Paramyxovirus fusion: Real-time measurement of parainfluenza virus 5 virus–cell fusion

Although cell–cell fusion assays are useful surrogate methods for studying virus fusion, differences between cell–cell and virus–cell fusion exist. To examine paramyxovirus fusion in real time, we labeled viruses with fluorescent lipid probes and monitored virus–cell fusion by fluorimetry. Two parainfluenza virus 5 (PIV5) isolates (W3A and SER) and PIV5 containing mutations within the fusion protein (F) were studied. Fusion was specific and temperature-dependent. Compared to many low pH-dependent viruses, the kinetics of PIV5 fusion was slow, approaching completion within several minutes. As predicted from cell–cell fusion assays, virus containing an F protein with an extended cytoplasmic tail (rSV5 F551) had reduced fusion compared to wild-type virus (W3A). In contrast, virus–cell fusion for SER occurred at near wild-type levels, despite the fact that this isolate exhibits a severely reduced cell–cell fusion phenotype. These results support the notion that virus–cell and cell–cell fusion have significant differences.