ASXL1/2 links MLL4 and BAP1 on enhancers

Histone H3K4 monomethyltransferases MLL3 and MLL4 contain a set of uncharacterized PHD fingers. By structural and biochemical assays, we found a novel function of the PHD2 and PHD3 (PHD2/3) fingers of MLL3 and MLL4, revealing their direct binding to the conserved MBH (MLL binding helix) region of ASXL1/2, components of the Polycomb repressive PR-DUB complex. In mouse embryonic stem cells, we observed that BAP1, the catalytic subunit of the PR-DUB complex, physically interacts with MLL4 in an ASXL1/2 MBH-dependent manner. Genomic studies demonstrate that the ASXL1/2 MBH is required for BAP1 binding on active enhancers and suggest that MLL4 facilitates BAP1 binding on active enhancers through ASXL1/2 MBH.