Proteomic Investigation of the HSV-1 Outer Tegument Protein pUL46: Interactions, Hyper-Phosphorylation, and Regulation by ICP0-Dependent Proteasomal Degradation

Abstract

Much like the host cells they infect, viruses must also regulate their life cycles. Herpes simples virus type 1 (HSV-1), a prominent member of the herpesvirus family, utilizes a promoter-rich genome in conjunction with multiple viral trans-activating factors. Following entry and uncoating, virion-associated outer tegument proteins pUL46 and pUL47 play non-structural roles by enhancing the ability of the trans-activating protein pUL48 to increase transcription of immediate-early (α) genes. Since pUL46 has gone largely unstudied, we employed a mass spectrometry-based approach to better understand how pUL46 modulates pUL48 and any other non-structural functions it might have. pUL46-containing protein complexes were immunoaffinity purified from infected cells, digested, and sequenced by tandem mass spectrometry (AP-MS/MS) to investigate protein-protein interactions and post-translational modifications of pUL46. We demonstrated that pUL46 is heavily phosphorylated in at least 23 sites, and that one of these matches a consensus 14-3-3 phospho-binding site. Indeed, our proteomic approach identified the phospho-binding 14-3-3 family, as well as host and viral kinases and the viral E3 ubiquitin ligase ICP0, as specific interacting partners of pUL46. Moreover, we found that pUL46 is partially degraded in a proteasome-mediated manner during infection, and that catalytic activity of ICP0 is responsible for this degradation. Together, these data imply that pUL46 levels are tightly controlled and that it may be important for temporal regulation of viral gene expression throughout the viral life cycle. Indeed, the phenomenon of a structural protein, pUL46, performing non-structural roles may reflect a theme common to many viruses, and a better understanding of how such proteins function and are regulated will be important in the development of novel therapeutics.