Enhancement of phospholipid transfer from Sendai virus to erythrocytes is mediated by target cell membrane.

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Abstract
Transfer of phospholipid from the envelope of Sendai virus to erythrocyte membrane was measured by using spin-labeled phosphatidylcholine. The transfer was enhanced autocatalytically above a threshold dose (about five adsorbed viruses per cell). There was an inflection point in the time course of transfer, after which the transfer was greatly accelerated. The time to reach the inflection point became shorter with increased viral dose. The transfer reaction was markedly enhanced above 19 degrees C and the inflection point was observed above this temperature. There was negligible transfer from trypsin-treated virus to erythrocyte membrane, but the transfer was greatly enhanced by intact virus. The enhancement was larger with increased amount of intact virus, and the inflection point was observed in the transfer curves. All the kinetic data can be satisfactorily analyzed by a model which assumes that the virus modifies the cell membrane at the attachment site, the modification is propagated in the membrane, and transfer of phospholipid to the modified sites is greatly enhanced. The propagation rate is estimated as approximately equal to 10(-11) cm2s-1 and the activation energy as 13 kcal mol-1 (54 kJ mol-1). The viral F glycoproteins are suggested as a possible entity for the modification and its propagation: they are introduced into the target cell membrane by envelope fusion, diffuse laterally, and enhance both phospholipid exchange and envelope fusion with viruses attached to the membrane sites.