Methyltransferase

A methyltransferase is an enzyme transferring a methyl group from a donor to an acceptor. The acceptor can be a small molecule, DNA, RNA or a protein; the family name specifies a type of chemical transfer, not a universal biological function. Many methyltransferases use S-adenosylmethionine as donor. ^[ref]

For example, DNA methylation changes selected bases, while histone methylation modifies amino acid side chains in a different macromolecule. Different acceptor positions can have different consequences. A specific activity assay therefore needs a defined donor, acceptor and product measurement: disappearance of donor alone does not establish the intended site or exclude another methyl-transfer reaction.

SUV39H1 is a human histone methyltransferase that modifies lysine 9 in the amino-terminal region of histone H3. Its catalytic machinery includes a SET domain and adjoining cysteine-rich regions. The corresponding gene encodes an enzyme acting on a protein component of chromatin.^[ref]

The original biochemical work established site specificity using histone H3 substrates and mapped the catalytic requirement to regions of SUV39H1. It also found that lysine 9 methylation interfered with phosphorylation of neighbouring serine 10. This gives an illuminating example of interacting histone modifications: changing one residue can affect a modification at the next residue, so the histone tail is a chemically coupled substrate.^[ref]

For an enzyme assay, the informative measurement is formation of the specified methylated histone product. Simply detecting SUV39H1 expression establishes the presence of its protein, while activity additionally depends on the reaction conditions and available substrate.