Transbilayer distribution and movement of cholesterol and phospholipid in the membrane of influenza virus.

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Abstract
The transfer of radioactive cholesterol from influenza virus to excess phosphatidylcholine-cholesterol vesicles has been studied. Viral cholesterol was found to exist in two pools, one rapidly exchangeable. Evidence is presented that the rapidly exchangeable pool corresponds to cholesterol present on the outer surface of the viral bilayer, while the slowly exchangeable pool corresponds to inner surface cholesterol. Approximately equal amounts are present in each pool, suggesting that cholesterol distribution is not markedly asymmetric in the viral bilayer. A half-time for the rate of equilibration between the two sides of the bilayer (flip-flop) was about 13 days at 37 degrees with a 90% confidence interval of 3.4- infinity days. Similar experiments were carried out which followed the time course of transfer of labeled phospholipids from the viral bilayer to phospholipid vesicles, catalyzed by phospholipid exchange protein from beef heart. From these experiments the half-times for the flip-flop of phosphatidyl-choline and spingomyelin were found to be indeterminately in excess of 10 and 30 days at 37 degrees, respectively. These results suggest that exchange of the major components of the viral bilayer between the two surfaces occurs very slowly relative to the turnover times of most membrane constituents, and provide a plausible mechanism for the maintenance of membrane asymmetry over biologically relevant time periods.