Cell culture evolution of a HSV-1/VZV UL34/ORF24 chimeric virus reveals novel functions for HSV genes in capsid nuclear egress

HSV and VZV are both members of the alphaherpesvirus subfamily, but are of different genera. Substitution of the HSV-1 UL34 coding sequence with that of its VZV homolog, ORF24, results in a virus that has defects in viral growth, spread, capsid egress, and nuclear lamina disruption very similar to those seen in a UL34-null virus despite normal interaction between ORF24 protein and HSV pUL31 and proper localization of the NEC at the nuclear envelope. Minimal selection for growth in cell culture resulted in viruses that grew and spread much more efficiently that the parental chimeric virus. These viruses varied in their ability to support nuclear lamina disruption, normal NEC localization and capsid de-envelopment. Single mutations that suppress the growth defect were mapped to the coding sequences of ORF24, ICP22 and ICP4 and one virus carried single mutations in each of the ICP22 and US3 coding sequences. The phenotypes of these viruses support a role for ICP22 in nuclear lamina disruption and a completely unexpected role for the major transcriptional regulator, ICP4, in capsid nuclear egress.