Like all alphaherpesviruses, varicella-zoster virus (VZV)
infection proceeds by both cell-cell spread and virion production. Virions are enveloped within vacuoles located near the trans-Golgi network (TGN), while in cell-cell spread,
surface glycoproteins fuse cells into syncytia. In this report, we delineate a potential role for
serine/
threonine phosphorylation of the cytoplasmic tail of the predominant VZV
glycoprotein, gE, in these processes. The fact that
VZV gE (formerly called gpI) is phosphorylated has been documented (E. A. Montalvo and C. Grose, Proc. Natl. Acad. Sci. USA 83:8967-8971, 1986), although respective roles of viral and cellular
protein kinases have never been delineated. VZV ORF47 is a viral
serine protein kinase that recognized a consensus sequence similar to that of
casein kinase II (CKII). During open reading frame 47 (ORF47)-specific in vitro
kinase assays, ORF47 phosphorylated four residues in the cytoplasmic tail of
VZV gE (S593, S595, T596, and T598), thus modifying the known phosphofurin acidic cluster sorting
protein 1 domain. CKII phosphorylated gE predominantly on the two
threonine residues. In wild-type-virus-infected cells, where ORF47-mediated phosphorylation predominated, gE endocytosed and relocalized to the TGN. In cells infected with a VZV ORF47-null mutant, internalized
VZV gE recycled to the plasma membrane and did not localize to the TGN. The mutant virus also formed larger syncytia than the wild-type virus, linking CKII-mediated gE phosphorylation with increased cell-cell spread. Thus, ORF47 and CKII behaved as "team players" in the phosphorylation of
VZV gE. Taken together, the results showed that phosphorylation of
VZV gE by ORF47 or CKII determined whether VZV
infection proceeded toward a pathway likely involved with either virion production or cell-cell spread.