Polycomblike PHD fingers mediate conserved interaction with enhancer of zeste protein

The products of Polycomb group (PcG) genes are required for the epigenetic repression of a number of important developmental regulatory genes, includi ng homeotic genes. Enhancer of zeste (E(Z)) is a Drosophila PcG protein tha t previously has been shown to bind directly to another PcG protein, Extra Sex Combs (ESC), and is present along with ESC in a 600-kDa complex in Dros ophila embryos. Using yeast two-hybrid and in vitro binding assays, we show that E(Z) binds directly to another PcG protein, Polycomblike (PCL). PCL-E (Z) interaction is shown to be mediated by the plant homeodomain (PHD) fing ers domain of PCL, providing evidence that this motif can act as an indepen dent protein interaction domain. An association was also observed between P HF1 and EZH2, human homologs of PCL and E(Z), respectively, demonstrating t he evolutionary conservation of this interaction. EM was found to not inter act with the PHD domains of three Drosophila trithorax group (trxG) protein s, which function to maintain the transcriptional activity of homeotic gene s, providing evidence for the specificity of the interaction of E(Z) with t he PCL PHD domain. Coimmunoprecipitation and gel filtration experiments dem onstrate in vivo association of PCL with EM and ESC in Drosophila embryos. We discuss the implications of PCL association with ESC.E(Z) complexes and the possibility that PCL may either be a subunit of a subset of ESC.E(Z) co mplexes or a subunit of a separate complex that interacts with ESC.E(Z) com plexes.