Man Mohan

Monomethylation of histone H3 on Lys 4 (H3K4me1) and acetylation of histone H3 on Lys 27 (H3K27ac) are histone modifications that are highly enriched over the body of actively transcribed genes and on enhancers. Although in yeast all H3K4 methylation patterns, including H3K4me1, are implemented by Set1/COMPASS (complex of proteins associated with Set1), there are three classes of COMPASS-like complexes in Drosophila that could carry out H3K4me1 on enhancers: dSet1, Trithorax, and Trithorax-related (Trr). Here, we report that Trr, the Drosophila homolog of the mammalian Mll3/4 COMPASS-like complexes, can function as a major H3K4 monomethyltransferase on enhancers in vivo. Loss of Trr results in a global decrease of H3K4me1 and H3K27ac levels in various tissues. Assays with the cut wing margin enhancer implied a functional role for Trr in enhancer-mediated processes. A genome-wide analysis demonstrated that Trr is required to maintain the H3K4me1 and H3K27ac chromatin signature that resembles the histone modification patterns described for enhancers. Furthermore, studies in the mammalian system suggested a role for the Trr homolog Mll3 in similar processes. Since Trr and mammalian Mll3/4 complexes are distinguished by bearing a unique subunit, the H3K27 demethylase UTX, we propose a model in which the H3K4 monomethyltransferases Trr/Mll3/Mll4 and the H3K27 demethylase UTX cooperate to regulate the transition from inactive/poised to active enhancers.

Epigenetic modifications of chromatin play an important role in the regulation of gene expression. KMT4/Dot1 is a conserved histone methyltransferase capable of methylating chromatin on Lys79 of histone H3 (H3K79). Here we report the identification of a multisubunit Dot1 complex (DotCom), which includes several of the mixed lineage leukemia (MLL) partners in leukemia such as ENL, AF9/MLLT3, AF17/MLLT6, and AF10/MLLT10, as well as the known Wnt pathway modifiers TRRAP, Skp1, and beta-catenin. We demonstrated that the human DotCom is indeed capable of trimethylating H3K79 and, given the association of beta-catenin, Skp1, and TRRAP, we investigated, and found, a role for Dot1 in Wnt/Wingless signaling in an in vivo model system. Knockdown of Dot1 in Drosophila results in decreased expression of a subset of Wingless target genes. Furthermore, the loss of expression for the Drosophila homologs of the Dot1-associated proteins involved in the regulation of H3K79 shows a similar reduction in expression of these Wingless targets. From yeast to human, specific trimethylation of H3K79 by Dot1 requires the monoubiquitination of histone H2B by the Rad6/Bre1 complex. Here, we demonstrate that depletion of Bre1, the E3 ligase required for H2B monoubiquitination, leads specifically to reduced bulk H3K79 trimethylation levels and a reduction in expression of many Wingless targets. Overall, our study describes for the first time the components of DotCom and links the specific regulation of H3K79 trimethylation by Dot1 and its associated factors to the Wnt/Wingless signaling pathway.

Methylation of histone H3 lysine 4 (H3K4) in Saccharomyces cerevisiae is implemented by Set1/COMPASS, which was originally purified based on the similarity of yeast Set1 to human MLL1 and Drosophila melanogaster Trithorax (Trx). While humans have six COMPASS family members, Drosophila possesses a representative of the three subclasses within COMPASS-like complexes: dSet1 (human SET1A/SET1B), Trx (human MLL1/2), and Trr (human MLL3/4). Here, we report the biochemical purification and molecular characterization of the Drosophila COMPASS family. We observed a one-to-one similarity in subunit composition with their mammalian counterparts, with the exception of LPT (lost plant homeodomains [PHDs] of Trr), which copurifies with the Trr complex. LPT is a previously uncharacterized protein that is homologous to the multiple PHD fingers found in the N-terminal regions of mammalian MLL3/4 but not Drosophila Trr, indicating that Trr and LPT constitute a split gene of an MLL3/4 ancestor. Our study demonstrates that all three complexes in Drosophila are H3K4 methyltransferases; however, dSet1/COMPASS is the major monoubiquitination-dependent H3K4 di- and trimethylase in Drosophila. Taken together, this study provides a springboard for the functional dissection of the COMPASS family members and their role in the regulation of histone H3K4 methylation throughout development in Drosophila.