The capsid architecture of channel catfish virus, an evolutionarily distant herpesvirus, is largely conserved in the absence of discernible sequence homology with herpes simplex virus.
Although herpesviruses have a wide host range and their genomes vary substantially in size, the nucleocapsid appears to be a conservative element of viral design. The capsid shell is icosahedrally symmetric (T = 16), and 125 nm in diameter and 15nm thick in the case of herpes simplex virus 1 (HSV-1). Channel catfish virus (CCV) has the gross morphology of a herpesvirus, although no relationship to other herpesviruses is evident from the sequences of its proteins. To examine CCV capsid architecture more closely, we have determined its structure by cryoelectron microscopy and three-dimensional image reconstruction. The CCV capsid is smaller than that of HSV-1, but its 12% smaller genome is packed to essentially the same average density; its icosahedral facets are flatter, and its shell is about 20% thinner, consistent with the smaller size of its major capsid protein. Otherwise, their major features are remarkably similar: CCV has the same triangulation number; its hexons and pentons also have chimney-like protrusions with an axial channel through each capsomer; and there are "triplexes" on the outer surface at the sites of local threefold symmetry. The basic herpesvirus capsid architecture is, therefore, remarkably well conserved in CCV and implies a utilitarian basis to this design. The protein composition of CCV mirrors that of HSV-1, except for the absence of the 12-kDa protein, VP26, which is dispensable for assembly in the HSV-1 system and, apparently, wholly dispensable for CCV.