. In contrast, at concentrations of 20 nM, lentivirus lytic peptide type 1 potently induces apoptosis. Thus, the mechanism by which human immunodeficiency virus type 1 mediates cell death, necrosis or apoptosis, may depend, in part, on the tissue concentration of transmembrane glycoprotein.

Lentivirus-derived antimicrobial peptides: increased potency by sequence engineering and dimerization

We have previously described a family of cationic amphipathic peptides derived from lentivirus envelope proteins that have properties similar to those of naturally occurring antimicrobial peptides. Here, we explored the effects of amino acid truncations and substitutions on the antimicrobial potency and selectivity of the prototype peptide, LLP1. Removal of seven residues from the C-terminus of LLP1 had little effect on potency, but abrogated haemolytic activity. Replacement of the two glutamic acid residues of LLP1 with arginine resulted in a peptide with greater bactericidal activity. We discovered that the cysteine-containing peptides spontaneously formed disulphide-linked dimers, which were 16-fold more bactericidal to Staphylococcus aureus. Monomeric and dimeric LLP1 possessed similar alpha helical contents, indicating that disulphide formation did not alter the peptide's secondary structure. The dimerization strategy was applied to magainin 2, enhancing its bactericidal activity eight-fold. By optimizing all three properties of LLP1, a highly potent and selective peptide, named TL-1, was produced. This peptide is significantly more potent than LLP1 against gram-positive bacteria while maintaining high activity against gram-negative organisms and low activity against eukaryotic cells. In addition to new antimicrobial peptides, these studies contribute useful information on which further peptide engineering efforts can be based.

Site-specific deuterium order parameters and membrane-bound behavior of a peptide fragment from the intracellular domain of HIV-1 gp41

The behavior of the cytolytic peptide fragment 828-848 (P828) from the carboxy-terminus of the envelope glycoprotein gp41 of HIV-1 in membranes was investigated by solid-state 2H NMR on P828 with the selectively deuterated isoleucines I3, I13, I16, and I20. The quadrupole splittings of the I3 side chain show significant sensitivity to the main phase-transition temperature of the lipid, consistent with partial penetration of the N-terminal peptide region into the hydrophobic core of the membrane. In contrast, the quadrupole splittings of I13, I16, and I20 are in agreement with a location of the C-terminal portion of the peptide near the lipid/water interface. The perturbation of the bilayer by the peptide was studied by 2H NMR on sn-1 chain deuterated 1-stearoyl-2-oleoyl-sn-glycero-3-phosphoserine membranes. Peptide incorporation results in a significant reduction of lipid chain order toward the bilayer center, but only a modest reduction near the lipid glycerol. These observations suggest a penetration of the partially structured peptide backbone into the membrane/water interface region that reduces lateral packing density and decreases order in the hydrophobic core. In addition, the structure of the peptide was investigated free in water and bound to SDS micelles by high-resolution NMR. P828 is unstructured in water but exists in a flexible partially helical conformation when bound to negatively charged liposomes or micelles. The flexible helix covers the first 14 residues of the peptide, whereas the C-terminus of the peptide, where three of the six positively charged arginine residues are located, appears to be unstructured. The peptide-induced changes in lipid chain order profiles indicate that membrane curvature stress is the driving force for the cytolytic behavior of P828.

Bax, but not Bcl-xL, decreases the lifetime of planar phospholipid bilayer membranes at subnanomolar concentrations

Release of proteins through the outer mitochondrial membrane can be a critical step in apoptosis, and the localization of apoptosis-regulating Bcl-2 family members there suggests they control this process. We used planar phospholipid membranes to test the effect of full-length Bax and Bcl-xL synthesized in vitro and native Bax purified from bovine thymocytes. Instead of forming pores with reproducible conductance levels expected for ionic channels, Bax, but not Bcl-xL, created arbitrary and continuously variable changes in membrane permeability and decreased the stability of the membrane, regardless of whether the source of the protein was synthetic or native. This breakdown of the membrane permeability barrier and destabilization of the bilayer was quantified by using membrane lifetime measurements. Bax decreased membrane lifetime in a voltage- and concentration-dependent manner. Bcl-xL did not protect against Bax-induced membrane destabilization, supporting the idea that these two proteins function independently. Corresponding to a physical theory for lipidic pore formation, Bax potently diminished the linear tension of the membrane (i.e., the energy required to form the edge of a new pore). We suggest that Bax acts directly by destabilizing the lipid bilayer structure of the outer mitochondrial membrane, promoting the formation of a pore-the apoptotic pore-large enough to allow mitochondrial proteins such as cytochrome c to be released into the cytosol. Bax could then enter and permeabilize the inner mitochondrial membrane through the same hole.

Role of lentiviral lytic polypeptide I (LLP-I) in the selective cytotoxicity of gamma-glutamylcysteine ethyl ester against human immunodeficiency virus type 1-producing cells

Selective cytotoxic effects of gamma-glutamylcysteine ethyl ester (gamma-GCE) against human immunodeficiency virus type 1 (HIV-1)-infected H-9 T lymphocytic cells were demonstrated previously. However, the mechanism of those effects remained unclear. Here, we report on enhanced cytotoxicity of the lentiviral lytic peptide I (LLP-I) of gp41, the envelope transmembrane glycoprotein of HIV-1, in the presence of gamma-GCE. Without gamma-GCE, the cytotoxic effect of LLP-I was transient, whereas with gamma-GCE, cell death induced by LLP-I remained continuous until termination. Of note, such effects by gamma-GCE were also observed with another unrelated amphipathic peptide toxin, melittin. These results suggest that the synergistic cytotoxic effect of gamma-GCE and LLP-I may play a central role in the molecular mechanism of the selective cytotoxicity of gamma-GCE in HIV-1-infected T lymphocytic cells.

Regulation of virus release by the macrophage-tropic human immunodeficiency virus type 1 AD8 isolate is redundant and can be controlled by either Vpu or Env

The human immunodeficiency virus type 1 (HIV-1) Vpu and Env proteins are expressed from a bicistronic mRNA. To address the biological significance of the coordinated expression of vpu and env, we compared the relative effects on particle release of HIV-1 isolates containing an intact vpu gene or carrying point mutations in its initiation codon or internal deletions, respectively. We found that the primary AD8 isolate, which is unable to express vpu due to a mutation in its translation initiation codon, was able to replicate in primary macrophages and peripheral blood mononuclear cells with efficiency similar to that of an isogenic variant expressing Vpu. Interestingly, AD8 lacking a vpu initiation codon produced higher levels of Env protein than its Vpu-expressing isogenic variant. In contrast, disabling Vpu without removing the vpu initiation codon did not alter Env expression but significantly reduced virus production. AD8 Env when provided in trans was capable of enhancing release not only of AD8 particles but also of viruses of the T-cell-tropic NL4-3 isolate. We conclude that AD8 Env encodes a Vpu-like activity similar to that previously reported for HIV-2 Env proteins and is thus able to augment virus secretion. When expressed at elevated levels, i.e., following mutation of the vpu initiation codon, AD8 Env was able to compensate for the lack of Vpu and thereby ensure efficient virus release. Thus, the ability to regulate virus release is redundant in AD8 and can be controlled by either Vpu or Env. Since Vpu controls several independent functions, including CD4 degradation, our results suggest that some HIV-1 isolates may have evolved a mechanism to regulate Vpu activity without compromising their ability to efficiently replicate in the host cells.

Impairment of excitatory amino acid transport in astroglial cells infected with the human immunodeficiency virus type 1

Perturbation of astrocyte functions by HIV-1 infection may contribute to the pathogenesis of AIDS dementia complex (ADC). The present study investigated the possibility that astroglial transport of glutamate and aspartate, the major excitatory amino acids (EAAs) in the mammalian central nervous system (CNS), is altered by HIV-1 infection. Human U251 glioma cells were infected with the brain isolate SF162 of HIV-1. HIV-1 persisted in glial cells over several months. This nonproductive infection of glial cells was characterized by persistent expression of Nef over the time of the infection, and the transient presence of structural viral proteins, including the viral transmembrane glycoprotein gp41, which was detected during the initial 2 weeks following HIV-1 infection. The presence of gp41 in acutely HIV-1-infected glial cells coincided with a 36% decrease in D-[3H]aspartate uptake, owing to a reduction in the maximal transport capacity (vmax) for D-aspartate. The expression of typical astrocytic glutamate transporters EAAT1 and EAAT2 in U251 glioma cells was not altered by HIV-1 infection. To determine whether viral protein gp120, gp41, or Nef was involved in the impairment of EAA transport in acutely HIV-1-infected glial cells, effects of lentiviral lytic peptide type 1 (LLP-1) (corresponding to the carboxy terminus of gp41), recombinant SF2 gp120, and recombinant LAI Nef on D-[3H]aspartate uptake and the release of glutamate in glial cells were investigated. Only LLP-1 reduced D-[3H]aspartate uptake and facilitated the release of glutamate from glial cells in a concentration-dependent manner. These results suggest that the carboxy terminus of gp41 impairs EAA transport in glial cells, which may contribute to excitotoxic damage to neurons in HIV-1 infection of the CNS.

Role of potassium in human immunodeficiency virus production and cytopathic effects

Acute infection of CD4+ lymphoid cells by human immunodeficiency virus type 1 (HIV-1) induces an increase in the intracellular concentration of potassium (K+). Media containing reduced or elevated concentrations of K+ were used to investigate the role of this ion in HIV-1 production and cytopathology. Incubation of CD4+ lymphoblastoid cells acutely infected by HIV-1 (strain LAI) in low K+ medium resulted in an approximately 50% decrease in HIV-1 production and markedly diminished HIV-1 induced cytopathic effects (CPE) relative to cells incubated in medium containing a normal K+ concentration (approximately 5 mM). Incubation of HIV-1 infected cells in media containing elevated concentrations of K+ medium. Cells mM) increased HIV-1 production by two- to fivefold over the amount produced in cells incubated in normal K+ medium. Cells incubated in high K+ media also displayed enhanced HIV-1-induced cytopathology. The decrease in HIV-1 production by low K+ medium and increase by high K+ media could be a accounted for by effects on HIV-1 reverse transcription. However, low K+ medium inhibited HIV-1 protein synthesis and high K+ media increased HIV-1 protein synthesis. These results suggest that the HIV-1-induced increase in intracellular is required for efficient viral replication and to induce cytopathology.

Interaction of a basic amphipathic peptide from the carboxyterminal part of the HIV envelope protein gp41 with negatively charged lipid surfaces

The interaction of the positively charged synthetic amphipathic peptide fragment gp41(828) corresponding to a segment from the carboxyterminal region of the HIV envelope glycoprotein gp41 with lipid monolayers spread at the air-water interface has been studied by film balance measurements. The peptide itself does not form a stable monolayer but interacts with phospholipids spread together on the aqueous surface. Upon compression of a mixed phosphatidylcholine-peptide monolayer the peptide is irreversibly squeezed out of the lipid-peptide monolayer. In contrast, with negatively charged phosphatidylglycerol stable lipid-peptide monolayers are formed even in the presence of up to 30 mol% peptide. The monolayer may be expanded and compressed repeatedly without significant loss of substance. After addition of calcium ions to the subphase of a phosphatidylglycerol-peptide monolayer the peptide is to some extent excluded from the monolayer. In contrast to phosphatidylcholine monolayers this process is partly reversible and the excluded material is reincorporated into the film during subsequent expansion. We conclude that attached to the headgroups of the lipid monolayer a peptide-layer stabilized at the surface by electrostatic interactions is formed. The surface action may lead to rigidified lipid-peptide domains causing an increased membrane permeability which might correspond to a cytopathologic function of the protein fragment.

The antiviral activity of a synthetic peptide derived from the envelope SU glycoprotein of feline immunodeficiency virus maps in correspondence of an amphipathic helical segment

In a previous paper (Lombardi et al., Virology 220, 274-284, 1996), we-reported that a 20-amino acid synthetic peptide derived from a conserved region of the SU glycoprotein of feline immunodeficiency virus (FIV), i.e., 225EGPTLGNWAREIWATLFKKA244, bound the surface of FIV-permissive cells and inhibited FIV infection of CrFK and lymphoid cells. In this paper, we report, by the use of N- and C-terminus deleted synthetic analogs and by glycine scanning experiments that the minimal sequence needed for the full antiviral activity of the peptide maps in correspondence of amino acids 229LGNWAREIWATL240 and that either tryptophans residues at sequence position 232 or 237 are essential for such activity. Circular dichroism (CD) studies indicate that in the presence of a hydrophobic environment the 225E-A244 peptide adopts a structure containing an amphipathic alpha-helical segment of approximately 7 residues, corresponding to 2 helical turns, likely in correspondence of the sequence 231(N)WAREIW(A)238. Such a helical segment of FIV SU glycoprotein may play a role in viral envelope fusion role with the host cell membrane, thus proving critical for cell infection.

Novel inhibitory effects of gamma-glutamylcysteine ethyl ester against human immunodeficiency virus type 1 production and propagation

The anti-human immunodeficiency virus type I (anti-HIV-1) effects of gamma-glutamylcysteine ethyl ester (gamma-GCE; TEI-2306) were examined in vitro. In initial studies using a vigorously HIV-1-producing human T-lymphocytic cell line, gamma-GCE displayed a novel biphasic repressive effect on chronic HIV-1 infection that was unlike that of other glutathione prodrugs or other reported antioxidants. In high doses, up to a concentration of 2.5 mM, at which neither glutathione (GSH) nor another GSH precursor has shown inhibitory effects, gamma-GCE potently inhibited the production of HIV-1 by a selective cytopathic effect against infected cells, while the viability and growth of uninfected cells were unaffected at the same gamma-GCE concentrations. At lower concentrations (200 to 400 microM), gamma-GCE significantly repressed the virus production from chronically HIV-1-expressing cells without affecting their viability. The discrepancy of the thresholds of the toxic doses between infected and uninfected cells was found to be more than 10-fold. Relatively high doses of gamma-GCE, utilized in acute HIV-1 infection of T-lymphocytic cells, entirely blocked the propagation of HIV-1 and rescued the cells from HIV-1-induced cell death. Furthermore, gamma-GCE at such concentrations was found to directly inhibit the infectivity of HIV-1 within 4 h. Repressive effects of gamma-GCE on acute HIV-1 infection in human primary human peripheral blood mononuclear cells were also demonstrated. Here, the anti-HIV-1 strategy utilizing gamma-GCE is removal of both HIV-1-producing cells and free infectious HIV-1 in vitro, in place of specific immunoclearance in vivo, which might lead to an arrest or slowing of viral propagation in HIV-1-infected individuals.

Apoptotic killing of CD4+ T lymphocytes in HIV-1-infected PHA-stimulated PBL cultures is mediated by CD8+ LAK cells

In vitro infection of PHA-stimulated, normal CD4+ human peripheral blood T lymphocytes (PBLs) with several HIV-1 isolates did not result in cytopathology, despite high levels of virus replication and the fact that some of these isolates were cytopathic in certain cell lines. In contrast, infection of unfractionated PBLs (containing CD8+ as well as CD4+ lymphocytes) with these isolates always resulted in death of the infected CD4+ T lymphocytes. It has been well documented that PHA stimulation and culture of PBLs in medium containing IL-2 generates lymphokine-activated killer (LAK) cell activity which can destroy many transformed cells and virus-infected normal cells. When CD8+ T lymphocytes from PHA-stimulated PBLs were added to HIV-1-infected purified CD4+ T lymphocytes, significant lysis occurred. This cytotoxicity was not MHC class I-restricted, and depletion of CD8+ T lymphocytes from unfractionated PBL cultures shortly after HIV infection largely abolished the killing of the infected CD4+ T lymphocytes. These results demonstrated that CD8+ LAK cells were killing the CD4+ T lymphocytes in unfractionated PBL cultures infected with these noncytopathic HIV-1 strains. Care is thus warranted when studying HIV cytopathology in unfractionated PBL cultures. Morphological and DNA gel electrophoretic analyses of HIV-infected CD4+ T lymphocytes being killed by CD8+ LAK cells demonstrated that apoptosis was the predominant mechanism of LAK cell-mediated killing. In contrast, necrosis was the major mechanism involved in killing of purified CD4+ T lymphocytes by HIV-1 strains which were directly cytopathic. These findings may explain some of the discrepancies in the literature concerning reports of either apoptotic or necrotic killing of cells by HIV in vitro. Moreover, these data strongly suggest that direct killing by replicating HIV-1 in vivo should reveal necrotic cells and immune effector cell killing should reveal apoptotic cells. Since the latter are much more frequently observed in vivo, perhaps immune effector-mediated depletion of CD4+ T lymphocytes is more important as a pathogenic mechanism.

Interruption of T-cell signal transduction by lentivirus lytic peptides from HIV-1 transmembrane protein

Two peptide segments designated LLP1 (residues 828-855) and LLP2 (residues 768-788) of the HIV-1 transmembrane (TM) envelope protein display structural and functional properties of calmodulin (CaM) binding. These LLP segments may contribute to cytopathogenesis by binding cellular CaM and inhibiting normal CaM-regulated signal transduction pathways. To determine whether these peptides could interrupt signal transduction in vivo, a cellular assay which uses a reporter gene linked to the nuclear factor of activated T cells (NF-AT) was used. Signal transduction perturbation was tested by exogenous addition of LLPs, W-7 or ionomycin; the LLPs inhibited NF-AT-mediated signal transduction as measured by reduced reporter activity. The LLP inhibition profile of NF-AT-driven luciferase activity was similar to the CaM inhibitor W-7. This was in direct contrast to ionomycin, a mobile calcium ion carrier which caused a significant increase in luciferase activity. These findings are consistent with the hypothesis that the CaM-binding properties of TM may contribute to defects in signal transduction leading to the T-cell anergy observed in patients infected with HIV-1.

A synthetic peptide corresponding to the carboxy terminus of human immunodeficiency virus type 1 transmembrane glycoprotein induces alterations in the ionic permeability of Xenopus laevis oocytes

The carboxy-terminal 29 amino acids of the human immunodeficiency virus type 1 transmembrane glycoprotein (HIV-1 TM) are referred to as lentivirus lytic peptide 1 (LLP-1). Synthetic peptides corresponding to LLP-1 have been shown to induce cytolysis and to alter the permeability of cultured cells to various small molecules. To address the mechanisms by which LLP-1 induces cytolysis and membrane permeability changes, various concentrations of LLP-1 were incubated with Xenopus laevis oocytes, and two-electrode, voltage-clamp recording measurements were performed. LLP-1 at concentrations of 75 nM and above induced dramatic alterations in the resting membrane potential and ionic permeability of Xenopus oocytes. These concentrations of LLP-1 appeared to induce a major disruption of plasma membrane electrophysiological integrity. In contrast, concentrations of LLP-1 of 20-50 nM induced changes in membrane ionic permeability that mimic changes induced by compounds, such as the bee venom peptide melittin, that are known to form channel-like structures in biological membranes at sublytic concentrations. An analog of LLP-1 with greatly reduced cytolytic activity failed to alter the electrophysiological properties of Xenopus oocytes. Thus, by altering plasma membrane ionic permeability, the carboxy terminus of TM may contribute to cytolysis of HIV-1-infected CD4+ cells.

Novel antimicrobial peptides derived from human immunodeficiency virus type 1 and other lentivirus transmembrane proteins

We have previously described a conserved set of peptides derived from lentiviral envelope transmembrane proteins that are similar to the natural antimicrobial peptides cecropins and magainins in overall structure but bear no sequence homology to them or other members of their class. We describe here an evaluation of the antimicrobial properties of these virally derived peptides, designated lentivirus lytic peptides (LLPs). The results of this study demonstrate that they are potent and selective antibacterial peptides: the prototype sequence, LLP1, is bactericidal to both gram-positive and gram-negative organisms at micromolar concentrations in 10 mM phosphate buffer. Furthermore, LLP1 kills bacteria quite rapidly, causing a 1,000-fold reduction in viable organisms within 50 s. Peptides corresponding to sequences from three lentivirus envelope proteins were synthesized and characterized. Several of these peptides are selective, killing bacteria at concentrations 50- to 100-fold lower than those required to lyse erythrocytes. Development of antimicrobial agents based on these peptides may lead to improved therapeutics for the management of a variety of infectious diseases.

Budding of enveloped viruses from the plasma membrane

Many enveloped viruses are released from infected cells by maturing and budding at the plasma membrane. During this process, viral core components are incorporated into membrane vesicles that contain viral transmembrane proteins, termed 'spike' proteins. For many years these spike proteins, which are required for infectivity, were believed to be incorporated into virions via a direct interaction between their cytoplasmic domains and viral core components. More recent evidence shows that, while such direct interactions drive budding of alphaviruses, this may not be the case for negative strand RNA viruses and retroviruses. These viruses can bud particles in the absence of spike proteins, using only viral core components to drive the process. In some cases the spike proteins, without the viral core, can be released as virus-like particles. Optimal budding and release may, therefore, depend on a 'push-and-pull' concerted action of core and spike, where oligomerization of both components plays a crucial role.

HIV/SIV glycoproteins: structure-function relationships

The various functions of human (HIV) and simian (SIV) immunodeficiency virus glycoproteins are similar, so it may be assumed that the overall structure of the folded proteins will be maintained. To preserve structure there must be constraints on sequence variation. The majority of mutations tolerated will be involved in immune escape but changes at some positions are known to have direct effects on glycoprotein expression and function. This allows the virus to change its phenotype and escape immune pressure. These properties will influence the fitness of the virus to infect and replicate in potential hosts. A better understanding of the structure-function relationships of HIV/SIV glycoproteins will assist in the development of vaccines and antivirals. Here, we identify similarities and differences between HIV-1 subtypes and HIV/SIV types that may be relevant to the phenotypes of the various groups. The results are discussed in relation to what is known of domain-function associations for HIV/SIV glycoproteins.

The cytopathicity of a simian immunodeficiency virus Mne variant is determined by mutations in Gag and Env

Previous studies suggested that the rapidly replicating, highly cytopathic, syncytium-inducing (rapid-high/SI) phenotype of simian immunodeficiency virus Mne variants that evolved in macaques inoculated with a slowly replicating, minimally cytopathic, non-syncytium-inducing (slow-low/NSI) molecular clone was not solely the result of changes in the envelope surface protein (Env SU). To define the viral determinants responsible for the change in phenotype, we molecularly cloned a rapid-high/SI variant (designated SIVMne170) derived from the peripheral blood mononuclear cells (PBMCs) of a pig-tailed macaque that was inoculated with a slow-low/NSI molecular clone, SIVMneCL8. SIVMne170 was SI and replicated with faster kinetics and was more cytopathic than the parent SIVMneCL8 in CEMx174 cells. Additionally, SIVMne170 was more cytopathic for the CD4+ T-cell population than SIVMneCL8 in macaque PBMCs. An analysis of chimeric viruses constructed between the variant SIVMne170 and the parent virus SIVMneCL8 demonstrated that there are determinants encoded within both the 5' and 3' halves of SIVMne170 that independently contribute to its rapid-high/SI phenotype. As we previously observed with other SIVMne variants, the Env SU of SIVMne170 was important for syncytium induction but was not a key determinant of cytopathicity. By contrast, the intracellular domain of the envelope transmembrane protein (Env TM) contributed to both the SI and cytopathic properties of SIVMne170. We also found that the minimal determinant within the 5' half of SIVMne170 that conferred its rapid replication kinetics and cytopathicity mapped to the capsid- and nucleocapsid-encoding regions of gag. Together, these data demonstrate that mutations selected in Gag and Env TM intracytoplasmic tail influence the replication and cytopathicity of SIVMne variants that evolve in the host.

Morphological changes induced by phospholipase C and by sphingomyelinase on large unilamellar vesicles: a cryo-transmission electron microscopy study of liposome fusion

Cryo-transmission electron microscopy has been applied to the study of the changes induced by phospholipase C on large unilamellar vesicles containing phosphatidylcholine, as well as to the action of sphingomyelinase on vesicles containing sphingomyelin. In both cases vesicle aggregation occurs as the earliest detectable phenomenon; later, each system behaves differently. Phospholipase C induces vesicle fusion through an intermediate consisting of aggregated and closely packed vesicles (the "honeycomb structure") that finally transforms into large spherical vesicles. The same honeycomb structure is also observed in the absence of enzyme when diacylglycerols are mixed with the other lipids in organic solution, before hydration. In this case the sample then evolves toward a cubic phase. The fact that the same honeycomb intermediate can lead to vesicle fusion (with enzyme-generated diacylglycerol) or to a cubic phase (when diacylglycerol is premixed with the lipids) is taken in support of the hypothesis according to which a highly curved lipid structure ("stalk") would act as a structural intermediate in membrane fusion. Sphingomyelinase produces complete leakage of vesicle aqueous contents and an increase in size of about one-third of the vesicles. A mechanism of vesicle opening and reassembling is proposed in this case.

A leucine zipper-like sequence from the cytoplasmic tail of the HIV-1 envelope glycoprotein binds and perturbs lipid bilayers

HIV-1 transmembrane envelope glycoprotein (gp41) has an unusually long cytoplasmic domain that has secondary associations with the inner leaflet of the membrane. Two highly amphiphatic alpha-helices in the cytoplasmic domain of gp41 have previously been shown to interact with lipid bilayers. We have detected a highly conserved leucine zipper-like sequence between the two alpha-helices. A peptide corresponding to this segment (residues 789-815, LLP-3) aggregates in aqueous solution, but spontaneously inserts into phospholipid membranes and dissociates into alpha-helical monomers. The peptide perturbs the bilayer structure resulting in the formation of micelles and other non-bilayer structures. Tryptophan fluorescence quenching experiments using brominated phospholipids revealed that the peptide penetrates deeply into the hydrophobic milieu of the membrane bilayer. The peptide interacts equally with zwitterionic and negatively-charged phospholipid membranes and is protected from proteolytic digestion in its membrane-bound state. Polarized attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy showed that the LLP-3 alpha-helix axis is about 70 degrees from the normal to the membrane plane. The ATR-FTIR CH2-stretching dichroic ratio increases when the peptide is incorporated into pure phospholipid membranes, further indicating that the peptide can deeply penetrate and perturb the bilayer structure. Integrating these data with what is already known about the membrane-associating features of adjacent segments, we propose a revised structural model in which a large portion of the cytoplasmic tail of the HIV-1 envelope glycoprotein is associated with the membrane.

Inner but not outer membrane leaflets control the transition from glycosylphosphatidylinositol-anchored influenza hemagglutinin-induced hemifusion to full fusion

Cells that express wild-type influenza hemagglutinin (HA) fully fuse to RBCs, while cells that express the HA-ectodomain anchored to membranes by glycosylphosphatidylinositol, rather than by a transmembrane domain, only hemifuse to RBCs. Amphipaths were inserted into inner and outer membrane leaflets to determine the contribution of each leaflet in the transition from hemifusion to fusion. When inserted into outer leaflets, amphipaths did not promote the transition, independent of whether the agent induces monolayers to bend outward (conferring positive spontaneous monolayer curvature) or inward (negative curvature). In contrast, when incorporated into inner leaflets, positive curvature agents led to full fusion. This suggests that fusion is completed when a lipidic fusion pore with net positive curvature is formed by the inner leaflets that compose a hemifusion diaphragm. Suboptimal fusion conditions were established for RBCs bound to cells expressing wild-type HA so that lipid but not aqueous dye spread was observed. While this is the same pattern of dye spread as in stable hemifusion, for this "stunted" fusion, lower concentrations of amphipaths in inner leaflets were required to promote transfer of aqueous dyes. Also, these amphipaths induced larger pores for stunted fusion than they generated within a stable hemifusion diaphragm. Therefore, spontaneous curvature of inner leaflets can affect formation and enlargement of fusion pores induced by HA. We propose that after the HA-ectodomain induces hemifusion, the transmembrane domain causes pore formation by conferring positive spontaneous curvature to leaflets of the hemifusion diaphragm.

Calmodulin-binding function of LLP segments from the HIV type 1 transmembrane protein is conserved among natural sequence variants

LLP1 is a peptide, derived from the cytoplasmic tail of HIV-1 TM glycoprotein, that binds and inhibits calmodulin; this region is generally conserved among isolates, but amino acid variation does exist both within clade B and among different clades, as well as SIV. In light of previous studies showing that selected single amino acid changes can have a qualitatively significant effect on the calmodulin-binding properties of this peptide, we sought to examine the properties of naturally occurring variant LLP1 sequences. Using a quantitative fluorescence-based method to measure dissociation constants of calmodulin-LLP1 complexes, a remarkable conservation of calmodulin-binding function among natural variants was revealed. In contrast, engineered nonconservative single amino acid changes altered the affinity of the peptide for calmodulin. The results show that the calmodulin-binding function is well preserved despite the sequence variation observed in nature, suggesting that this region of the TM protein is important to viral replication.

Amphipathic domains in the C terminus of the transmembrane protein (gp41) permeabilize HIV-1 virions: a molecular mechanism underlying natural endogenous reverse transcription

Reverse transcription of HIV-1, without detergent or amphipathic peptide-induced permeability of the viral envelope, has been demonstrated to occur in the intact HIV-1 virion. In this report, we demonstrate that the amphipathic domains in the C terminus of the transmembrane glycoprotein (gp41) account for the natural permeability of the HIV-1 envelope to deoxyribonucleoside triphosphates, the substrates for DNA polymerization. In addition, nonphysiological deoxyribonucleoside triphosphates, such as 3'-azido-3'-deoxythymidine 5'-triphosphate and 3'-deoxythymidine 5'-triphosphate, can also penetrate the viral envelope, incorporate into, and irreversibly terminate reverse transcripts. As a result, viral infectivity is potently inhibited. Since the lentiviral envelope with these newly demonstrated characteristics can serve as a delivery pathway for anti-reverse transcription agents, we propose a unique strategy to prevent HIV-1 interand, possibly, intrahost transmission.

Membrane glycolipids and human immunodeficiency virus infection of primary macrophages

The membrane glycolipids galactosylceramide (GalCer) and sulfatide (SGalCer) have been reported to act as receptors of human immunodeficiency virus (HIV) on CD4- cell lines. We show here that these glycolipids are present on CD4+ cells purified from human blood and on in vitro-differentiated monocyte-derived macrophages (MDMs). We investigated the role they could play in HIV infection. Glycolipids of MDMs were characterized at the molecular level by immunolabeling and thin-layer chromatography immune overlay, using a panel of human-, rabbit-, or murine-specific antibodies. GalCer and SGalCer were expressed at the surface of MDMs as assessed by indirect immunofluorescence and flow cytometry analysis, and they could be characterized with specific antibodies in the cellular glycolipid extracts in addition to GM1, GM3, and GD1b gangliosides. Recombinant 125-I-labeled gp160 specifically bound to GalCer, SGalCer, GM1, and GM3 as well as to phospholipids (phosphatidylethanolamine and phosphatidylserine) from MDM extracts. Anti-SGalCer monoclonal antibodies (MAbs), but not anti-GalCer antibodies, entailed limited (30-40%) but significant inhibition of gp160 binding to MDMs. However, the four human anti-SGalCer MAbs and the three murine or rabbit ant-GalCer antibodies tested did not inhibit HIV infection of MDMs, in contrast to CD4 antibody anti-Leu3a tested in parallel. These findings suggests that although HIV envelope glycoprotein can bind to SGalCer and GalCer from CD4+ MDM extracts, these glycolipids do not apparently act as HIV coreceptors nor are they involved in HIV infection of these cells.

Endogenous reverse transcription of human immunodeficiency virus type 1 in physiological microenviroments: an important stage for viral infection of nondividing cells

Endogenous reverse transcription (ERT) of retroviruses has long been considered a somewhat artificial process which only mimics reverse transcription occurring in target cells, as detergents or amphipathic peptides have classically been used to make the envelopes of retroviruses in these reaction systems permeable. Recently, several studies suggested that ERT of human immunodeficiency virus type 1 (HIV-1) might occur without detergent treatment. However, this phenomenon could be due to damage of the retroviral envelope during the process of virion purification or freezing and thawing. In this report, intravirion HIV-1 ERT, without detergent-induced permeabilization, is demonstrated to occur in the natural microenvironments of HIV-1 virions and is not caused by artificial processes. Therefore, this stage of the viral life cycle was termed natural ERT (NERT). The efficiency of NERT in HIV-1 virions was markedly augmented by several physiological substances in the extracellular milieu, such as polyamines and deoxyribonucleoside triphosphates. In addition, HIV-1 virions in seminal plasma samples harbored dramatically higher levels of full-length or nearly full-length reverse transcripts than virions isolated from peripheral blood plasma samples of HIV-1-seropositive men. When HIV-1 virions were incubated with seminal plasma samples, infectivity in initially nondividing cells was also significantly enhanced. Thus, we suggest that HIV-1 virions are actively altered by the extracellular microenvironment and that NERT may play an important role in viral infection of nondividing cells.

Identification of a novel splice acceptor in the HIV-1 genome: independent expression of the cytoplasmic tail of the envelope protein

Multiple splicing sites exist in the RNA genome of the human immunodeficiency virus type 1 (HIV-1). In a screen for subgenomic forms of the HIV-1 genome that could be transferred to fresh cells by virus infection, we identified a novel spliced variant of HIV-1 RNA that uses a hitherto unknown splice acceptor site within the envelope (Env) gene. We demonstrate that this splice acceptor is infrequently used in HIV-infected T cells. Interestingly, an AUG initiator codon is created at this splice junction which has the potential to direct the synthesis of the cytoplasmic tail of the Env gp41 protein. Transient transfection experiments with the new cDNA cloned in an expression vector demonstrated efficient utilization of this start codon and the C-terminus of the Env open reading frame. Independent expression of the 152 amino acid long, intracellular Env domain provides novel regulatory mechanisms for modulating viral infectivity and perhaps pathogenicity.

Human immunodeficiency virus type 2 glycoprotein enhancement of particle budding: role of the cytoplasmic domain

Previous studies have shown that the glycoprotein cytoplasmic domains of human immunodeficiency virus type 2 (HIV-2) or simian immunodeficiency virus of macaques modulate biological activities of the viral glycoprotein complex, including syncytium formation, exterior glycoprotein conformation, and glycoprotein incorporation into budding virus particles. We have now utilized a recombinant expression system to study interactions of full-length or truncated HIV-2 glycoproteins with coexpressed HIV-2 Gag proteins which self-assemble and bud as virus-like particles. Interestingly, budding of HIV-2 virus-like particles from cells was enhanced 5- to 24-fold when Gag was coexpressed with the full-length HIV-2 glycoprotein, compared with Gag expressed either alone or with a truncated HIV-2 glycoprotein. The results obtained in this model system indicate that an additional effect of the lengthy cytoplasmic domain of the glycoprotein of HIV-2 is enhancement of particle budding. We speculate that the cytoplasmic domain of the viral glycoprotein of HIV-2 enhances budding by (i) potentiation of Gag structure or function or (ii) membrane modulation.

Molecular determinants of acute single-cell lysis by human immunodeficiency virus type 1

Human immunodeficiency virus type 1 (HIV-1) infection of CD4-positive lymphocytes is accompanied by acute cytopathic effects, i.e., syncytium formation and single-cell lysis. Syncytium formation involves cell-cell fusion mediated by viral envelope glycoproteins on the surface of infected cells and by CD4 glycoproteins on adjacent cells. The molecular basis for the lysis of single-HIV-1 infected cells is unclear. Here we report that the expression of functional envelope glycoproteins from primary and laboratory-adapted HIV-1 isolates resulted in the lysis of single CD4-positive lymphocytes. As was previously observed in HIV-1 infected cultures, single-cell lysis in this system primarily involved necrosis and was not inhibited by soluble CD4. Binding of the viral envelope glycoproteins to the CD4 glycoprotein facilitated, but was not sufficient for, cytolysis. Importantly, the ability of the HIV-1 envelope glycoproteins to mediate membrane fusion was essential for single-cell killing. By contrast, the long cytoplasmic tail of the gp41 transmembrane envelope glycoprotein was neither necessary nor sufficient for single-cell lysis. These results suggest that intracellular envelope glycoprotein-CD4 interactions initiate autofusion events that disrupt cell membrane integrity, leading to single-cell lysis by HIV-1.

Anti-idiotypic antibody to the V3 domain of gp120 binds to vimentin: a possible role of intermediate filaments in the early steps of HIV-1 infection cycle

Although the CD4 molecule is the major cellular receptor for human immunodeficiency virus (HIV), several lines of evidence suggest participation of additional molecules that are engaged after the binding of HIV to the CD4 receptor and that may facilitate viral entry into the target cell. Some of the post-CD4 binding, perfusion events involve the third hypervariable region (V3 loop) of the viral envelope protein gp120. To identify cellular proteins that interact with the V3 loop, we chose as a probe an antiidiotypic monoclonal antibody (MAb), anti-id2, which was prepared against the neutralizing MAb 110.4 that binds the V3 domain in the envelope glycoprotein gp120 of the LAI isolate of HIV-1. Anti-id2 reacted specifically with a 55- to 60-kDa protein in human T cell and monocytoid cell lines, and in a mouse melanoma cell line. This protein was identified immunologically and by protein sequence analysis as vimentin, an intermediate filament protein of lymphoid and other cells of mesodermal origin. Antiserum raised against vimentin inhibited nuclear translocation of HIV-1 DNA following infection of monocytes and CD4+ T cells with live virus, and reduced the amount of HIV-1 gag-specific RNA in the nuclei of monocytes following inoculation with HIV-1 pseudovirions. These data suggest that vimentin may participate in the early steps of HIV-1 replication, perhaps during the uptake of HIV-1 preintegration complexes into the nuclear compartment.

Effect of amino acid substitutions on calmodulin binding and cytolytic properties of the LLP-1 peptide segment of human immunodeficiency virus type 1 transmembrane protein

Previous studies have identified two highly basic amphipathic helical regions in the human immunodeficiency virus type 1 transmembrane protein that, in vitro, display both cytolytic and calmodulin-binding and -inhibitory properties that could contribute to cellular dysfunctions and cytopathogenesis during a persistent viral infection. In the current study, the structural specificity of the cytolytic and calmodulin-binding activities of the human immunodeficiency virus type 1 lentivirus lytic peptide (LLP-1) are examined with synthetic peptide homologs and analogs. The results of these studies demonstrate that even minor changes in LLP-1 amino acid content can markedly affect these properties, suggesting that sequence variation in these highly conserved LLP sequences may correlate with alterations in viral cytopathic properties.

Molecular mimicry in channel-protein structure

The approach to probing the sequence-structure relationship of ion-channel proteins using small peptides stems from the abundance of sequence information and the virtual absence of structures at atomic resolution. It is anticipated that model peptides may fold predictably into stable structures and reproduce functional properties of specific proteins. Model peptides are well suited to the application of NMR methods to determine protein structure in a membrane environment or to high-resolution X-ray diffraction analysis. It is timely to ask what we have learned through this strategy and where it may lead in our quest to understand the sequence-structure determinism.