Molecular basis for chromatin binding and regulation of MLL5

The human mixed lineage leukemia 5 (MLL5) protein mediates hematopoietic cell homeostasis, cell cycle and survival, however, the molecular basis underlying MLL5 activities remains unknown. Here, we show that MLL5 is recruited to gene-rich euchromatic regions via the interaction of its PHD finger with the histone mark H3K4me3. The 1.48 Å resolution crystal structure of the MLL5 PHD finger in complex with the H3K4me3 peptide reveals a non-canonical binding mechanism, whereby K4me3 is recognized through a single aromatic residue and an aspartate. The binding induces a His-Asp swapping rearrangement mediated by a C-terminal α-helix. Phosphorylation of H3T3 and H3T6 abrogates the association with H3K4me3 in vitro and in vivo, releasing MLL5 from chromatin in mitosis. This regulatory switch is conserved in the Drosophila ortholog of MLL5, MLL5, and suggests the developmental control for targeting of H3K4me3. Together, our findings provide insight into the molecular basis for the recruitment, exclusion and regulation of MLL5 at chromatin. For determaning MLL5 chromatin profile, DamID-MLL5 chromatin profiling was determined in C2C12 cells . Three biological replicates were performed. Normalized data was averaged and HMM approach was applied to establish the bound regions (Straub, T., Grimaud, C., Gilfillan, G. D., Mitterweger, A., and Becker, P. B. (2008). PLoS Genet 4, e1000302)