Cryo-electron tomography of the herpesvirus procapsid reveals interactions of the portal with the scaffold and a shift on maturation
Herpes simplex virus type 1 (HSV-1) requires seven proteins to package its genome through a vertex in its capsid, one of which is the portal protein, pUL6. The portal protein is also thought to facilitate assembly of the procapsid. While the portal has been visualized in mature capsids, we aimed to elucidate its role in the assembly and maturation of procapsids using cryo-electron tomography. We identified the portal vertex in individual procapsids, calculated a subtomogram average, and compared that with the portal vertex in empty mature capsids (A-capsids). The resulting maps show the portal on the interior surface with its narrower end facing outwards, while maintaining close contact with the capsid shell. In the procapsid, the portal is embedded in the underlying scaffold, suggesting that assembly involves a portal - scaffold complex. During maturation, the capsid shell angularizes with a corresponding outward movement of the vertices. We found that in A-capsids, the portal translocates further than the adjacent capsomers and strengthens its contacts with the capsid shell. Our methodology also allowed us to determine the number of portal vertices in each capsid, with most having one per capsid, but some none or two, and rarely three. The predominance of a single portal per capsid supports facilitation of the assembly of the procapsid. ImportanceHerpes simplex virus type 1 (HSV-1) infects a majority of humans, causing mostly mild disease but in some cases progressing towards life-threatening encephalitis. Understanding the life cycle of the virus is important to devise countermeasures. Production of the virion starts with the assembly of an icosahedral procapsid including DNA packaging proteins at a vertex, one of which is the dodecameric portal protein. The procapsid then undergoes maturation and DNA packaging through the portal, driven by a terminase complex. We used cryo-electron tomography to visualize the portal in procapsids and compare them to mature empty capsids. We found the portal located inside one vertex interacting with the scaffold protein in the procapsid. On maturation, the scaffold dissociates, the capsid angularizes, and the portal moves outward, associating closely with the capsid shell. These transformations may provide a basis for the development of drugs to prevent HSV-1 infections.