Characterization of Methyltransferase Properties of <i>Escherichia coli</i> YabC Protein with an Enzyme-Coupled Colorimetric Assay

An enzyme-coupled colorimetric assay for S-adenosylmethionine-dependent methyltransferase (MT) was established to characterize the enzymatic identity of YabC protein from Escherichia coli. Results showed that the MT activity of YabC is able to be effectively detected with this coupling assay system when filamentous cell lysates from an S-adenosylmethionine synthetase mutant, MEW402metK84, were employed as the source of methylation target, whereas no activity exhibited under same conditions from lysates from E. coli normal shape cells. Moreover, YabB, whose gene always clustered together with yabC, was demonstrated to be an inhibitor, but not a substrate of YabC protein. Analysis of each component's effect on methylation process further confirms that increase of MT's common product - S-adenosylhomocysteine - shows a methylation pattern depending on the protein concentration of cell lysates, which indicates that methylation target of YabC is the rate-limiting factor under current coupling assay conditions. Characterization of YabC's MT properties with the coupling assay system is consistent with the similar results identified with LC/MS and knockout strains by Kimura and Suzuki [Nucleic Acids Res 2010;38:1341-1352].