Single VLP lipid-mixing measurements confirm off-pathway state in dengue virus fusion mechanism

Dengue virus (DENV) is the causative agent of dengue fever and exerts a substantial healthcare burden worldwide. Like other flaviviruses, DENV must undergo membrane fusion with the host cell in order to initiate infection. This membrane fusion occurs after acidification during endocytosis and is pH dependent. Here, we interrogate whether the mechanism of DENV fusion contains an off-pathway state, such has been reported previously for two other flaviviruses-Zika virus and West Nile virus. To do this, we utilize single-particle lipid-mixing measurements of DENV virus-like particles (VLPs) to tethered liposomes, together with computational modeling inspired by chemical kinetics. By observing and then modeling the pH dependence of single-VLP fusion kinetics, we provide evidence that the DENV fusion mechanism must contain an off-pathway state. Measuring the proportion of VLPs undergoing hemi-fusion over time, we also demonstrate that the off-pathway state appears to be slowly reversible over tens of minutes, at least for some virions. Additionally, we find that late endosomal anionic lipids do not appear to influence the off-pathway mechanism to any great extent. In conjunction with the prior reports on Zika virus and West Nile virus, this work indicates that an off-pathway fusion state may be a feature of flavivirus fusion more broadly. We also note that the platform and mechanistic model described in this study may be useful in elucidating the mechanism of action of small molecule inhibitors of flavivirus fusion developed by our group and others.