In Situ Structural Characterisation of Herpesvirus Nuclear Egress using Cryo Focused Ion Beam-Scanning Electron Microscopy and Electron Cryo-Tomography

During exit from the nucleus, newly-assembled herpesvirus capsids are transported across the nuclear membranes via a process known as nuclear egress. This process is mediated by two proteins, pUL31 and pUL34, that together form the nuclear egress complex (NEC). We recently determined the architecture of the NEC inside vesicles in the perinuclear space, revealing that the NEC coat forms a predominantly hexagonal lattice. These studies were performed in the absence of capsid cargos in pUL31/pUL34 overexpressing cells. Using a combined approach of focussed ion beam milling/scanning electron cryo- microscopy (cryoFIB/SEM), electron cryo-tomography (cryoET) and sub-volume averaging, we imaged capsids egressing from the cell nucleus in the frozen hydrated state, providing unprecedented details of this process. We directly visualised capsids during different stages of primary envelopment/de-envelopment and performed respective structure determination. We show the structural changes between different stages of nuclear egress, describe how the NEC ensures separation of nucleoplasm and cytosol and present a model for how the interaction between the capsid and the NEC is mediated.