title: The delta psi- and Hsp70/MIM44-dependent reaction cycle driving early steps of protein import into mitochondria.
abstract: New steps in the reaction cycle that drives protein translocation into the mitochondrial matrix have been defined. The membrane potential (delta psi)- and the mtHsp70/MIM44-dependent import machinery cooperate in the transfer of the presequence across the inner membrane. Translocation intermediates, arrested at a stage where only the presequence could form a complex with mtHsp70, still required delta psi for further import. Delta psi at this stage prevented retrograde movement, since mtHsp70 did not bind to the presequence with sufficient affinity. In contrast, mature regions of incoming chains adjacent to the presequence were bound by mtHsp70 tightly enough to stabilize them in the matrix. Cycling of the mtHsp70 on and off incoming chains is a continuous process in the presence of matrix ATP. Both MIM44-bound and free forms of mtHsp70 were found in association with the incoming chains. These data are consistent with a reaction pathway in which the mtHsp70/MIM44 complex acts as a molecular ratchet on the cis side of the inner membrane to drive protein translocation into the matrix.
authors.affiliation: Institut für Physiologische Chemie der Universität München, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Ungermann
authors.forename: B
authors.initials: B
authors.lastname: Guiard
authors.forename: W
authors.initials: W
authors.lastname: Neupert
authors.forename: D M
authors.initials: DM
authors.lastname: Cyr
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D001693:Biological Transport, Active; D002352:Carrier Proteins / Q000378:metabolism*; D003432:Cross-Linking Reagents; D005656:Fungal Proteins / Q000378:metabolism*; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism; D008564:Membrane Potentials; D008565:Membrane Proteins / Q000378:metabolism*; D008928:Mitochondria / Q000378:metabolism*; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D012441:Saccharomyces cerevisiae; D029701:Saccharomyces cerevisiae Proteins
chemical_list: D002352:Carrier Proteins; D003432:Cross-Linking Reagents; D005656:Fungal Proteins; D018840:HSP70 Heat-Shock Proteins; D008565:Membrane Proteins; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D029701:Saccharomyces cerevisiae Proteins; C077016:TIM44 protein, S cerevisiae; D000255:Adenosine Triphosphate