Biphasic exocytosis of herpesvirus from hippocampal neurons and mechanistic implication to membrane fusion.

Exocytosis is a crucial cellular process involved in the release of neural transmitters or signaling hormones, and disposal of waste or toxic materials. The relationship between structural transition and temporal progression of this process is poorly understood, partly due to lack of adequate tools to resolve such dynamic structures at sufficient resolution in 3D. Exocytosis can be hijacked by some viruses, exemplified by the widely used model α-herpesvirus pseudorabies virus (PRV), which relies on exocytosis for trans-synaptic spread across neurons. Here, we have used cryo electron tomography (cryoET) to capture 199 events of PRV exocytosis from cultured hippocampal neurons. We established cumulative frequency analysis to estimate the relative duration of an exocytosis stage based on the frequency of observed viral particles at that stage. This analysis revealed that PRV exocytosis is biphasic, including a fast, "release phase" driven by fusion proteins and fused membranes, and a slow, "recovery phase" driven by flattening of curved membranes. The biphasic property of exocytosis discovered here appears to be conserved for membrane fusion during viral entry, and our approach of cumulative frequency analysis should have general utility for characterizing other membrane fusion events.