ORF11 Protein Interacts with the ORF9 Essential Tegument Protein in Varicella-Zoster Virus Infection

The tegument proteins encoded by ORF11 and ORF9 of varicella-zoster virus (VZV) are conserved among all alphaherpesvirus. We previously demonstrated that the ORF9 gene is essential, whereas ORF11 is dispensable in vitro but its deletion severely impairs VZV infection of skin xenografts in the SCID mouse model in vivo. Here we report that ORF11 protein interacts with ORF9 protein in infected cells as well as in the absence of other viral proteins, and we have mapped the ORF11 protein domain involved in their interaction. Although ORF11 is an RNA binding protein, the interaction between ORF11 and ORF9 proteins was not mediated by RNA or DNA bridging. VZV recombinants with mutations preventing ORF11 protein binding to ORF9 protein had no effect on 6-day growth kinetics based on plaque numbers, but plaque sizes were reduced in vitro. However, disruption of the ORF11 and ORF9 protein interaction was associated with failure to replicate in skin xenografts in vivo. Further, we demonstrate that in the absence of their interaction, the ORF9 protein displays an identical cellular localization, accumulating in the trans-Golgi region, whereas the ORF11 protein exhibits aberrant localization, dispersing throughout the cytoplasm. Overall, our observations suggest that while complete tegument assembly may not be necessary for VZV replication in vitro, the interaction between the ORF11 and ORF9 proteins appears to be critical for the proper localization of ORF11 protein to the assembly complex and for production of infectious virus during VZV pathogenesis in skin.