Gene regulation and function of ICP0 in herpes simplex virus infected cells

Herpes simplex virus (HSV) is a clinically important virus, whose life cycle alternates between productive replication and latency. Infected cell protein 0 (ICP0) is generally believed to play a key role in determining the outcome of HSV infections. Synthesis of ICP0 promotes the productive replication of HSV, whereas absence of ICP0 renders HSV prone to establish latent infections. In the first part of the dissertation, I attempt to address the question how is ICP0 gene regulated. To tackle this question, we constructed recombinant HSV that encodes GFP-tagged ICP0 so that the regulation of ICP0 gene can be visualized in real time. Using this reagent, we found that ICP0 gene was subject to potent repression immediately following infection. Surprisingly, HSV\u27s major transcriptional regulator, ICP4, was necessary and sufficient to repress ICP0 gene, and did so in an ICP4-binding-site dependent manner. Synthesis of ICP0 alleviated the ICP4-dependent repression of ICP0 gene. ICP4 co-immunoprecipitated with FLAG-tagged ICP0, thus, a physical interaction between ICP0 and ICP4 likely explains how ICP0 antagonizes ICP4\u27s capacity to silence the ICP0 gene. Therefore, our findings suggest that ICP0 gene is differentially regulated by virus-encoded repressor ICP4 and virus-encoded antirepressor ICP0. In the second part of the dissertation, I attempt to address the question what function does ICP0 assume. Since the discovery of ICP0, the nuclear function of ICP0 has been the focal point of studies, whereas the cytoplasmic function of ICP0 is unknown. While pursuing our first study, we unexpectedly found that GFP-tagged ICP0 was predominantly localized to the cytoplasm during infections. Taking advantage of live-cell imaging, we found that ICP0 translocated from nucleus to cytoplasm during early phase of HSV infections, where it bundled and dispersed microtubules. Synthesis of ICP0 was proved to be necessary and sufficient to dismantle microtubules in HSV-infected and transfected cells. Therefore, our findings suggest ICP0 might play a previously unrecognized role in the cytoplasm through dismantling microtubule networks of the host cells. Furthermore, our study represents the first report showing a virus-encoded E3 ligase disrupts host cell microtubule networks, thus suggests a general function of many other viral E3 ligases