Varicella Zoster Virus ORF9p binding to cellular adaptor protein complex-1 is important for viral infectivity.

ORF9p (homologous to HSV-1 VP22) is a varicella-zoster virus (VZV) tegument protein essential for viral replication. Even though its precise functions are far from being fully described, a role in the secondary envelopment of the virus has long been suggested. We performed a yeast two-hybrid screen to identify cellular proteins interacting with ORF9p that might be important for this function. We found thirty-one ORF9p interaction partners, among which AP1M1, the mu subunit of the adaptor protein complex-1 (AP-1). AP-1 is a heterotetramer involved in intracellular vesicle-mediated transport and regulates the shuttling of cargo proteins between endosomes and the TGN via clathrin-coated vesicles. We confirmed that AP-1 interacts with ORF9p in infected cells and mapped potential interaction motifs within ORF9p. We generated VZV mutants in which each of these motifs is individually impaired and identified leucine 231 in ORF9p as critical to interact with AP-1. Disrupting ORF9p binding to AP-1 by mutating leucine 231 to alanine in ORF9p strongly impaired viral growth, most likely by preventing efficient secondary envelopment of the virus. Leucine 231 is part of a di-leucine motif conserved among alphaherpesviruses and we showed that VP22 of MDV and HSV-2 also interact with AP-1. This indicates that the function of this interaction in the secondary envelopment might be conserved as well.IMPORTANCE Herpesviruses are responsible for infections that, especially in immunocompromised patients, can lead to severe complications, including neurological symptoms and strokes. The constant emergence of viral strains resistant to classical antivirals (mainly acyclovir and its derivatives) pleads for the identification of new targets for future antiviral treatments. Cellular adaptor protein complexes (AP) have been implicated in the correct addressing of herpesvirus glycoproteins in infected cells and the discovery that a major constituent of varicella-zoster virus tegument is interacting with AP-1 reveals a previously unsuspected role of this tegument protein. Unraveling the complex mechanisms leading to virion production will certainly be an important step in the discovery of future therapeutic targets.