title: Characterization of the signal that directs Tom20 to the mitochondrial outer membrane.
abstract: Tom20 is a major receptor of the mitochondrial preprotein translocation system and is bound to the outer membrane through the NH(2)-terminal transmembrane domain (TMD) in an Nin-Ccyt orientation. We analyzed the mitochondria-targeting signal of rat Tom20 (rTom20) in COS-7 cells, using green fluorescent protein (GFP) as the reporter by systematically introducing deletions or mutations into the TMD or the flanking regions. Moderate TMD hydrophobicity and a net positive charge within five residues of the COOH-terminal flanking region were both critical for mitochondria targeting. Constructs without net positive charges within the flanking region, as well as those with high TMD hydrophobicity, were targeted to the ER-Golgi compartments. Intracellular localization of rTom20-GFP fusions, determined by fluorescence microscopy, was further verified by cell fractionation. The signal recognition particle (SRP)-induced translation arrest and photo-cross-linking demonstrated that SRP recognized the TMD of rTom20-GFP, but with reduced affinity, while the positive charge at the COOH-terminal flanking segment inhibited the translation arrest. The mitochondria-targeting signal identified in vivo also functioned in the in vitro system. We conclude that NH(2)-terminal TMD with a moderate hydrophobicity and a net positive charge in the COOH-terminal flanking region function as the mitochondria-targeting signal of the outer membrane proteins, evading SRP-dependent ER targeting.
authors.affiliation: Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka 812-8582, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Kanaji
authors.forename: J
authors.initials: J
authors.lastname: Iwahashi
authors.forename: Y
authors.initials: Y
authors.lastname: Kida
authors.forename: M
authors.initials: M
authors.lastname: Sakaguchi
authors.forename: K
authors.initials: K
authors.lastname: Mihara
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport; D019556:COS Cells; D002451:Cell Compartmentation; D003546:Cysteine Endopeptidases / Q000187:drug effects; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D049452:Green Fluorescent Proteins; D007425:Intracellular Membranes / Q000378:metabolism*; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D026901:Membrane Transport Proteins; D008928:Mitochondria / Q000378:metabolism*; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000187:drug effects; D046988:Proteasome Endopeptidase Complex; D021382:Protein Sorting Signals; D017434:Protein Structure, Tertiary; D051381:Rats; D011956:Receptors, Cell Surface; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D018271:Signal Recognition Particle / Q000378:metabolism
chemical_list: D015853:Cysteine Proteinase Inhibitors; D008164:Luminescent Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D009097:Multienzyme Complexes; D021382:Protein Sorting Signals; D011956:Receptors, Cell Surface; D011993:Recombinant Fusion Proteins; D018271:Signal Recognition Particle; C489998:Tomm20 protein, rat; D049452:Green Fluorescent Proteins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex