Varicella-zoster virus Fc receptor component gI is phosphorylated on its endodomain by a cyclin-dependent kinase

Varicella-zoster virus (VZV) glycoprotein gI is a type 1 transmembrane glyc oprotein which is one component of the heterodimeric gE:gI Fc receptor comp lex. Like VZV gE, VZV gI was phosphorylated in both VZV-infected cells and gI-transfected cells. Preliminary studies demonstrated that a serine 343-pr oline 344 sequence located within the gI cytoplasmic tail was the most like ly phosphorylation site. To determine which protein kinase catalyzed the gI phosphorylation event, we constructed a fusion protein, consisting of glut athione-S-transferase (GST) and the gI cytoplasmic tail, called GST-gI-wt. When this fusion protein was used as a substrate for gI phosphorylation in vitro, the results demonstrated that GST-gI-wt fusion protein was phosphory lated by a representative cyclin-dependent kinase (CDK) called P-TEFb, a ho mologue of CDK1 (cdc2). When serine 343 within the serine-proline phosphory lation site was replaced with an alanine residue, the level of phosphorylat ion of the gI fusion protein was greatly reduced. Subsequent experiments wi th individually immunoprecipitated mammalian CDKs revealed that the VZV gI fusion protein was phosphorylated best by CDK1, to a lesser degree by CDK2, and not at all by CDK6. Transient-transfection assays carried out in the p resence of the specific CDK inhibitor roscovitine strongly supported the pr ior results by demonstrating a marked decrease in gI phosphorylation while gI protein expression was unaffected. Finally, the possibility that VZV gI contained a CDK phosphorylation site in its endodomain was of further inter est because its partner, gE, contains a casein kinase gI phosphorylation si te in its endodomain; prior studies have established that CDK1 can phosphor ylate casein kinase II.