Methylates histone H3 at 'Lys-4' (H3K4me), seems able to perform both mono-, di-, and trimethylation. 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. Expression seems mostly restricted to heart and skeletal muscle. By serum response factor SRF and myogenin. SRF binds to the CArG site and MYOG binds to the E-box element on SMYD1 promoter. 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.