The protein import motor of mitochondria: unfolding and trapping of preproteins are distinct and separable functions of matrix Hsp70. |
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Authors: | C Voisine E A Craig N Zufall O von Ahsen N Pfanner W Voos |
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Affiliation: | Department of Biomolecular Chemistry, University of Wisconsin-Madison, 53706, USA. |
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Abstract: | Mitochondrial heat shock protein 70 (mtHsp70) functions in unfolding, translocation, and folding of imported proteins. Controversial models of mtHsp70 action have been discussed: (1) physical trapping of preproteins is sufficient to explain the various mtHsp70 functions, and (2) unfolding of preproteins requires an active motor function of mtHsp70 ("pulling"). Intragenic suppressors of a mutant mtHsp70 separate two functions: a nonlethal folding defect caused by enhanced trapping of preproteins, and a conditionally lethal unfolding defect caused by an impaired interaction of mtHsp70 with the membrane anchor Tim44. Even enhanced trapping in wild-type mitochondria does not generate a pulling force. The motor function of mtHsp70 cannot be explained by passive trapping alone but includes an essential ATP-dependent interaction with Tim44 to generate a pulling force and unfold preproteins. |
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