A method to site-specifically incorporate methyl-lysine analogues into recombinant proteins

The site-specific and degree-specific methylation of histone lysine residues is important for the regulation of chromatin. To study the biochemical roles of lysine methylation, several approaches have been developed to reconstitute chromatin fibers in vitro with well-defined methylation patterns. Here, we describe the installation of methyl-lysine analogues (MLAs) as a simple and scalable method to introduce mono-, di-, or trimethylation at specific sites of recombinantly expressed histones. In this method, a histone is engineered to harbor a lysine-to-cysteine mutation at the desired site of modification. These mutant histones are treated with halo-ethylamines that react with the cysteine side chain, providing high yields of N-methylated aminoethylcysteines, analogues of N-methylated lysine residues. These MLA histones have been used to construct well-defined chromatin templates to study the direct biochemical consequences of histone lysine methylation in a variety of contexts.