Phosphatidylserine vesicle lysis by sendai virus at low pH is not due to virus-vesicle fusion

As a model of the fusion of Sendai virus with red cells, the interaction of the virus with phosphatidylserine (PS) vesicles at pH 5 was quantitated by the release of a trapped marker from target vesicles and by mixing of lipids of the virus and the vesicles. Release of the marker was measured as dequenching of calcein trapped at a self-quenched concentration and lipid mixing was measured as a decrease in energy transfer between fluorescent phospholipid analogs in the target membrane. At comparable virus:vesicle ratios both calcein release and lipid mixing were maximal at pH 5 and significantly reduced after trypsin, but not chymotrypsin, treatment. In contrast, these two effects differed in their PS dependence, time course, and temperature dependence, indicating that calcein release is not a consequence of the fusion of a permeable virus membrane with an impermeable target membrane. Vesicles composed of 25 to 100% PS released similar amounts of calcein, whereas fusion increased linearly as a function of PS content of the target vesicles. The half-time was 15 s for calcein release but 1.5 min for fusion. The temperature coefficient of fusion was at least three times greater than that of calcein release. These results indicate that calcein release at pH 5 may signify an interaction of the virus with PS target membranes which precedes but does not necessarily culminate in fusion, given too low a temperature or an inappropriate target membrane composition.