Functional homologies between avian and human alphaherpesvirus VP22 proteins in cell-to-cell spreading as revealed by a new cis-complementation assay.

VP22, encoded by the UL49 gene of Marek's disease virus (MDV), is indispensable for virus cell-to-cell spreading. We show herein that MDV UL49 can be functionally replaced with avian and human viral orthologs. Replacement of MDV VP22 with that of avian gallid herpesvirus 3 or herpesvirus of turkey, whose residue identity with MDV is close to 60%, resulted in 73 and 131% changes in viral spreading, respectively. In contrast, VP22 replacement with human herpes simplex virus type 1 resulted in 14% plaque formation. Therefore, heterologous avian and human VP22 proteins share sufficient structural homology to support MDV cell-to-cell spreading, albeit with different efficiencies.