Kinetic studies of Sendai virus-target membrane interactions: independent analysis of binding and fusion

Fusion between Sendai virus and liposomes containing phosphatidylethanolamine (PE) and different mole fractions of ganglioside GD1a has been investigated. At different times after mixing the virus and liposomes, the mixture was diluted with a sucrose solution and centrifuged in an airfuge to separate the free and virus-associated liposomes. Since the HN protein of the virus was sensitive to the reducing reagent, inclusion of dithiothreitol in the sucrose solution dissociated the bound but not the fused liposomes. Thus, the kinetics of liposome-virus binding and fusion could be independently measured. The validity of the assay was confirmed by electron microscopic observation of the virus-liposome mixtures. With trypsin-treated Sendai virus, in which the F glycoprotein of the virus had been selectively removed, only virus-liposome binding but not fusion was observed. The kinetic experiments were done under the condition of virus in large excess. Following a very fast initial binding phase, which was completed at the "zero time" of the measurement, the virus-liposome binding followed pseudo-first-order kinetics. The subsequent fusion step was zero order. Judging from the discontinuity in the Arrhenius plots, both binding and fusion events were sensitive to the gel-liquid-crystalline phase transition of the target membrane. The binding rate constants had activation energies between 16 and 23 kcal/mol at temperatures above the transition. They were not sensitive to temperature change at temperatures below the transition. On the other hand, the fusion rate constants were not sensitive to temperature change above the transition, except for 6.3% GD1a liposomes.(ABSTRACT TRUNCATED AT 250 WORDS)