Sendai virus (SeV) has long been studied as a model paramyxovirus and is closely related to other respiroviruses such as human parainfluenza viruses 1 and 3. It also has important applications as a viral vector. SeV membrane fusion is catalyzed by the viral fusion (F) protein, which itself is triggered by binding of the viral hemagglutinin-neuraminidase (HN) protein to sialic acid conjugated receptors in the host membrane. Additionally, the F protein must be proteolytically cleaved to be properly activated for fusion. However, despite knowledge of these key aspects of membrane fusion, important gaps remain in our understanding of the SeV fusion mechanism. Here, we study the effect of proteolytic cleavage, different viral receptors, and variations in membrane chemical composition on SeV fusion using a single-virus fusion (lipid mixing) assay. Synthetically created model membranes consisting of tethered vesicles are constructed within microfluidic flow cells, and the binding and fusion of single viral particles are observed via fluorescence microscopy. We observe that SeV lipid mixing occurs on a long time-scale (tens of minutes) and has a single rate-limiting step, as suggested by the exponential distribution of binding-to-lipid mixing wait times. We demonstrate that while the extent of lipid mixing is modulated by treatment with the serine protease trypsin and viral receptor identity, the lipid mixing kinetics are unaffected. We also investigate the effect of varying the lipid composition of the target membrane on lipid mixing. We show that both cholesterol and sphingomyelin concentrations alter lipid mixing extent. On the other hand, kinetics are shifted in the presence of sphingomyelin while remaining unaltered by cholesterol. We discuss the insights that these results provide into the mechanism of SeV fusion.
Ji et al. (Sun,) studied this question.