Methanogenesis marker protein 10 (Mmp10) from Methanosarcina acetivorans is a radical S-adenosylmethionine methylase that unexpectedly requires cobalamin

Abstract

Methyl coenzyme M reductase (MCR) catalyzes the last step in the biological production of methane by methanogenic archaea, as well as the first step in the anaerobic oxidation of methane to methanol by methanotrophic archaea. MCR contains a number of unique post-translational modifications in its α subunit, including thioglycine, 1-<i>N</i>-methylhistidine, <i>S</i>-methylcysteine, 5-C-(<i>S</i>)-methylarginine, and 2-C-(<i>S</i>)-methylglutamine. Recently, genes responsible for the thioglycine and methylarginine modifications have been identified in bioinformatics studies and <i>in vivo</i> complementation of select mutants; however, none of these reactions has been verified <i>in vitro</i> Herein, we purified and biochemically characterized the radical <i>S</i>-adenosylmethionine (SAM) protein <i>Ma</i>Mmp10, the product of the methanogenesis marker protein 10 gene in the methane-producing archaea <i>Methanosarcina acetivorans</i> Using an array of approaches, including kinetic assays, LC-MS-based quantification, and MALDI TOF-TOF MS analyses, we found that <i>Ma</i>Mmp10 catalyzes the methylation of the equivalent of Arg²⁸⁵ in a peptide substrate surrogate, but only in the presence of cobalamin. We noted that the methyl group derives from SAM, with cobalamin acting as an intermediate carrier, and that <i>Ma</i>Mmp10 contains a C-terminal cobalamin-binding domain. Given that Mmp10 has not been annotated as a cobalamin-binding protein, these findings suggest that cobalamin-dependent radical SAM proteins are more prevalent than previously thought.