Single-Virus Lipid-Mixing Study of Sendai Virus Provides Insight into Fusion Mechanism.

Single-virus studies have proven useful to interrogate the entry mechanism for several viral families. Here, we employ a fluorescence microscopy-based single-virus assay to study the fusion (lipid mixing) of Sendai virus to model membranes, the first for any paramyxovirus to our knowledge. We find that fusion wait times following binding are exponentially distributed, suggesting a single rate-limiting step. Compared to previously studied viruses, fusion is relatively slow (tens of minutes) and inefficient (only a small fraction of virions undergo fusion). Trypsin treatment of the virus or different viral receptors in the target alter the efficiency, although the wait time distribution remains unchanged in both cases. This provides constraints on the fusion mechanism and the identity of the rate-limiting step. Together, our data paint a picture of Sendai virus as a comparatively inefficient and slow fusion "machine" and set the stage for the investigation of other paramyxoviruses.