Histone recognition and nuclear receptor co-activator functions ofDrosophilaCara Mitad, a homolog of the N-terminal portion of mammalian MLL2 and MLL3

Abstract

SUMMARYMLL2 and MLL3 histone lysine methyltransferases are conserved components of COMPASS-like co-activator complexes. In vertebrates, the paralogous MLL2 and MLL3 contain multiple domains required for epigenetic reading and writing of the histone code involved in hormone-stimulated gene programming, including receptor-binding motifs, SET methyltransferase, HMG and PHD domains. The genes encoding MLL2 and MLL3 arose from a common ancestor. Phylogenetic analyses reveal that the ancestral gene underwent a fission event in some Brachycera dipterans, including Drosophila species, creating two independent genes corresponding to the N-and C-terminal portions. In Drosophila, the C-terminal SET domain is encoded by trithorax-related (trr), which is required for hormone-dependent gene activation. We identified the cara mitad (cmi) gene, which encodes the previously undiscovered N-terminal region consisting of PHD and HMG domains and receptor-binding motifs. The cmi gene is essential and its functions are dosage sensitive. CMI associates with TRR, as well as the EcR-USP receptor, and is required for hormone-dependent transcription. Unexpectedly, although the CMI and MLL2 PHDf3 domains could bind histone H3, neither showed preference for trimethylated lysine 4. Genetic tests reveal that cmi is required for proper global trimethylation of H3K4 and that hormone-stimulated transcription requires chromatin binding by CMI, methylation of H3K4 by TRR and demethylation of H3K27 by the demethylase UTX. The evolutionary split of MLL2 into two distinct genes in Drosophila provides important insight into distinct epigenetic functions of conserved readers and writers of the histone code.

The MLL3/MLL4 Branches of the COMPASS Family Function as Major Histone H3K4 Monomethylases at Enhancers

“…We identified Drosophila COMPASS family member Trr as a major H3K4 monomethylase functioning at the enhancer regions and showed that it is required in enhancer-promoter communication for faithful and tissue-specific gene expression (16,21). While Trr has long been considered a divergent form of MLL3 and MLL4 with homology restricted to the C-terminal halves of these proteins, it was recently found that a second gene encodes a protein with homology to the N-terminal halves of MLL3 and MLL4 (14,37). This protein, named Lpt, associates with Trr in a COMPASS-like complex and is also required for bulk H3K4me1 (14,21).…”

Trithorax monomethylates histone H3K4 and interacts directly with CBP to promote H3K27 acetylation and antagonize Polycomb silencing

“…MLL1 and MLL4 are human orthologs of TRX, and MLL2/ALR and MLL3 are human orthologs of TRR (Ardehali et al, 2011;Chauhan et al, 2012) (supplementary material Fig. S5).…”

Setd1b, encoding a histone 3 lysine 4 methyltransferase, is a maternal effect gene required for the oogenic gene expression program

“…These six come in three pairs: Setd1a and Setd1b, which are orthologous to the sole yeast H3K4 methyltransferase Set1 (Roguev et al, 2001); Mll1 (Kmt2a) and Mll2, which are homologous to Drosophila Trithorax (Francis and Kingston, 2001), and Mll3 (Kmt2c) and Mll4 (Kmt2d), which are homologous to a fusion of Drosophila Cara Mitad (Lpt) and trithorax-related (Chauhan et al, 2012;Herz et al, 2012). Each of the six appears to reside in its own protein complex.…”