pH Dependence of Zika Membrane Fusion Kinetics Reveals an Off-Pathway State

[Image: see text] The recent spread of Zika virus stimulated extensive research on its structure, pathogenesis, and immunology, but mechanistic study of entry has lagged behind, in part due to the lack of a defined reconstituted system. Here, we report Zika membrane fusion measured using a platform that bypasses these barriers, enabling observation of single-virus fusion kinetics without receptor reconstitution. Surprisingly, target membrane binding and low pH are sufficient to trigger viral hemifusion to liposomes containing only neutral lipids. Second, although the extent of hemifusion strongly depends on pH, hemifusion rates are relatively insensitive to pH. Kinetic analysis shows that an off-pathway state is required to capture this pH-dependence and suggests this may be related to viral inactivation. Our surrogate-receptor approach thus yields new understanding of flaviviral entry mechanisms and should be applicable to many emerging viruses.