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

Phorbol esters reactivate Epstein-Barr virus (EBV) from latently infected c ells via transcriptional activation of the viral immediate-early gene BZLF1 . BZLF1 is a member of the extended AP-1 family of transcription factors th at 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 tr anscriptional level as well as through post-translational modifications. Re cently, 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. 7 2, 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 an d in vivo after stimulation with 12-O-tetradecanoylphorbol-13-acetate (TPA) . Here, we identified RACK1 as a binding partner of BZLF1 in a yeast intera ction trap assay. RACK stands for receptor of activated C-kinase and is inv olved in targeting activated PKCs and other signaling proteins. In vivo, RA CK1 binds directly to the transactivation domain of BZLF1. Although a funct ional 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 a s a scaffolding protein on BZLF1 independently of activated PKCs.