Roles of the Phosphorylation of Herpes Simplex Virus 1 UL51 at a Specific Site in Viral Replication and Pathogenicity.

Herpes simplex virus 1 (HSV-1) UL51 is a phosphoprotein that functions in the final envelopment in the cytoplasm and viral cell-cell spread, leading to efficient viral replication in cell cultures. To clarify the mechanism by which UL51 is regulated in HSV-1-infected cells, we focused on the phosphorylation of UL51. Mass spectrometry analysis of purified UL51 identified five phosphorylation sites in UL51. Alanine replacement of one of the identified phosphorylation sites in UL51, serine 184 (Ser-184), but not the other identified phosphorylation sites, significantly reduced viral replication and cell-cell spread in HaCaT cells. This mutation induced membranous invaginations adjacent to the nuclear membrane, the accumulation of primary enveloped virions in the invaginations and perinuclear space, and mislocalized UL34 and UL31 in punctate structures at the nuclear membrane; however, it had no effect on final envelopment in the cytoplasm of HaCaT cells. Of note, the alanine mutation in UL51 Ser-184 significantly reduced the mortality of mice following ocular infection. Phosphomimetic mutation in UL51 Ser-184 partly restored the wild-type phenotype in cell cultures and in mice. Based on these results, we concluded that some UL51 functions are specifically regulated by phosphorylation at Ser-184 and that this regulation is critical for HSV-1 replication in cell cultures and pathogenicity HSV-1 UL51 is conserved in all members of the family. This viral protein is phosphorylated and functions in viral cell-cell spread and cytoplasmic virion maturation in HSV-1-infected cells. Although the downstream effects of HSV-1 UL51 have been clarified, there is a lack of information on how this viral protein is regulated as well as the significance of the phosphorylation of this protein in HSV-1-infected cells. In this study, we show that the phosphorylation of UL51 at Ser-184 promotes viral replication, cell-cell spread, and nuclear egress in cell cultures and viral pathogenicity in mice. This is the first report to identify the mechanism by which UL51 is regulated as well as the significance of UL51 phosphorylation in HSV-1 infection. Our study may provide insights into the regulatory mechanisms of other herpesviral UL51 homologs.