Assembly of herpes simplex virus type 1 capsids using a panel of recombinant baculoviruses

Immature or B capsids of herpes simplex virus type 1 (HSV-1) are composed of seven proteins encoded by six viral genes. The proteins encoded by UL18 (VP23), UL19 (VP5), UL35 (VP26) and UL38 (VP19C) are components of the outer capsid shell whereas those specified by UL26 (VP21 and VP24) and UL26.5 (VP22a), are involved in scaffold formation. We have used a panel of recombinant baculoviruses, each expressing one of the capsid protein genes, to examine the requirements for capsid assembly. Coexpression of the six genes in insect cells resulted in the formation of capsids that were indistinguishable in appearance and protein composition from those made during HSV-1 infection of mammalian cells. This demonstrates that the proteins encoded by the known capsid genes contain all the structural information necessary for capsid assembly and that other virus-encoded proteins are not required for this process. Omission of single recombinant baculoviruses from this system allowed the role of individual HSV-1 proteins in capsid assembly to be determined. Thus, capsid assembly did not take place in the absence of VP23, VP5 or VP19C, whereas lack of VP26 had no discernible effect on capsid formation. Capsids assembled in the absence of the UL26 gene products had a large-cored phenotype resembling that previously described for the HSV-1 mutant ts1201 which has a lesion in this gene. Some apparently intact capsid shells were also made in the absence of the major scaffolding protein, VP22a, whereas the omission of both UL26 and UL26.5 resulted in the appearance of large numbers of partial and deformed capsid shells.