Phage Assembly

MIT subaward from Baylor College of Medicine as part of the NIH Award #AI0175208 titled, "Structures of the Portal Vertex in dsDNA viruses and virus-infected cells"

Herpes viruses are major pathogens in human populations, causing disease in normal and immunocompromised individuals. Double stranded DNA (dsDNA) bacterial viruses and dsDNA herpes viruses share surprising similarities in capsid assembly. In both viral classes, a precursor (procapsid) shell is formed with the help of scaffolding proteins. In both, the newly replicated dsDNA is inserted into the precursor procapsid through a unique vertex called the portal, which is a multimeric ring formed by a specific portal protein. The similarities in assembly mechanism and in the structure of the capsid protein suggest that dsDNA herpes viruses and dsDNA phage arose from a common ancestor that also had a portal vertex. They appear to retain common features in both their DNA packaging processes during viron assembly, and in their DNA ejection processes required for successful infection of the host cell. Of particular interest is the role of portal and perhaps pilot proteins in the transport of the DNA genome from the virion across the bacterial envelope for phage, and across the nuclear envelope for herpes virus. Until very recently the structures of such singular complexes - lacking incosahedral symmetry - could not be resolved by any structure analysis methods. Recent advances in asymmetric single particle reconstructions from electron cryo-microscopy (cryoEM) images of the infectious virions of epsilon15 and P22 phages reveal details of their portal and associated protein components (referred to as portal vertex). However, the structural organization of the portal vertex before DNA entry and the opening of the portal vertex to allow later DNA exit remains to be determined. The herpes virus portal shares the cyclic symmetry of the phage portals, but has only been visualized at low resolution. The herpes virus portal vertex may act like the phage portal vertex, and rearrange to release or deliver DNA into the cell nucleus. Such an apparatus presents a new target for the development of anti-viral therapies. We propose to use advances in electron cryo-microscopy and electron cryo-tomography to determine the structural rearrangements of the phage and herpes virus portal complexes during the process of DNA ejection into host cells. The structures to be determined include not only the isolated virus particles but the forms generated upon interaction with the host cell and release of DNA. The simultaneous study of both viruses will allow us to rapidly apply advances in imaging technology developed with the phage capsids, to the more complex herpes virion and capsid structures. The imaging and computational tools developed for the study of infection by these viruses should be generally applicable to the infection processes of many other viruses.