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J. Biol. Chem., Vol. 283, Issue 1, 120-127, January 4, 2008

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Biogenesis of the Mitochondrial TOM Complex

Mim1 PROMOTES INSERTION AND ASSEMBLY OF SIGNAL-ANCHORED RECEPTORS^*

Thomas Becker, Sylvia Pfannschmidt, Bernard Guiard, Diana Stojanovski¹, Dusanka Milenkovic, Stephan Kutik^¶2, Nikolaus Pfanner³, Chris Meisinger, and Nils Wiedemann

From the Institut für Biochemie und Molekularbiologie, Zentrum für Biochemie und Molekulare Zellforschung, Universität Freiburg, D-79104 Freiburg, Germany, the Centre de Génétique Moléculaire, CNRS, F-91190 Gif-sur-Yvette, France, and the ^¶Fakultät für Biologie, Universität Freiburg, D-79104 Freiburg, Germany

The translocase of the outer membrane (TOM complex) is the central entry gate for nuclear-encoded mitochondrial precursor proteins. All Tom proteins are also encoded by nuclear genes and synthesized as precursors in the cytosol. The channel-forming β-barrel protein Tom40 is targeted to mitochondria via Tom receptors and inserted into the outer membrane by the sorting and assembly machinery (SAM complex). A further outer membrane protein, Mim1, plays a less defined role in assembly of Tom40 into the TOM complex. The three receptors Tom20, Tom22, and Tom70 are anchored in the outer membrane by a single transmembrane -helix, located at the N terminus in the case of Tom20 and Tom70 (signal-anchored) or in the C-terminal portion in the case of Tom22 (tail-anchored). Insertion of the precursor of Tom22 into the outer membrane requires pre-existing Tom receptors while the import pathway of the precursors of Tom20 and Tom70 is only poorly understood. We report that Mim1 is required for efficient membrane insertion and assembly of Tom20 and Tom70, but not Tom22. We show that Mim1 associates with SAM_core components to a large SAM complex, explaining its role in late steps of the assembly pathway of Tom40. We conclude that Mim1 is not only required for biogenesis of the β-barrel protein Tom40 but also for membrane insertion and assembly of signal-anchored Tom receptors. Thus, Mim1 plays an important role in the efficient assembly of the mitochondrial TOM complex.

Received for publication, August 21, 2007 , and in revised form, October 25, 2007.

^* This work was supported in part by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 746, Gottfried Wilhelm Leibniz Program, Max Planck Research Award, Bundesministerium für Bildung und Forschung, and the Fonds der Chemischen Industrie. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Supported by a postdoctoral fellowship from the Alexander von Humboldt Foundation.

² Supported by a predoctoral fellowship from the Boehringer Ingelheim Fonds.

³ To whom correspondence should be addressed: Institut für Biochemie und Molekularbiologie, Zentrum für Biochemie und Molekulare Zellforschung, Universität Freiburg, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany. Tel.: 49-761-203-5224; Fax: 49-761-203-5261; E-mail: nikolaus.pfanner{at}biochemie.uni-freiburg.de.

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