Tailed bacteriophages (order Caudovirales) are characterised by an icosahedral capsid, which encloses a double-stranded DNA genome. These phages and herpesviruses share a common assembly pathway for prohead formation and genome packaging. In both viruses, DNA incorporation and ejection is mediated by a machinery built by similar components, including the portal protein and a motor protein complex called terminase, which provides the energy for DNA translocation and has nuclease activity. The portal is a large oligomeric ring-shaped protein located at a unique pentameric vertex of the capsid. It acts as an initiator for capsid assembly and it is also a critical part of the DNA packaging and ejection machinery. In phages, the portal is also involved in tail assembly.

Using X-ray crystallography and high-resolution cryo-electron microscopy, we have solved the structure of a bacteriophage portal, in different conformations, and a tail complex. Our findings point to a molecular mechanism for DNA retention and ejection. A herpesvirus terminase subunit and a portal have also been solved and will be compared with their phage equivalent parts.