In situ structure determination of virus capsids imaged within cell nuclei by correlative light and cryo-electron tomography.

Abstract

Cryo electron microscopy (cryo-EM), a key method for structure determination involves imaging purified material embedded in vitreous ice. Images are then computationally processed to obtain three-dimensional structures at atomic resolution. There is increasing interest in extending structural studies by cryo-EM into the cell, where biological structures and processes may be imaged in context. The limited penetrating power of electrons prevents imaging of thick specimens (>500 nm) however. Cryo-sectioning methods employed to overcome this are technically challenging, subject to artefacts or involve specialised equipment of limited availability. Here we describe the first structure of herpesvirus capsids determined by sub-tomogram averaging from nuclei of eukaryotic cells, achieved by cryo-electron tomography (cryo-ET) of re-vitrified cell sections prepared using the Tokuyasu method. Our reconstructions reveal that the capsid associated tegument complex is present on capsids prior to nuclear egress. We show that this approach to cryogenic imaging of cells is suited to both correlative light/electron microscopy and 3D structure determination.