Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA-3C) is one of five viral nuclear proteins that are essential for EBV-induced immortalization of primary human B lymphocytes in vitro. Previous studies have implied that EBNA-3C acts as a transcription factor. Using transient transfection assays, we demonstrate that EBNA-3C has two effects on reporter genes that are linked to the latent membrane protein 1 promoter, (i) low-level activation by EBNA-3C alone, as well as potentiation of EBNA-2-mediated transactivation, and (ii) inhibition of the normally strong activation mediated by EBNA-2. These two disparate effects seem to be mediated at different stages following cell feeding. The inhibitory effect of EBNA-3C was localized to a known EBNA-2 response element that had previously been shown to be recognized by the DNA-binding protein RBP-J kappa. In addition, direct interaction between RBP-J kappa and EBNA-3C was observed by coimmunoprecipitation. Activation by EBNA-3C, however, seems to be achieved via sequences that are distinct from RBP-J kappa sites, since activation remained even after these sites had been mutated. Consistent with its ability to activate transcription, a region of EBNA-3C which has homology to the glutamine-rich activation domain of Sp1 can function as a transcription activation domain when it is fused to the heterologous DNA-binding domain of Ga14 and can partially restore the activity of a mutant EBNA-2 protein with a deletion in the transactivation domain. Collectively, these data strongly support the role of EBNA-3C as a transcriptional regulator.
