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J. Biol. Chem., Vol. 283, Issue 7, 3799-3807, February 15, 2008

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Tom20 and Tom22 Share the Common Signal Recognition Pathway in Mitochondrial Protein Import^*

Koji Yamano¹², Yoh-ichi Yatsukawa¹, Masatoshi Esaki³, Alyson E. Aiken Hobbs, Robert E. Jensen, and Toshiya Endo^¶||4

From the Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan, the Department of Cell Biology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, and the ^¶Institute for Advanced Research and ^||Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan

Precise targeting of mitochondrial precursor proteins to mitochondria requires receptor functions of Tom20, Tom22, and Tom70 on the mitochondrial surface. Tom20 is a major import receptor that recognizes preferentially mitochondrial presequences, and Tom70 is a specialized receptor that recognizes presequence-less inner membrane proteins. The cytosolic domain of Tom22 appears to function as a receptor in cooperation with Tom20, but how its substrate specificity differs from that of Tom20 remains unclear. To reveal possible differences in substrate specificities between Tom20 and Tom22, if any, we deleted the receptor domain of Tom20 or Tom22 in mitochondria in vitro by introducing cleavage sites for a tobacco etch virus protease between the receptor domains and transmembrane segments of Tom20 and Tom22. Then mitochondria without the receptor domain of Tom20 or Tom22 were analyzed for their abilities to import various mitochondrial precursor proteins targeted to different mitochondrial subcompartments in vitro. The effects of deletion of the receptor domains on the import of different mitochondrial proteins for different import pathways were quite similar between Tom20 and Tom22. Therefore Tom20 and Tom22 are apparently involved in the same step or sequential steps along the same pathway of targeting signal recognition in import.

Received for publication, October 9, 2007 , and in revised form, December 5, 2007.

^* This work was supported in part by grants-in-aid for scientific research from the Ministry of Education, Science, Sports, and Culture of Japan. 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 Both authors contributed equally to this work.

² Research Fellow of the Japan Society for the Promotion of Science.

³ Present address: Div. of Molecular Cell Biology, Inst. of Molecular Embryology and Genetics, Kumamoto University, 2-2-1 Honjo, Kumamoto 860-0811, Japan.

⁴ To whom correspondence should be addressed: Dept. of Chemistry, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan. Tel.: 81-52-789-2490; Fax: 81-52-789-2947; E-mail: endo{at}biochem.chem.nagoya-u.ac.jp.

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