Capsid assembly and DNA packaging in herpes simplex virus

The genome of HSV-1 contains 80-85 open reading frames. Genetic and biochemical evidence suggests that at least 39 of these genes encode proteins that are components of the HSV-1 virion. The architecture of the HSV-1 virion consists of a trilaminar lipid envelope, an amorphous layer known as the tegument, a capsid shell, and a DNA-containing core. The capsid is an icosahedral shell whose major morphological features are 162 capsomers. It is composed of a major capsid protein called VP5 and three less abundant proteins, VP19C, VP23 and VP26. VP5 is the structural subunit of all 162 capsomers while VP19C and VP23 are located in the space between the capsomers. In addition to the structural proteins, capsid assembly involves participation of the HSV-1-encoded protease and the scaffolding protein, preVP22a. DNA packaging involves participation of DNA, empty capsids, and at least seven additional HSV-1-encoded proteins. Considerable advances have been made in understanding the structure of the capsid shell, largely as the result of applying cryoelectron microscopy techniques. Use of recombinant baculoviruses has allowed for a detailed analysis of the proteins required for capsid assembly. More recently, an in vitro system has been developed which has aided in defining the assembly pathway by identifying intermediates in the assembly of intact capsids. The in vitro system has identified a fragile roundish procapsid which matures into the polyhedral capsid in a transition similar to that undergone by bacteriophage proheads. This review is a summary of our present knowledge with respect to the structure and assembly of the HSV-1 capsid and what is known about the seven genes involved in DNA packaging. Copyright 1997 John Wiley & Sons Ltd.