Identification of a novel higher molecular weight isoform of USP7/HAUSP that interacts with the Herpes simplex virus type-1 immediate early protein ICP0

The Herpes simplex virus type-1 (HSV-1) regulatory protein ICP0, a RING-finger E3 ubiquitin ligase, stimulates the onset of viral lytic replication and the reactivation of quiescent viral genomes from latency. Like many ubiquitin ligases ICP0 induces its own ubiquitination, a process that can lead to its proteasome-dependent degradation. ICP0 counteracts this activity by recruiting the cellular ubiquitin-specific protease USP7/HAUSP. Here we show that ICP0 can also interact with a previously unidentified isoform of USP7 (termed here USP7β). This isoform is not a predominantly ubiquitinated, SUMO-modified, or phosphorylated species of USP7 but is constitutively expressed in a number of different cell types. Like USP7, USP7β binds specifically to an electrophilic ubiquitin probe, indicating that it contains an accessible catalytic core with potential ubiquitin-protease activity. The interaction formed between ICP0 and USP7β requires ICP0 to have an intact USP7-binding domain and results in its susceptibility to ICP0-mediated degradation during HSV-1 infection.