Polycomb (Pc) is part of a Pc group (PcG) protein complex that is involved in repression of gene activity during Drosophila and vertebrate development. To identify proteins that interact with vertebrate Pc homologs, we performed two-hybrid screens with Xenopus Pc (XPc) and human Pc 2 (HPC2). We find that the C-terminal binding protein (CtBP) interacts with XPc and HPC2, that CtBP and HPC2 coimmunoprecipitate, and that CtBP and HPC2 partially colocalize in large PcG domains in interphase nuclei. CtBP is a protein with unknown function that binds to a conserved 6-amino-acid motif in the C terminus of the adenovirus E1A protein. Also, the Drosophila CtBP homolog interacts, through this conserved amino acid motif, with several segmentation proteins that act as repressors. Similarly, we find that CtBP binds with HPC2 and XPc through the conserved 6-amino-acid motif. Importantly, CtBP does not interact with another vertebrate Pc homolog, M33, which lacks this amino acid motif, indicating specificity among vertebrate Pc homologs. Finally, we show that CtBP is a transcriptional repressor. The results are discussed in terms of a model that brings together PcG-mediated repression and repression systems that require corepressors such as CtBP.
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