The two functions of herpes simplex virus 1 ICP0, inhibition of silencing by the CoREST/REST/HDAC complex and degradation of PML, are executed in tandem.

ICP0, an alpha (immediate-early) protein of herpes simplex virus 1, performs at least two key functions. It blocks inhibition of viral-gene expression by interferon, a function dependent on the degradation of the ND10 components PML and SP100 by the ubiquitin ligase expressed by the RING finger (RF), and it blocks silencing of viral DNA mediated by the HDAC1/2-CoREST-REST complex. In the latter case, a mutant CoREST lacking the HDAC1 binding site compensates totally or in part for the absence of ICP0 in a cell-type-dependent manner. Here, we compare the phenotypes of an ICP0 mutant containing disabling amino acid substitutions in the RF with those of a mutant with substitutions in the CoREST binding site (R8507). We report the following: (i) the onset of replication of both mutants was delayed, but the RF mutant yields did not reach wild-type virus levels even as late as 48 h after infection, and (ii) in infected cells, PML is rapidly degraded by wild-type virus, with some delay by the R8507 mutant, and is spared by the RF mutant. The translocation of ICP0 to the cytoplasm is impaired in cells infected with the RF mutant or delayed in cells infected with the R8507 mutant. Finally, in contrast to wild-type viruses, both mutants are inhibited by alpha or gamma interferon. The results indicate that both sets of events, the degradation of PML and the blocking of silencing, are interdependent and in large measure dependent on events in the ND10 nuclear bodies.