RamA, a Protein Required for Reductive Activation of Corrinoid-dependent Methylamine Methyltransferase Reactions in Methanogenic Archaea

• Published in: The Journal of biological chemistry 2009-01, Vol.284 (4), p.2285-2295
• Main Authors: Ferguson, Tsuneo, Soares, Jitesh A., Lienard, Tanja, Gottschalk, Gerhard, Krzycki, Joseph A.
• Format: Article
• Language: English
• Subjects: Adenosine Triphosphate - metabolism; Archaea; Archaeal Proteins - genetics; Archaeal Proteins - isolation & purification; Archaeal Proteins - metabolism; Corrinoids - metabolism; Enzyme Activation; Enzyme Catalysis and Regulation; Ferredoxins - genetics; Ferredoxins - metabolism; Genome, Archaeal - genetics; Methanosarcina barkeri; Methanosarcina barkeri - metabolism; Methylation; Methyltransferases - metabolism; Time Factors
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M807392200

Archaeal methane formation from methylamines is initiated by distinct methyltransferases with specificity for monomethylamine, dimethylamine, or trimethylamine. Each methylamine methyltransferase methylates a cognate corrinoid protein, which is subsequently demethylated by a second methyltransferase to form methyl-coenzyme M, the direct methane precursor. Methylation of the corrinoid protein requires reduction of the central cobalt to the highly reducing and nucleophilic Co(I) state. RamA, a 60-kDa monomeric iron-sulfur protein, was isolated from Methanosarcina barkeri and is required for in vitro ATP-dependent reductive activation of methylamine:CoM methyl transfer from all three methylamines. In the absence of the methyltransferases, highly purified RamA was shown to mediate the ATP-dependent reductive activation of Co(II) corrinoid to the Co(I) state for the monomethylamine corrinoid protein, MtmC. The ramA gene is located near a cluster of genes required for monomethylamine methyltransferase activity, including MtbA, the methylamine-specific CoM methylase and the pyl operon required for co-translational insertion of pyrrolysine into the active site of methylamine methyltransferases. RamA possesses a C-terminal ferredoxin-like domain capable of binding two tetranuclear iron-sulfur proteins. Mutliple ramA homologs were identified in genomes of methanogenic Archaea, often encoded near methyltrophic methyltransferase genes. RamA homologs are also encoded in a diverse selection of bacterial genomes, often located near genes for corrinoid-dependent methyltransferases. These results suggest that RamA mediates reductive activation of corrinoid proteins and that it is the first functional archetype of COG3894, a family of redox proteins of unknown function.