Multiple transactivation domains of EZH2 bind to the TAZ2 domain of p300 and stimulate the acetyltransferase function of p300

The H3K27me-specific methyltransferase enhancer of zeste homologue 2 (EZH2) is the catalytic subunit of the repressive complex Polycomb repressive complex 2. EZH2 is typically implicated in transcriptional silencing, but it can also activate gene expression. Here, we show that EZH2 contains three adjacent transactivation domains (EZH2_TAD) that are recognized by the TAZ2 domain of the transcriptional coactivator and acetyltransferase p300 (p300_TAZ2). Binding interfaces identified by chemical shift perturbations in NMR experiments, measurements of binding affinities, and analysis of the complex formation by mass photometry demonstrate that each EZH2_TAD can be concomitantly bound by a separate p300_TAZ2. Interaction of EZH2_TADs with p300_TAZ2 stimulates H3K18- and H3K27-specific acetyltransferase activity of p300. We show that in 22Rv1 prostate cancer cells, EZH2 occupies a large set of gene loci lacking H3K27me3, and these non-canonical genomic sites are instead co-occupied by p300, RNA Polymerase II and BRD4 and are rich in histone marks associated with transcriptional activation. Our findings shed light on the potential basis for such a high degree of genetic co-localization through the direct association of p300_TAZ2 with EZH2_TADs.