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.