Depletion and exchange of cholesterol from the membrane of vesicular stomatitis virus by interaction with serum lipoproteins or poly(vinylpyrrolidone) complexed with bovine serum albumin

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Abstract
Cholesterol was depleted from the membrane of vesicular stomatitis virus [VSV] by exposing virion suspensions to serum lipoproteins enriched with phospholipids. Unlike the reaction of virions with phospholipid vesicles, nonspecific adherence of lipoproteins and exogenous lipids to the envelope of the virus was minimal. The extent of cholesterol depletion was dependent on the type of phospholipid complexed with interacting lipoprotein; sphingomyelin and dipalmitoyllecithin were highly effective depleters of cholesterol compared to egg phosphatidylcholine, phosphatidylethanolamine or phosphatidylserine. Similar depletion of cholesterol from the virion membrane was also observed when VSV was exposed to a complex of poly(vinylpyrrolidone) and bovine serum albumin coated with egg phosphatidylcholine or dioleoylphosphatidylcholine. Cholesterol depletion altered the morphology but not the membrane integrity of the virus. Directly correlated with depletion of cholesterol was a substantial loss in the anisotropy of the viral membrane as determined by fluorescence depolarization of the lipophilic probe 1,6-diphenyl-1,3,5-hexatriene. Interaction with poly(vinylpyrrolidone) complexed with albumin, phosphatidylcholine and cholesterol resulted in exchange of cholesterol from the virion membrane which followed biphasic kinetics with a rapid and a slow phase, indicating that 75-85% of viral membrane cholesterol is present in the outer monolayer, and 15-25% is located in the inner monolayer. Depletion of cholesterol from the virion membrane resulted in a significant drop in the infectivity of the virus as measured by plating efficiency on L-cell [mouse neoplastic fibroblast cell] monolayers. This effect was not observed when virion cholesterol was exchanged without net reduction in the concentration of viral membrane cholesterol. Part of the loss in infectivity following depletion of cholesterol could be restored by reincorporation of cholesterol in the membrane, thus demonstrating that membrane cholesterol partly contributes to the infectivity of VSV.