FUNCTIONAL ANTAGONISM BETWEEN THE RETINOIC ACID RECEPTOR AND THE VIRAL TRANSACTIVATOR BZLF1 IS MEDIATED BY PROTEIN-PROTEIN INTERACTIONS

The Epstein-Barr virus-encoded protein BZLF1 is a member of the basic leucine zipper (bZip) family of transcription factors. Like several ot her members of the bZip family, transcriptional activity of BZLF1 is m odulated by retinoic acid receptors (RARs). We present evidence that t he RAR alpha and BZLF1 can reciprocally repress each other's transcrip tional activation by a newly discovered mechanism. Analysis of RAR alp ha mutants in transfection studies reveals that the DNA binding domain is sufficient for inhibition of BZLF1 activity. Analysis of BZLF1 mut ants indicates that both the coiled-coil dimerization domain and a reg ion containing the transcriptional activation domain of BZLF1 are requ ired for transrepression. Coimmunoprecipitation experiments demonstrat e physical interactions between RAR alpha and BZLF1 in vivo. Furthermo re, glutathione S-transferase-pulldown assays reveal that these protei n-protein interactions are mediated by the coiled-coil dimerization do main of BZLF1 and the DNA binding domain of RAR alpha. While RAR alpha is unable to recognize BZLF1 binding sites, the RAR alpha can be teth ered to the DNA by forming a heteromeric complex with BZLF1 bound to D NA. Tethering RARs via protein-protein interactions onto promoter DNA suggest a mechanism through which RARs might gain additional levels of transcriptional regulation.