ICP0 induces the accumulation of colocalizing conjugated ubiquitin

Herpes simplex virus type 1 (HSV-1) immediate-early protein ICP0 is a gener al activator of viral gene expression which stimulates the initiation of ly tic infection and reactivation from quiescence and latency. The importance of ICP0 to the biology of HSV-1 infection has stimulated interest in its mo de of action. Previous studies have reported its interactions with other vi ral regulatory molecules, with the translation apparatus, with cyclin D3, a nd with a ubiquitin-specific protease, It has been demonstrated that ICP0 i s able to induce the proteasome-dependent degradation of a number of cellul ar proteins, including components of centromeres and small nuclear substruc tures known as ND10 or PML nuclear bodies. ICP0 has a RING finger zinc-bind ing domain which is essential for its functions. In view of several recent examples of other RING finger proteins which modulate the stability of spec ific target proteins by acting as components of E3 ubiquitin ligase complex es, this study has explored whether ICP0 might operate via a similar mechan ism. Evidence that the foci of accumulated ICP0 in transfected and infected cells contain enhanced levels of conjugated ubiquitin is presented. This e ffect was dependent on the RING finger region of ICP0, and comparison of th e properties of a number of ICP0 mutants revealed an excellent correlation between previously established functions of ICP0 and its ability to induce concentrations of colocalizing conjugated ubiquitin. These results strongly support the hypothesis that a major factor in the mechanism by which ICP0 influences virus infection is its ability to induce the degradation of spec ific cellular targets by interaction with the ubiquitin-proteasome pathway.