Structural insights into SETD3-mediated histidine methylation on beta-actin.

SETD3 is a member of SET (Su(var)3-9, Enhancer of zeste, and Trithorax) domain protein superfamily and plays important roles in hypoxic pulmonary hypertension, muscle differentiation, and carcinogenesis. Recently, we have identified SETD3 as the actin-specific methyltransferase that methylates the N3 of His73 on β-actin. Here we present two structures of S-adenosyl-L-homocysteine-bound SETD3 in complex with either an unmodified β-actin peptide or its His-methylated variant. Structural analyses supported by the site-directed mutagenesis experiments and the enzyme activity assays indicated that the recognition and methylation of β-actin by SETD3 is highly sequence specific, and both SETD3 and β-actin adopt pronounce conformational changes upon binding to each other. In conclusion, the data show for the first time a catalytic mechanism of SETD3-mediated histidine methylation in β-actin, which not only throws light on protein histidine methylation phenomenon, but also facilitates the design of small molecule inhibitors of SETD3.