No abstract

This paper presents a study on the membrane fusion activity of a 23-residue synthetic peptide, representing the N-terminus of gp41 of the human immunodeficiency virus type I (HIV-1; LAV1a strain), in a model system involving large unilamellar vesicles (LUV) composed of the negatively charged 1-palmitoyl-2-oleoylphosphatidylglycerol (POPG). The peptide (HIVarg) induced fusion of POPG LUV as evidenced by (i) mixing of membrane lipids, (ii) mixing of aqueous vesicle contents, and (iii) an irreversible increase in vesicle size. Fusion could be induced only in the presence of millimolar concentrations of Ca2+ or Mg2+, needed for induction of vesicle aggregation; the divalent cations by themselves did not induce any fusion. The rate constant of the fusion reaction, as determined by simulation of the process according to a kinetic model, increased dramatically with the peptide-to-lipid molar ratio, indicating that the peptide was the mediator of the process. In the absence of divalent cations, the HIVarg peptide induced leakage of small molecules due to formation of pores in the membrane of single vesicles. Final extents and kinetics of this leakage process could be simulated adequately by model calculations for peptide-to-lipid ratios ranging from 1:25 to 1:750. Experiments, in which the order of peptide and Ca2+ addition to the vesicles was varied, indicated that the peptide is likely to adopt two different structures, one in the absence of Ca2+, primarily supporting leakage by formation of pores in separate vesicles, and one in the presence of Ca2+, primarily supporting fusion. Once a final structure had been established, it persisted even upon addition or removal of Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

We have identified a region within the ectodomain of the fusogenic human immunodeficiency virus type 1 (HIV-1) gp41, different from the fusion peptide, that interacts strongly with membranes. This conserved sequence, which immediately precedes the transmembrane anchor, is not highly hydrophobic according to the Kyte-Doolittle hydropathy prediction algorithm, yet it shows a high tendency to partition into the membrane interface, as revealed by the Wimley-White interfacial hydrophobicity scale. We have investigated here the membrane effects induced by NH 2 -DKWASLWNWFNITNWLWYIK-CONH 2 (HIV c ), the membrane interfacepartitioning region at the C terminus of the gp41 ectodomain, in comparison to those caused by NH 2 -AVGIGALFLGFLGAAGSTMGARS-CONH 2 (HIV n ), the fusion peptide at the N terminus of the subunit. Both HIV c and HIV n were seen to induce membrane fusion and permeabilization, although lower doses of HIV c were required for comparable effects to be detected. Experiments in which equimolar mixtures of HIV c and HIV n were used indicated that both peptides may act in a cooperative way. Peptide-membrane and peptide-peptide interactions underlying those effects were further confirmed by analyzing the changes in fluorescence of peptide Trp residues. Replacement of the first three Trp residues by Ala, known to render a defective gp41 phenotype unable to mediate both cell-cell fusion and virus entry, also abrogated the HIV c ability to induce membrane fusion or form complexes with HIV n but not its ability to associate with vesicles. Hydropathy analysis indicated that the presence of two membrane-partitioning stretches separated by a collapsible intervening sequence is a common structural motif among other viral envelope proteins. Moreover, sequences with membrane surfaceresiding residues preceding the transmembrane anchor appeared to be a common feature in viral fusion proteins of several virus families. According to our experimental results, such a feature might be related to their fusogenic function.

Enveloped animal viruses, such as Semliki Forest virus (SFV), utilize a membrane fusion strategy to deposit their genome into the cytosol of the host cell. SFV enters cells through receptor-mediated endocytosis, fusion of the viral envelope occurring subsequently from within acidic endosomes. Fusion of SFV has been demonstrated before to be strictly dependent on the presence of cholesterol in the target membrane. Here, utilizing a variety of membrane fusion assays, including an on-line fluorescence assay involving pyrene-labeled virus, we demonstrate that low-pH-induced fusion of SFV with cholesterol-containing liposomal model membranes requires the presence of sphingomyelin or other sphingolipids in the target membrane. The miinimal molecular characteristics essential for supporting SFV fusion are encompassed by a ceramide. The action of the sphingolipids is confined to the actual fusion event, cholesterol being necessary and sufficient for low-pHdependent binding of the virus to target membranes. Complex formation of the sphingolipids with cholesterol is unlikely to be important for the induction of SFV-liposome fusion, as sphingolipids that do not interact appreciably with cholesterol, such as galactosylceramide, effectively support the process. The remarkably low levels of sphingomyelin required for halfmaximal fusion (1-2 mole%) suggest that sphingolipids do not play a structural role in the SFV fusion process, but rather act as a cofactor, possibly activating the viral fusion protein in a specific manner.

The peptide HIV(arg), corresponding to a sequence of 23 amino acid residues at the N-terminus of HIV-1 gp41 (LAV1a strain), has the capacity to destabilize negatively charged large unilamellar vesicles. As revealed by infrared spectroscopy, the peptide associated with those vesicles showed conformational polymorphism: in the absence of cations the main structure was a pore-forming alpha-helix, whereas in the presence of Ca2+ the conformation switched to a fusogenic, predominantly extended beta-type structure. Here we show that an extended structure can also be involved in electrically neutral vesicle destabilization induced by the HIV-1 fusion peptide when it binds the vesicle from the aqueous phase. In the absence of cations, neutral liposomes composed of phosphatidylcholine, phosphatidylethanolamine, and cholesterol (molar ratio 1:1:1) selected for an extended structure that became fusogenic in a dose-dependent fashion. At subfusogenic doses this structure caused the release of trapped 8-aminonaphtalene-1,3,6-trisulfonic acid sodium salt/p-xylenebis(pyridinium)bromide from liposomes, indicating the existence of a peptide-mediated membrane destabilizing process before and independent of the development of fusion. When compared to HIV(arg), the fusion activity of HIV(ala) (bearing the R22 --> A substitution) was reduced by 70%. Fusogenicity was completely abolished when a second substitution (V2 --> E) was included to generate HIV(ala-E2), a sequence representing the N-terminus of an inactive gp41. However, the three sequences associated with vesicles to the same extent, and the three adopted a similar extended structure in the membrane. Whereas 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene emission anisotropy was unaffected by the three peptides, DPH emission anisotropy in membranes was increased only by the fusogenic sequences. Taken together, our observations strongly argue that it is not an alpha-helical but an extended structure adopted by the HIV-1 fusion peptide what actively destabilizes cholesterol-containing, electrically neutral membranes. Moreover, membrane destabilization is modulated by the amino acid sequence in the extended structure. The effect displayed by the aforementioned V2 --> E substitution suggests that the fusion process described here could be reflecting a physiologically relevant phenomenon.