The Polycomb group protein (PcG) SUZ12 forms Polycomb repressive complexes together with histone methyltransferase EZH2. Although the complexes have been demonstrated to be involved in epigenetic maintenance of gene expression in a transcriptional repressive state, it is unclear how they are recruited to the target genes. Here we report that SUZ12 directly interacts with site-specific transcriptional repressor E2F6 and forms a complex together with EZH2. SUZ12 interacts with E2F6 selectively among the E2F family proteins and E2F6- containing SUZ12-EZH2 complex was biochemically purified from HEK293 cells stably expressing Flag-tagged SUZ12. Chromatin immunoprecipitation assays revealed the target genes of the E2F6-SUZ12-EZH2 complex. Contrary to expectation, the promoter regions of these genes are not or only weakly tri-methylated at histone H3-K27, and their expression is down-regulated by depletion of EZH2. Given that the transactivation function of SUZ12-EZH2 has been previously reported, the inhibitory effect on E2F6-mediated transcriptional repression by physical interaction can be considered a candidate mechanism of gene activation by these PcGs.
Keywords: E2F transcription factor; chromatin immunoprecipitation; histone methylation; polycomb group protein; transcriptional regulation.