Mutations in herpes simplex virus type 1 genes encoding VP5 and VP23 abrogate capsid formation and cleavage of replicated DNA

The herpes simplex virus type 1 capsid is composed of seven capsid proteins which are termed VP5, VP19c, VP21, VP22a, VP23, VP24, and VP26. Major capsid protein VP5 is encoded by the gene UL19. UL18, whose transcript is 3' coterminal with that of VP5, specifies capsid protein VP23. Vero cell lines have been isolated that are transformed with either the BglII N (UL19) or EcoRI G (UL16 to UL21) fragment of KOS. These cell lines, selected for the ability to support the replication of a temperature-sensitive VP5 mutant, were used to isolate VP5 and VP23 null mutants. The mutations in VP5 (K5ΔZ) and VP23 (K23Z) were generated by insertion of the lacZ gene at the beginning of the coding sequences of the genes. Both mutants failed to form plaques on the nonpermissive cell line, and therefore, VP23, like VP5, is an essential gene product for virus replication. Both mutants expressed wild-type levels of infected-cell proteins upon infection of permissive and nonpermissive cell lines. However, the VP5 (150-kDa) and VP23 (33-kDa) polypeptides were absent in lysates prepared from K5ΔZ- and K23Z-infected Vero cells, respectively. No capsid structures were observed by electron microscopic analysis of thin sections of K5ΔZ- and K23Z-infected Vero cells. Following sedimentation of lysates from cells infected by the mutants, capsid proteins were not observed in the fractions where capsids normally sediment. The amounts of DNA replicated in the VP5 and VP23 mutant and in KOS-infected Vero cells were the same as in permissive cells. However, genomic ends were not evident in Vero cells infected with the mutants, suggesting that the DNA remains in concatemers and is not processed into unit length genomes.