Herpesvirus capsid assembly: insights from structural analysis

In all herpesviruses, the capsid is icosahedral in shape, composed of 162 capsomers, and assembled in the infected cell nucleus. Once a closed capsid is formed, it is packaged with the virus DNA and transported to the cytoplasm where further morphogenetic events take place. Herpesvirus capsid populations are highly uniform in shape, and this property has made them attractive for structural analysis particularly by cryo electron microscopy followed by three-dimensional image reconstruction. Here we describe what is known about herpesvirus capsid structure and assembly with emphasis on herpes simplex virus and on the contribution of structural studies. The overall analysis has demonstrated that herpesvirus capsids are formed by a pathway resembling that established for dsDNA bacteriophage such as P22 and HK97. For example herpes capsid assembly is found to: (1) involve a scaffolding protein not present in the mature virus; (2) proceed through a fragile, spherical procapsid intermediate; and (3) result in the incorporation of a portal complex at a unique capsid vertex.

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► X-ray crystallographic structure determined for part of the HSV-1 major capsid protein.
► HSV-1 capsid assembly occurs by way of a procapsid intermediate as observed in dsDNA bacteriophage.
► Use of cryoelectron microscopy to identify steps in the procapsid maturation pathway.
► HSV-1 portal identified at a unique capsid vertex.
► Portal involved in the initiation of HSV-1 capsid assembly.