Mitochondrial import driving forces: enhanced trapping by matrix Hsp70 stimulates translocation and reduces the membrane potential dependence of loosely folded preproteins.
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.
Geissler
A
A
Institut für Biochemie und Molekularbiologie, Universität Freiburg, D-79104 Freiburg, Germany.
Rassow
J
J
Pfanner
N
N
Voos
W
W
D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism*; D002945:Cisplatin / Q000494:pharmacology; D003432:Cross-Linking Reagents / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D005656:Fungal Proteins / Q000378:metabolism; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism*; D006360:Heat-Shock Proteins; D008564:Membrane Potentials; D008565:Membrane Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins; D008928:Mitochondria / Q000378:metabolism*; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D018832:Molecular Chaperones / Q000378:metabolism; D009154:Mutation; D011233:Precipitin Tests; D021381:Protein Transport; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D013997:Time Factors
D002352:Carrier Proteins; D003432:Cross-Linking Reagents; D005656:Fungal Proteins; D018840:HSP70 Heat-Shock Proteins; D006360:Heat-Shock Proteins; C086039:MGE1 protein, S cerevisiae; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D018832:Molecular Chaperones; C101767:SSC1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C077016:TIM44 protein, S cerevisiae; D000252:Calcium-Transporting ATPases; D002945:Cisplatin