A novel automethylation reaction in the Aspergillus nidulans LaeA protein generates S‐methylmethionine (555.2)

The filamentous fungi in the genus Aspergillus are opportunistic plant and animal pathogens that produce secondary metabolites including lovastatin, penicillin, and aflatoxin. The synthesis of these small molecules is dependent on gene clusters that are globally regulated by the LaeA protein. Null mutants of LaeA show decreased virulence coupled with reduced secondary metabolism. Although LaeA contains the motifs characteristic of seven‐beta‐strand methyltransferases, a methyl‐accepting substrate of LaeA has not been identified. In this work, we did not find a methyl‐accepting substrate in A. nidulans with various assays including in vivo S‐adenosyl‐[methyl‐3H]methionine labeling, targeted in vitro methylation experiments using putative protein substrates, or in vitro methylation assays using whole cell extracts grown under different conditions. However, in each experiment LaeA was shown to self‐methylate. Amino acid hydrolysis of radioactively‐labeled LaeA followed by cation exchange and reverse phase chromatography identified methionine as the modified residue. Point mutations show that the major site of modification of LaeA is on methionine‐207. LaeA is the first protein to exhibit automethylation at a methionine residue. These findings not only indicate LaeA may perform novel chemistry with S‐adenosylmethionine, but also provide insights into the physiological function of LaeA. Grant Funding Source : Supported by Ruth L. Kirschstein National Research Service Award GM007185