Histone methyltransferase that acts as a coactivator for the ecdysone receptor during development. Specifically trimethylates 'Lys-4' of histone H3, a specific tag for epigenetic transcriptional activation. Recruited by EcR in an ecdysone-dependent manner causing H3 'Lys-4' trimethylation at ecdysone-inducible promoters, leading to activate expression. Plays a central role in the developing compound eye, during the progression of the morphogenetic furrow and in post-furrow differentiation of the retinal epithelium, notably by activating expression of hh. Also required for wing and abdominal development. L-lysyl(4)-[histone H3] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl(4)-[histone H3] + 3 S-adenosyl-L-homocysteine Component of the MLL3/4 complex composed at least of the catalytic subunit trr, ash2, Rbbp5, Dpy-30L1, wds, hcf, ptip, Pa1, Utx, Lpt and Ncoa6. Interacts with nuclear receptor EcR in an ecdysone-dependent manner. Interacts with ash2; the interaction stabilizes trr. Widely expressed. Expressed both maternally and zygotically. Abundantly expressed in 1-7 than in 7-20 hours old embryos. Expressed uniformly at the preblastoderm stage, prior to the onset of zygotic transcription. In ovaries, it is not expressed in ovarian stem cells, oogonia or early cysts and is first detectable at stage 8 in nurse cells. At stage 10, it is expressed in the anterior end of the oocyte, and is uniformly distributed later on. Expressed almost uniformly in embryos from precellular blastoderm stage to the germband extended stage. At the germband extended stage, it is enriched in the mesoderm. During germband retraction, it is strongly expressed in the anterior and posterior midgut. At the germband retracted stage, it becomes less abundant and is mainly localized to the ventral nerve cord and the brain. In third instar larvae, it is strongly and almost ubiquitously expressed in all imaginal disks. Also weakly expressed expression in salivary glands. Not expressed in larval brain and gut tissues. Contains 3 Leu-Xaa-Xaa-Leu-Leu (LXXLL) motifs. LXXLL motif 2 is essential for the association with nuclear receptor EcR. Defects in mechanosensory bristle spacing and differentiation in wings and lack of chaetes and macrochaetes in abdominal a4 and a5 segments. Produced by alternative promoter usage. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. TRX/MLL subfamily. Truncated N-terminus. Truncated N-terminus.