Effect of HSV-1 ICP0 E3 ubiquitin ligase on CIN85 endosomes

Herpes simplex virus-1 (HSV-1) has infected more than 67% of the world population causing diseases that range in severity from benign cold sores to encephalitis. To infect and persist in the host, HSV-1 has evolved sophisticated strategies to counteract antiviral responses. The viral immediate early protein ICP0 (Infected Cells Protein 0) plays a fundamental role in this process. ICP0 is an E3 ubiquitin ligase that is required for successful onset of lytic infection and productive reactivation of viral genomes from latency. Most ICP0 studies have been focused on its nuclear functions, but late gene expression drives accumulation of ICP0 to the cytoplasm where its functions are unknown. We recently demonstrated that cytoplasmic ICP0 localizes on the surface of endosomes through its interaction with CIN85 (Cbl-interacting protein of 85 kDa), a scaffolding protein for factors involved in endocytosis. This interaction is required for extrusion of host antiviral factors and immunoevasion. Here, we report that the E3 ubiquitin ligase activity of ICP0 is also required for these effects, as disruption of ICP0 RING finger domain, which disrupts its catalytic activity, resulted in an increase in the number of CIN85 endosomes that cannot be exocytosed. These endosomes colocalize with autophagosome components, but they do not enter the lysosomal pathway for degradation. Mutant viruses that cannot exocytose CIN85 endosomes produce EVs that are more potent at activating antiviral responses.