The PcG protein HPC2 inhibits RBP‐J‐mediated transcription by interacting with LIM protein KyoT2

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Abstract
The DNA‐binding protein recombination signal‐binding protein‐Jk (RBP‐J) plays a key role in transcriptional regulation by targeting the intracellular domain of Notch (NIC) and the Epstein–Barr virus nuclear antigen 2 (EBNA2) to specific promoters. In the absence of the Notch signaling, RBP‐J acts as a transcriptional suppressor through recruiting co‐suppressors such as histone deacetylase (HDAC). KyoT2 is a LIM domain protein that suppresses the RBP‐J‐mediated transcriptional activation. In the current study, we show that the polycomb group (PcG) protein HPC2, which functions as a transcriptional suppressor, is a candidate of KyoT2‐binding proteins. To confirm the physical and functional interaction between KyoT2 and HPC2, we carried out yeast two‐hybrid, GST‐pull down, co‐immunoprecipitation, as well as mammalian two‐hybrid assays. Our results showed HPC2 and KyoT2 interacted both in vitro and in vivo, probably through the C‐terminal fragment of HPC2 and LIM domains of KyoT2. In addition, we also found that overexpression of HPC2, not only inhibited transactivation of a RBP‐J‐dependent promoter by NIC, but also transactivation by RBP‐J–VP16, a constitutively active form of RBP‐J. Taken together, our results suggested that KyoT2 might inhibit the RBP‐J‐mediated transactivation through NIC by recruiting co‐suppressors such as HPC2.