Tri-methylation of lysine-5 of histone H3 (H3K4) has been linked to transcriptional activation in a variety of eukaryotic species (Ruthenberg et al. 2007). Several H3K4 methyltransferases have been identified in mammals, predominantly members of the Mixed Lineage Leukemia (MLL) protein family. Five of these, KMT2A (MML1), KMT2D (MLL2), KMT2C (MLL3), KMT2B (MLL4) and SETD1A (KMT2F) have been shown to display H3K4 mono-, di- and tri-methyltransferase activity (Milne et al. 2002, Hughes et al. 2004, Cho et al. 2007, Wysocka et al. 2003). KMT2G (SETD1B) is believed to have similar activity on the basis of sequence homology (Ruthenberg et al. 2007). MLLs are a component of large multiprotein complexes that also include WDR5, RBBP5, ASH2 and DPY30, assembled to form the core MLL complex (Nakamura et al. 2002, Hughes et al. 2004, Dou et al. 2006, Tremblay et al. 2014). The WD40 domain of WDR5 recognizes and binds the histone H3 N-terminus, presenting the lysine-4 side chain for methylation by one of the catalytically active MLL family (Couture et al. 2006, Ruthenburg et al. 2006). Histone H3 recognition by WDR5 is regulated by the methylation state of adjacent arginine (H3R2) residue. H3R2 methylation abolishes WDR5 interaction with the H3 histone tail (Couture et al. 2006); H3K4 di-/trimethylation and H3R2 methylation have an inverse relationship (Guccione et al. 2006).
SETD7 (KMT7, SET9, SET7/9) is an H3K4 mono-methytransferase (Wang et al. 2001, Xiao et al. 2003, Hu & Zhang 2006) that can also methylate a wide range of non-histone proteins (Dhayalan et al. 2011). SETD3 can mono- and di-methylate H3K4 and H3K36 (Eom et al. 2011).