title: The import route of ADP/ATP carrier into mitochondria separates from the general import pathway of cleavable preproteins at the trans side of the outer membrane.
abstract: The ADP/ATP carrier (AAC) of the mitochondrial inner membrane is synthesized in the cytosol without a cleavable presequence. The preprotein preferentially binds to the mitochondrial surface receptor Tom70 and joins the import pathway of presequence-carrying preproteins at the cis side of the outer membrane. Little is known about the translocation of the AAC across the outer membrane and where its import route separates from that of cleavable preproteins. Here we have characterized a translocation intermediate of AAC during transfer across the outer membrane. The major portion of the preprotein is exposed to the intermembrane space, while a short segment is still accessible to externally added protease. This intermediate can be quantitatively chased to the fully imported form in the inner membrane. Its accumulation depends on Tom7, but not on the intermembrane space domain of Tom22 in contrast to cleavable preproteins. Moreover, opening of the intermembrane space inhibits the import of AAC, but not that of cleavable preproteins into mitoplasts. We conclude that the import route of AAC diverges from the general import pathway of cleavable preproteins already at the trans side of the outer membrane.
authors.affiliation: Institut für Biochemie und Molekularbiologie, Universität Freiburg, Hermann-Herder-Strabetae 7, D-79104 Freiburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Kübrich
authors.forename: J
authors.initials: J
authors.lastname: Rassow
authors.forename: W
authors.initials: W
authors.lastname: Voos
authors.forename: N
authors.initials: N
authors.lastname: Pfanner
authors.forename: A
authors.initials: A
authors.lastname: Hönlinger
mesh_terms: D002352:Carrier Proteins / Q000378:metabolism; D005656:Fungal Proteins / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins; D008928:Mitochondria / Q000378:metabolism*; D000226:Mitochondrial ADP, ATP Translocases / Q000378:metabolism*; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D011956:Receptors, Cell Surface; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins
chemical_list: C037688:ADP-ATP carrier receptor; D002352:Carrier Proteins; D005656:Fungal Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D011956:Receptors, Cell Surface; D018160:Receptors, Cytoplasmic and Nuclear; D029701:Saccharomyces cerevisiae Proteins; C102549:TIM22 protein, S cerevisiae; C090857:TOM22 protein, S cerevisiae; C099634:TOM7 protein, S cerevisiae; D000226:Mitochondrial ADP, ATP Translocases