MCM family proteins are only found in methanogenic bacteria. These systems are comprised of a protein with an ABC2 organization. A protein encoded by the atw gene was found to be essential for the in vitro activity of the nickel enzyme methyl-coenzyme M reductase, catalyzing the terminal step of methane formation in the energy metabolism of all methanogenic archaea ( Pubmed : 3980447 ) , ( Pubmed : 8491734 ) . This gene was located apart from the mcr operon encoding the subunits of methyl-coenzyme M reductase. More recent purification procedures demonstrated that this protein was dispensable and was probably a contaminant ( Pubmed : 2506016 ) . The function of MCM proteins is therefore unknown.