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.

Cholesterol was depleted from the membrane of vesicular stomatitis virus by exposing virion suspensions to serum lipoproteins enriched with phospholipids. Unlike the reaction of virions with phospholipid vesicle, nonspecific adherence of lipoproteins and exogenous lipids to the envelope of the virus was found to be minimal. The extent of cholesterol depletion was dependent upon the type of phospholipid complexed with interacting lipoprotein; sphingomyelin and dipalmitoyllecithin were found to be highly effective depleters of cholesterol compared to egg phosphatidylcholine, phosphatidylethanolamine, or phosphatidylserine. Similar depletion of cholesterol from the virion membrane was also observed when vesicular stomatitis virus was exposed to a complex of poly(vinylpyrrolidone) and bovine serum albumin coated with egg phosphatidylcholine or dioleoylphosphatidylcholine. Cholesterol depletion was found to alter 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 following biphasic kinetics with a rapid and a slow phase; these data indicate 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 monolayers. Such an 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 vesicular stomatitis virus.