Post-translational thioamidation of methyl-coenzyme M reductase, a key enzyme in methanogenic and methanotrophic Archaea

The enzyme methyl-coenzyme M reductase (MCR), found in strictly anaerobic methanogenic and methanotrophic archaea, catalyzes a reversible reaction involved in the production and consumption of the potent greenhouse gas methane. The α subunit of this enzyme (McrA) contains several unusual post-translational modifications, including an exceptionally rare thioamidation of glycine. Based on the presumed function of homologous genes involved in the biosynthesis of thioamide-containing natural products, we hypothesized that the archaeal tfuA and ycaO genes would be responsible for post-translational installation of thioglycine into McrA. Mass spectrometric characterization of McrA in a Δ ycaO-tfuA mutant of the methanogenic archaeon Methanosarcina acetivorans revealed the presence of glycine, rather than thioglycine, supporting this hypothesis. Physiological characterization of this mutant suggested a new role for the thioglycine modification in enhancing protein stability, as opposed to playing a direct catalytic role. The universal conservation of this modification suggests that MCR arose in a thermophilic ancestor.