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Tom40 forms the hydrophilic channel of the mitochondrial import pore for preproteins

Abstract

The mitochondrial outer membrane contains machinery for the import of preproteins encoded by nuclear genes1,2,3. Eight different Tom (translocase of outer membrane) proteins have been identified that function as receptors and/or are related to a hypothetical general import pore. Many mitochondrial membrane channel activities have been described4,5,6,7, including one related to Tim23 of the inner-membrane protein-import system5; however, the pore-forming subunit(s) of the Tom machinery have not been identified until now. Here we describe the expression and functional reconstitution of Tom40, an integral membrane protein with mainly β-sheet structure. Tom40 forms a cation-selective high-conductance channel that specifically binds to and transports mitochondrial-targeting sequences added to the cis side of the membrane. We conclude that Tom40 is the pore-forming subunit of the mitochondrial general import pore and that it constitutes a hydrophilic, 22 Å wide channel for the import of preproteins.

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Figure 1: Expression of yeast Tom40 and its reconstitution into liposomes.
Figure 2: Tom40 forms a high-conductance cation-selective channel.
Figure 3: The Tom40 channel is sensitive to and transports a presequence peptide.

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Acknowledgements

We thank P. J. T. Dekker, B. Guiard, M. Kiebler, W. Kühlbrandt, D. Mills, J. Brix, S. C. Hinnah and H. Martin for materials, advice and discussion. This work was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 388, the Fonds der Chemischen Industrie (N.P.), the Sonderforschungsbereich 171 (R.W.) and a longterm fellowship of the Alexander-von-Humboldt Foundation (M.T.R.)

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Correspondence to Nikolaus Pfanner.

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Hill, K., Model, K., Ryan, M. et al. Tom40 forms the hydrophilic channel of the mitochondrial import pore for preproteins. Nature 395, 516–521 (1998). https://doi.org/10.1038/26780

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