VP16 Interacts Via Its Activation Domain with VP22, a Tegument Protein of Herpes Simplex Virus, and is Relocated to a Novel Macromolecular Assembly in Coexpressing Cells

In addition to its function as a powerful transactivator of viral immediate-early transcription, VP16 is an essential component of the herpes simplex virus (HSV) virion. As such, VP16 is introduced into cells, to effect its function in transactivation, as part of the virus tegument. Here we examine the potential for VP16 protein-protein interactions specific to virus-infected cells and show that VP16 copurifies in a highly enriched fraction with a single major polypeptide which we identify as the virus-encoded structural protein VP22. We further show that in vitro-translated VP22 binds specifically to purified VP16. The activation domain of VP16 was required and largely sufficient for this binding. Mutations within this domain, which disrupt its transactivation function, also affected VP22 binding. Furthermore, we show that while VP16 and VP22 showed distinct patterns of compartmentalization in vivo, coexpression of both proteins resulted in a profound reorganization from their normal locations to a novel macromolecular assembly. The colocalization was also dependent on the activation domain of VP16 but required additional determinants within the N terminus. These results are discussed in the context of VP16 regulation of transcription both early in infection during delivery of tegument proteins and at late times during virus assembly.