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  • The EMBO Journal (1997) 16, 6727 - 6736
  • doi:10.1093/emboj/16.22.6727

Active unfolding of precursor proteins during mitochondrial protein import

Andreas Matouschek1, Abdussalam Azem2, Kevin Ratliff1, Benjamin S. Glick3, Karl Schmid2 and Gottfried Schatz2

  1. Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern University, 2153 Sheridan Road, Evanston, IL 60208-3500, USA
  2. Department of Biochemistry, Biozentrum der Universität Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland
  3. Department of Molecular Genetics and Cell Biology, The University of Chicago, 920 East 58th Street, Chicago, IL 60637, USA

Received 11 April 1997; Revised 5 September 1997

Precursor proteins made in the cytoplasm must be in an unfolded conformation during import into mitochondria. Some precursor proteins have tightly folded domains but are imported faster than they unfold spontaneously, implying that mitochondria can unfold proteins. We measured the import rates of artificial precursors containing presequences of varying length fused to either mouse dihydrofolate reductase or bacterial barnase, and found that unfolding of a precursor at the mitochondrial surface is dramatically accelerated when its presequence is long enough to span both membranes and to interact with mhsp70 in the mitochondrial matrix. If the presequence is too short, import is slow but can be strongly accelerated by urea-induced unfolding, suggesting that import of these 'short' precursors is limited by spontaneous unfolding at the mitochondrial surface. With precursors that have sufficiently long presequences, unfolding by the inner membrane import machinery can be orders of magnitude faster than spontaneous unfolding, suggesting that mhsp70 can act as an ATP-driven force-generating motor during protein import.

  • Keywords:

    • heat shock protein 70,
    • hsp70,
    • import,
    • motor proteins,
    • protein translocation