The herpes simplex virus 1 ICP0 is a regulatory protein. Early in infection ICP0 localizes in ND10 bodies and performs two functions: As an E3 ligase in conjunction with E2 UbcH5a conjugating enzyme, it degrades the ND10 components PML and SP100. Concurrently, it suppresses the silencing of viral DNA by dispersing the HDAC1/ CoREST/REST/LSD1 repressor complex. Subsequently, ICP0 is exported to the cytoplasm. In cells treated with HDAC inhibitors or transfected with irrelevant DNA, the export is delayed in a DNA dose-dependent fashion. Here, we follow up an observation that ICP0 binds cyclin D3 and that ICP0 mutants unable to bind cyclin D3 are not exported. Moreover, in infected cells cdk4 is activated, but cdk2 is not. We report that (i) cyclin D1, D2, or D3 colocalize with ND10 bodies and ICP0 early in infection and ultimately become incorporated into viral replication compartments, (ii) each of the D cyclins partially rescues ⌬ICP0 mutants, and (iii) inhibition of cdk4 by inhibitor I sequesters ICP0 in the nucleus. A key finding is that overexpression of cyclin D3 enables the transport of ICP0 to the cytoplasm. We conclude that (i) ICP0 facilitates the recruitment of cyclin D3 to the sites of viral DNA synthesis, (ii) until its functions are completed, ICP0 is retained in the nucleus, and (iii) a common signal that results in the export of ICP0 to the cytoplasm is the accumulation of a viral DNA-synthesis-dependent late protein.ND10 bodies ͉ nuclear-cytoplasmic translocation ͉ PML I nfected cell protein no. 0 (ICP0) is a major multifunctional herpes simplex virus 1 (HSV-1) regulatory protein made immediately after infection. ICP0 is a promiscuous transactivator, and in cells infected at low multiplicity with ⌬ICP0 mutants, the transition from ␣ (immediate early) to genes expressed later in infection does not ensue (1). In wild-type virus-infected cells, it accumulates initially at ND10 bodies, where it performs at least two functions. As an E3 ligase, it degrades PML and SP100 in conjunction with the UbcH5a ubiquitin-conjugating enzyme (2, 3). At the same time, it blocks the silencing of the viral DNA by a host repressor complex consisting of several proteins, including HDACs 1 or 2, CoREST, REST, and LSD1 (4, 5). Indeed, a dominant negative CoREST retaining the binding site of ICP0 but lacking the HDAC binding site significantly increases the yields of ⌬ICP0 mutant in cells that do not support its replication (4).The interaction of ICP0 with cyclin D3 but not with cyclin D1 was observed initially in a yeast two-hybrid system and confirmed in pull-down experiments (6). The binding site was mapped to Glu199. ICP0 carrying the substitution D199A was not translocated to the cytoplasm. Furthermore, the virus carrying this mutation failed to replicate as well as wild-type in contact-inhibited, resting primary human fibroblasts and was less pathogenic in mice when administered i.p (7). Moreover, in cells infected with this mutant, cyclin D3 was less stable than in wild-type virus-infected cells (7). Earlier studies...
