Methylates histone H3 at 'Lys-4' (H3K4me). Acts as a transcriptional repressor. Essential for cardiomyocyte differentiation and cardiac morphogenesis. 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 Interacts with HDAC1, HDAC2 and HDAC3. Interacts (via MYND-type zinc finger) with NACA isoform skNAC. Expressed in cardiac and skeletal muscle, lymphocytes and thymus. The SET domain is split between the S-sequence (residues 1-49) and the core SET domain (residues 181-258), however the two segments still come together to form a conserved SET domain fold. Belongs to the class V-like SAM-binding methyltransferase superfamily. Truncated N-terminus. Truncated N-terminus.