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The nuclear lamina is a major obstacle encountered by herpesvirus nucleocapsids in their passage from the nucleus to the cytoplasm (nuclear egress). We found that the human cytomegalovirus (HCMV)-encoded protein kinase UL97, which is required for efficient nuclear egress, phosphorylates the nuclear lamina component lamin A/C in vitro on sites targeted by Cdc2/cyclin-dependent kinase 1, the enzyme that is responsible for breaking down the nuclear lamina during mitosis. Quantitative mass spectrometry analyses, comparing lamin A/C isolated from cells infected with viruses either expressing or lacking UL97 activity, revealed UL97-dependent phosphorylation of lamin A/C on the serine at residue 22 (Ser22). Transient treatment of HCMV-infected cells with maribavir, an inhibitor of UL97 kinase activity, reduced lamin A/C phosphorylation by approximately 50%, consistent with UL97 directly phosphorylating lamin A/C during HCMV replication. Phosphorylation of lamin A/C during viral replication was accompanied by changes in the shape of the nucleus, as well as thinning, invaginations, and discrete breaks in the nuclear lamina, all of which required UL97 activity. As Ser22 is a phosphorylation site of particularly strong relevance for lamin A/C disassembly, our data support a model wherein viral mimicry of a mitotic host cell kinase activity promotes nuclear egress while accommodating viral arrest of the cell cycle.

Human cytomegalovirus encodes an unusual protein kinase, UL97, that activates the established antiviral drug ganciclovir and is specifically inhibited by a new antiviral drug, maribavir. We used maribavir and a UL97 null mutant, which is severely deficient in viral replication, to determine what stage of virus infection critically requires UL97. Compared with wild-type virus, there was little or no decrease in immediate-early gene expression, viral DNA synthesis, late gene expression, or packaging of viral DNA into nuclease-resistant structures in mutant-infected or maribavir-treated cells under conditions where the virus yield was severely impaired. Electron microscopy studies revealed similar proportions of various capsid forms, including DNAcontaining capsids, in the nuclei of wild-type-and mutant-infected cells. However, capsids were rare in the cytoplasm of mutant-infected or maribavir-treated cells; the magnitudes of these decreases in cytoplasmic capsids were similar to those for virus yield. Thus, genetic and pharmacological evidence indicates that UL97 is required at the stage of infection when nucleocapsids exit from the nucleus (nuclear egress), and this poorly understood stage of virus infection can be targeted by antiviral drugs. Understanding UL97 function and maribavir action should help elucidate this interesting biological process and help identify new antiviral drug targets for an important pathogen in immunocompromised patients.Protein kinases are crucial for specific stages in the replication of many different viruses. Human cytomegalovirus (HCMV), a herpesvirus that causes severe disease in newborns and in immunocompromised adults (reviewed in reference 27), encodes an unusual protein kinase known as UL97 (so designated by the open reading frame in the HCMV genome that encodes it [5,12]). This enzyme, remarkably, phosphorylates and thereby activates the antiviral nucleoside analogs ganciclovir and acyclovir (17,37,38). On peptide substrates, it exhibits an unusual dependence on the identity of the residue five positions C terminal to the site of phosphorylation (1). Although not absolutely essential, UL97 is critical for viral replication in cell culture inasmuch as a UL97 deletion mutation causes a ϳ100-fold decrease in viral yield (28).The stages of HCMV infection resemble those of other herpesviruses. Following attachment to cells and entry, the internalized nucleocapsid travels to the nuclear membrane and deposits its DNA in the nucleus, wherein gene expression and DNA replication ensue (reviewed in reference 23). Viral genes are expressed in three temporally regulated classes: immediate early, early, and late. Viral DNA replication proceeds via highmolecular-weight, concatemeric intermediates, which are cleaved to genome-length units during packaging into capsids. Three capsid forms are observed in the nuclei of infected cells:nonproductive forms thought to result from failed packaging attempts (A capsids), productive intermediates that contain a scaffolding protein (B capsids), and ass...

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