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Vol. 11, Issue 11, 3977-3991, November 2000

Membrane Potential-Driven Protein Import into Mitochondria
The Sorting Sequence of Cytochrome b₂ Modulates the -Dependence of Translocation of the Matrix-targeting Sequence

Andreas Geissler,^* Thomas Krimmer,^* Ulf Bömer,^* Bernard Guiard, Joachim Rassow,^*§ and Nikolaus Pfanner^*

 ^*Institut für Biochemie und Molekularbiologie and  Fakultät für Biologie, Universität Freiburg, D-79104 Freiburg, Germany;  Centre de Génétique Moleculaire CNRS, Université Pierre et Marie Curie, 91190 Gif-sur-Yvette, France; and  ^§Institut für Mikrobiologie, Universität Hohenheim, D-70593 Stuttgart, Germany

The transport of preproteins into or across the mitochondrial inner membrane requires the membrane potential across this membrane. Two roles of in the import of cleavable preproteins have been described: an electrophoretic effect on the positively charged matrix-targeting sequences and the activation of the translocase subunit Tim23. We report the unexpected finding that deletion of a segment within the sorting sequence of cytochrome b₂, which is located behind the matrix-targeting sequence, strongly influenced the -dependence of import. The differential -dependence was independent of the submitochondrial destination of the preprotein and was not attributable to the requirement for mitochondrial Hsp70 or Tim23. With a series of preprotein constructs, the net charge of the sorting sequence was altered, but the -dependence of import was not affected. These results suggested that the sorting sequence contributed to the import driving mechanism in a manner distinct from the two known roles of . Indeed, a charge-neutral amino acid exchange in the hydrophobic segment of the sorting sequence generated a preprotein with an even better import, i.e. one with lower -dependence than the wild-type preprotein. The sorting sequence functioned early in the import pathway since it strongly influenced the efficiency of translocation of the matrix-targeting sequence across the inner membrane. These results suggest a model whereby an electrophoretic effect of on the matrix-targeting sequence is complemented by an import-stimulating activity of the sorting sequence.

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Corresponding author. E-mail address: pfanner{at}uni-freiburg.de

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Molecular Biology of the Cell
Vol. 11, 3977-3991, November 2000
Copyright © 2000 by The American Society for Cell Biology

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