The PKC targeting protein RACK1 interacts with the Epstein–Barr virus activator protein BZLF1

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Abstract
Phorbol esters reactivate Epstein–Barr virus (EBV) from latently infected cells via transcriptional activation of the viral immediate‐early gene BZLF1. BZLF1 is a member of the extended AP‐1 family of transcription factors that binds to specific BZLF1‐binding motifs within early EBV promoters and to consensus AP‐1 sites. Regulation of BZLF1's activity is achieved at the transcriptional level as well as through post‐translational modifications. Recently, we reported that the transcriptional activity of BZLF1 is augmented by TPA [Baumann, M., Mischak, H., Dammeier, S., Kolch, W., Gires, O., Pich, D., Zeidler, R., Delecluse, H. J. & Hammerschmidt, W., (1998) J. Virol.72, 8105–8114]. The increase of BZLF1's activity depends on a single serine residue (S186) that is phosphorylated by protein kinase C (PKC) in vitro and in vivo after stimulation with 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA). Here, we identified RACK1 as a binding partner of BZLF1 in a yeast interaction trap assay. RACK stands for receptor of activated C‐kinase and is involved in targeting activated PKCs and other signaling proteins. In vivo, RACK1 binds directly to the transactivation domain of BZLF1. Although a functional relationship between BZLF1 and PKC could be mediated by RACKs, RACK1 did not have a detectable effect on the phosphorylation status of BZLF1 in in vitro or in vivo phosphorylation assays. We suggest that RACK1 may act as a scaffolding protein on BZLF1 independently of activated PKCs.