HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 (HIV-1) FUSION WITH MODEL MEMBRANES - KINETIC-ANALYSIS AND THE ROLE OF LIPID-COMPOSITION, PH AND DIVALENT-CATIONS

The kinetics and extent of HIV-1 fusion with model membranes was studi ed. HIV-1 was labeled with octadecyl rhodamine B chloride, and fusion was monitored continuously as the dilution of the probe into target me mbranes. The results were analyzed by a mass action model which yielde d good simulations and predictions for the kinetics and final extents of fluorescence increase. The model determined the percents of virions capable of fusing and rate constants of fusion, aggregation and disso ciation. Ultrastructural analysis of the virus and reaction products b y electron microscopy also provided evidence of HIV-1 fusion with memb ranes lacking CD4. HIV-1 fusion activity depends on the target membran e lipid composition according to the sequence: cardiolipin (CL) > > ph osphatidylinositol > CL/dioleoylphosphatidylcholine (DOPC) (3:7), phos phatidic acid > phosphatidylserine (PS), PS/cholesterol (2:1) > PS/PC (1:1), PS/phosphatidylethanolamine (1:1) > DOPC, erythrocyte ghosts. R eduction of pH from 7.5 generally enhances the rate and extent of HIV- 1 fusion. Physiologi cally relevant concentrations of calcium stimulat e HIV-1 fusion with several liposome compositions and with erythrocyte ghost membranes. The fusion products of HIV-1 with liposomes consist of a single virus and several liposomes. The mass action analysis reve aled that, compared to intact virions, the fusion products show a stri king reduction in the fusion rate constant. Like influenza and Sendai viruses, HIV-1 fusion with membranes containing its own envelope glyco protein(s) is strongly inhibited. Unlike these viruses, HIV-1 fusion i s promoted by physiological levels of calcium. HIV-1 fusion with lipos omes is qualitatively similar to simian immunodeficiency virus fusion.