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Molecular and Cellular Biology, October 2001, p. 7097-7104, Vol. 21, No. 20
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.20.7097-7104.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Mitochondrial Import Driving Forces: Enhanced Trapping by Matrix Hsp70 Stimulates Translocation and Reduces the Membrane Potential Dependence of Loosely Folded Preproteins

Andreas Geissler,^1,2 Joachim Rassow,³ Nikolaus Pfanner,¹ and Wolfgang Voos^1,*

Institut für Biochemie und Molekularbiologie¹ and Fakultät für Biologie,² Universität Freiburg, D-79104 Freiburg, and Institut für Mikrobiologie, Universität Hohenheim, D-70593 Stuttgart,³ Germany

Received 9 March 2001/Returned for modification 12 April 2001/Accepted 19 July 2001

The mitochondrial heat shock protein Hsp70 (mtHsp70) is essential for driving translocation of preproteins into the matrix. Two models, trapping and pulling by mtHsp70, are discussed, but positive evidence for either model has not been found so far. We have analyzed a mutant mtHsp70, Ssc1-2, that shows a reduced interaction with the membrane anchor Tim44, but an enhanced trapping of preproteins. Unexpectedly, at a low inner membrane potential, ssc1-2 mitochondria imported loosely folded preproteins more efficiently than wild-type mitochondria. The import of a tightly folded preprotein, however, was not increased in ssc1-2 mitochondria. Thus, enhanced trapping by mtHsp70 stimulates the import of loosely folded preproteins and reduces the dependence on the import-driving activity of the membrane potential, directly demonstrating that trapping is one of the molecular mechanisms of mtHsp70 action.

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^* Corresponding author. Mailing address: Institut für Biochemie und Molekularbiologie, Universität Freiburg, Hermann-Herder-Strasse 7, Universität Freiburg, D-79104 Freiburg, Germany. Phone: 49-761-203-5269. Fax: 49-761-203-5261. E-mail: voos{at}ruf.uni-freiburg.de.

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Molecular and Cellular Biology, October 2001, p. 7097-7104, Vol. 21, No. 20
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.20.7097-7104.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

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