Electron transport during aceticlastic methanogenesis by Methanosarcina acetivorans involves a sodium-translocating Rnf complex

The anaerobic methanogenic archaeon Methanosarcina acetivorans lives under extreme energy limitation. Methanogenesis from acetate as carried out by M. acetivorans involves an anaerobic electron transport chain with ferredoxin as electron donor and heterodisulfide as electron acceptor, and so far only the heterodisulfide reductase has been shown to translocate H+. Here, we describe a second Na+-translocating coupling site in this electron transport chain. Inside-out membrane vesicles of M. acetivorans catalyzed Na+ transport coupled to an electron transport catalyzed by the ferredoxin:heterodisulfide oxidoreductase activity. Ionophore studies revealed that Na+ transport was primary and electrogenic. A ?rnf mutant was unable to grow on acetate and the ferredoxin:heterodisulfide oxidoreductase-coupled Na+ transport was abolished. These data are consistent with the hypothesis that the Rnf complex of M. acetivorans is an Na+-translocating coupling site and the entry point of electrons derived from reduced ferredoxin into the electron transport chain leading to the heterodisulfide.