The lytic switch protein of KSHV activates gene expression via functional interaction with RBP-Jkappa (CSL), the target of the Notch signaling pathway.

The RTA protein of the Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is responsible for the switch from latency to lytic replication, a reaction essential for viral spread and KS pathogenesis. RTA is a sequence-specific transcriptional activator, but the diversity of its target sites suggests it may act via interaction with host DNA-binding proteins as well. Here we show that KSHV RTA interacts with the RBP-Jkappa protein, the primary target of the Notch signaling pathway. This interaction targets RTA to RBP-Jkappa recognition sites on DNA and results in the replacement of RBP-Jkappa's intrinsic repressive action with activation mediated by the C-terminal domain of RTA. Mutation of such sites in target promoters strongly impairs RTA responsiveness. Similarly, such target genes are induced poorly or not at all by RTA in fibroblasts derived from RBP-Jkappa(-/-) mice, a defect that can be reversed by expression of RBP-Jkappa. In vitro, RTA binds to two adjacent regions of RBP-Jkappa, one of which is identical to the central repression domain that binds the Notch effector fragment. These results indicate that KSHV has evolved a ligand-independent mechanism for constitutive activation of the Notch pathway as a part of its strategy for reactivation from latency.