INTERACTION OF THE HIV-1 FUSION PEPTIDE WITH PHOSPHOLIPID-VESICLES - DIFFERENT STRUCTURAL REQUIREMENTS FOR FUSION AND LEAKAGE

This paper presents a study on the membrane fusion activity of a 23-re sidue synthetic peptide, representing the N-terminus of gp41 of the hu man immunodeficiency virus type I (HIV-1; LAV(1a) strain), in a model system involving large unilamellar vesicles (LUV) composed of the nega tively charged 1-palmitoyl-2-oleoylphosphatidylglycerol (POPG). The pe ptide (HIVarg) induced fusion of POPG LUV as evidenced by (i) mixing o f 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 f or induction of vesicle aggregation; the divalent cations by themselve s did not induce any fusion. The rate constant of the fusion reaction, as determined by simulation of the process according to a kinetic mod el, increased dramatically with the peptide-to-lipid molar ratio, indi cating that the peptide was the mediator of the process. In the absenc e of divalent cations, the HIVarg peptide induced leakage of small mol ecules due to formation of pores in the membrane of single vesicles. F inal extents and kinetics of this leakage process could be simulated a dequately by model calculations for peptide-to-lipid ratios ranging fr om 1:25 to 1:750. Experiments, in which the order of peptide and Ca2addition to the vesicles was varied, indicated that the peptide is lik ely to adopt two different structures, one in the absence of Ca2+, pri marily supporting leakage by formation of pores in separate vesicles, and one in the presence of Ca2+, primarily supporting fusion. Once a f inal structure had been established, it persisted even upon addition o r removal of Ca2+. From the infrared spectroscopy of the peptide at eq uilibrium with POPG vesicles it is concluded that the structure formed in the absence of Ca2+, supporting leakage, represents predominantly an alpha-helix, whereas in the presence of Ca2+, i.e., under condition s supporting fusion, the peptide adopts mostly an extended antiparalle l beta-structure.