RING1 inhibits transactivation of RBP-J by Notch through interaction with LIM protein KyoT2.

The DNA-binding protein recombination signal binding protein-Jkappa (RBP-J) mediates transcriptional activation of the Notch intracellular domain (NIC). In the absence of transcriptional activators, RBP-J suppresses transcription by recruiting co-suppressors. KyoT2 is a LIM domain protein that inhibits the RBP-J-mediated transcriptional activation. Here we provide evidence that the polycomb group protein RING1 interacts with the LIM domains of KyoT2 in yeast and mammalian cells. The interaction between KyoT2 and RING1 was detected both in vitro and in vivo. By using a co-immunoprecipitation assay, we also showed that, though RING1 and RBP-J did not associate directly, the two molecules could be co-precipitated simultaneously by KyoT2, probably through the LIM domains and the RBP-J-binding motif of KyoT2, respectively. These results suggested the formation of a three-molecule complex consisting of RBP-J, KyoT2 and RING1 in cells. Moreover, we found that overexpression of RING1 together with KyoT2 in cells inhibited transactivation of RBP-J by NIC. Suppression of the NIC- mediated transactivation of RBP-J by RING1 was abrogated by overexpression of KBP1, a molecule that competed with RING1 for binding to LIM domains of KyoT2, suggesting that suppression of RBP-J by RING1 was dependent on its associating with KyoT2. Taken together, our data suggested that there might be at least two ways of the KyoT2-mediated suppression of RBP-J, namely competition for binding sites with transactivators, and recruitment of suppressors such as RING1.