To modulate expression of target genes, nuclear receptors cooperate with coactivators and corepressors that are recruited to promoters by their interactions with nuclear receptors. Mixed-lineage leukemia 2 (MLL2) and MLL3 are mammalian nuclear receptor coactivators that contain many conserved elements, including a histone methyltransferase (Set) domain, an HMG-I binding domain, a nuclear receptor binding domain, and five zinc fingers. These MLLs are part of COMPASS (complex proteins associated with Set1)–like coactivator complexes that are required for nuclear receptor–mediated gene regulation in several contexts. Trithorax-related (TRR) is the closest relative of these proteins in the fruit fly Drosophila melanogaster, but TRR is much smaller than its mammalian counterparts, with homology only to the C-terminal histone methyltransferase domain of the MLLs. Chauhan et al. searched the Drosophila genome and found a gene they named cara mitad (cmi) that encoded a protein homologous to the N-terminal half of MLL2 and MLL3, containing several zinc finger motifs, an HMG-I binding domain, and putative nuclear receptor–binding motifs. The authors identified single genes encoding MLL homologs in sea anemone, nematodes, and beetles, suggesting that an ancestral MLL gene underwent a fission event in the fly lineage. Mutational analysis, RNA interference, and genetic interaction experiments were consistent with CMI cooperating with TRR to mediate developmental responses to the steroid hormone ecdysone, and induction of ecdysone-responsive genes was reduced when cmi was knocked down in cultured cells and in vivo. Knocking down ultraspiracle (usp), which encodes the heterodimeric partner of the ecdysone receptor (EcR), rescued phenotypes induced by overexpression of cmi in the wing. CMI acted as a transcriptional coactivator with USP and EcR in an in vivo reporter assay, was recruited to an endogenous ecdysone-regulated promoter in vivo, and formed complexes with TRR, EcR, and USP in vitro. CMI bound to histone H3 in vitro and was required for histone methylation mediated by TRR and for histone demethylation mediated by the fly homolog of UTX, which is present in mammalian COMPASS-like complexes. CMI activity depended on the presence of other putative components of the fly COMPASS-like complex, implying that the components of this complex work together in the fly in a manner similar to their function in mammals. Although the genes encoding CMI and TRR are located on separate chromosomes, they apparently come together to reconstitute full nuclear receptor coactivator function.
C. Chauhan, C. B. Zraly, M. Parilla, M. O. Diaz, A. K. Dingwall, Histone recognition and nuclear receptor co-activator functions of Drosophila Cara Mitad, a homolog of the N-terminal portion of mammalian MLL2 and MLL3. Development 139, 1997–2008 (2012). [Abstract] [Full Text]