Vaccinia Virus (VV), the prototype member of the poxviridae a family of complex DNA viruses, encodes for more than 200 proteins. The structural details underlying these controlled events are, however, poorly understood for most viruses. These cellular cues activate and induce conformational changes to critical viral factors required for virus-disassembly and the subsequent release of the viral genome.
To coordinate disassembly and assembly viruses rely on specific signals that program them towards uncoating these can be receptor binding, reducing or low pH environment of the cytoplasm or endosomes, respectively. They fall apart to release their genome for replication and then assemble into stable structures many hours later within the same host cell. When viruses enter cells they undergo a programmed sequence of events that finally leads to the production of new infectious progeny. Generally, this is the first study that employs whole cell cryo-ET to address structural details of pathogen-host cell interaction. We propose that the cell surface induced changes, in particular the decondensation of the viral genome, are a prerequisite for the subsequent release of the vaccinia DNA into the cytoplasm, which is followed by its cytoplasmic replication. Binding to the cell surface induced distinct structural rearrangements of the core, such as a shape change, the rearrangement of its surface spikes and de-condensation of the viral DNA. Here we study the disassembly of VV by cryo-ET on intact, rapidly frozen, mammalian cells, infected for up to 60 minutes. Cryo-electron tomography (cryo-ET), a unique method to investigate large and asymmetric structures at the near molecular resolution, was previously used to study the complex structure of vaccinia virus (VV). For most viruses, the structural details that underlie these disassembly and assembly reactions are poorly understood. At each round of infection, viruses fall apart to release their genome for replication, and then reassemble into stable particles within the same host cell.