title: The mitochondrial amidoxime-reducing component (mARC1) is a novel signal-anchored protein of the outer mitochondrial membrane.
abstract: The mitochondrial amidoxime-reducing component (mARC) was recently discovered as the fifth eukaryotic molybdenum cofactor-containing enzyme. The human genome encodes two mARC proteins, mARC1 and mARC2, sharing significant homologies with respect to sequence and function. Whereas mARC2 was identified as a mitochondrial enzyme, the subcellular localization of mARC1 has remained uncharacterized, although the similarity of both proteins suggested identical subcellular localizations. In addition, neither mARC1 nor mARC2 could be attributed unambiguously to one of the four mitochondrial subcompartments. Accordingly, mechanisms triggering the subcellular distribution of both enzymes have been unexplored so far. Here, we shed light on the subcellular localization of mARC1 and demonstrate that it is integrated into the outer mitochondrial membrane. The C-terminal catalytic domain of the protein remains exposed to the cytosol and confers an N((in))-C((out)) membrane orientation of mARC1. This localization is triggered by the N terminus of the enzyme, being composed of a weak N-terminal mitochondrial targeting signal and a downstream transmembrane helix. We demonstrate the transmembrane domain of mARC1 to be sufficient for mitochondrial targeting and the N-terminal targeting signal to function as a supportive receptor for the outer mitochondrial membrane. According to its localization and targeting mechanism, we classify mARC1 as a novel signal-anchored mitochondrial protein. During mitochondrial import, mARC1 is not processed, and membrane integration proceeds membrane potential independently but requires external ATP, which finally results in the assembly of mARC1 into high oligomeric protein complexes.
authors.affiliation: Institute of Biochemistry, Department of Chemistry and Center for Molecular Medicine, University of Cologne, 50674 Cologne, Germany.
authors.forename: Julian M
authors.initials: JM
authors.lastname: Klein
authors.forename: Jakob D
authors.initials: JD
authors.lastname: Busch
authors.forename: Christoph
authors.initials: C
authors.lastname: Potting
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Baker
authors.forename: Thomas
authors.initials: T
authors.lastname: Langer
authors.forename: Guenter
authors.initials: G
authors.lastname: Schwarz
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D002460:Cell Line; D003067:Coenzymes; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D053078:Membrane Potential, Mitochondrial; D008667:Metalloproteins; D051336:Mitochondrial Membranes / Q000378:metabolism*; D024101:Mitochondrial Proteins / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D000090006:Molybdenum Cofactors; D010088:Oxidoreductases / Q000737:chemistry / Q000378:metabolism*; D021382:Protein Sorting Signals; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D011621:Pteridines; D013347:Subcellular Fractions / Q000378:metabolism
chemical_list: D000255:Adenosine Triphosphate; D003067:Coenzymes; D008667:Metalloproteins; D024101:Mitochondrial Proteins; D000090006:Molybdenum Cofactors; D010088:Oxidoreductases; D021382:Protein Sorting Signals; D011621:Pteridines; C022261:molybdopterin; C554553:MTARC1 protein, human; C554552:mitochondrial amidoxime reducing component 2, human