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J. Biol. Chem., Vol. 281, Issue 32, 22554-22565, August 11, 2006

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Effect of Mutations in Tom40 on Stability of the Translocase of the Outer Mitochondrial Membrane (TOM) Complex, Assembly of Tom40, and Import of Mitochondrial Preproteins^*

From the Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada

Mitochondrial preproteins synthesized in the cytosol are imported through the mitochondrial outer membrane by the translocase of the outer mitochondrial membrane (TOM) complex. Tom40 is the major component of the complex and is essential for cell viability. We generated 21 different mutations in conserved regions of the Neurospora crassa Tom40 protein. The mutant genes were transformed into a tom40 null nucleus maintained in a sheltered heterokaryon, and 17 of the mutant genes gave rise to viable strains. All mutations reduced the efficiency of the altered Tom40 molecules to assemble into the TOM complex. Mitochondria isolated from seven of the mutant strains had defects for importing mitochondrial preproteins. Only one strain had a general import defect for all preproteins examined. Another mutation resulted in defects in the import of a matrix-destined preprotein and an outer membrane -barrel protein, but import of the ADP/ATP carrier to the inner membrane was unaffected. Five strains showed deficiencies in the import of -barrel proteins. The latter results suggest that the TOM complex distinguishes -barrel proteins from other classes of preprotein during import. This supports the idea that the TOM complex plays an active role in the transfer of preproteins to subsequent translocases for insertion into the correct mitochondrial subcompartment.

Received for publication, February 21, 2006 , and in revised form, May 8, 2006.

^* This work was supported in part by a grant from the Canadian Institutes of Health Research (to F. E. N.). 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. 1.

¹ Present address: Dept. of Agriculture, Food, and Nutritional Science, 4-10 Agriculture/Forestry Center, University of Alberta, Edmonton, Alberta T6G 2P5, Canada. E-mail: evanveen{at}ualberta.ca.

² Recipient of scholarships from the Natural Sciences and Engineering Research Council of Canada and the Alberta Heritage Foundation for Medical Research. Present address: Bldg. 21, Central Experimental Farm, 960 Carling Ave., Ottawa, Ontario K1A 0C6, Canada. E-mail: TaylorR{at}AGR.GC.CA.

³ To whom correspondence should be addressed. Tel.: 780-492-5375; Fax (780) 492-9234; E-mail: frank.nargang{at}ualberta.ca.

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