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Published online 16 October 2000. doi:10.1083/jcb.151.2.277
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© The Rockefeller University Press, 0021-9525/2000//277 $5.00
The Journal of Cell Biology, Volume 151, Number 2, , 2000 277-288

Original Article

Characterization of the Signal That Directs Tom20 to the Mitochondrial Outer Membrane



Sachiko Kanajia, Jun Iwahashia, Yuichiro Kidaa, Masao Sakaguchia, and Katsuyoshi Miharaa

a Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka 812-8582, Japan
Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka 812-8582, Japan.81-92-642-618381-92-642-6176

Tom20 is a major receptor of the mitochondrial preprotein translocation system and is bound to the outer membrane through the NH2-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 NH2-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.

Key Words: mitochondria • protein import • sorting signal • signal recognition particle • signal-anchor sequence

© 2000 The Rockefeller University Press

Drs. Kanaji and Iwahashi contributed equally to this paper and are co–first authors.

Abbreviations used in this paper: GFP, green fluorescent protein; RNC, ribosome-nascent chain-complex; SRP, signal recognition particle; TMD, transmembrane domain.


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