title: Active unfolding of precursor proteins during mitochondrial protein import.
abstract: 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.
authors.affiliation: Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern University, 2153 Sheridan Road, Evanston, IL 60208-3500, USA.
authors.forename: A
authors.initials: A
authors.lastname: Matouschek
authors.forename: A
authors.initials: A
authors.lastname: Azem
authors.forename: K
authors.initials: K
authors.lastname: Ratliff
authors.forename: B S
authors.initials: BS
authors.lastname: Glick
authors.forename: K
authors.initials: K
authors.lastname: Schmid
authors.forename: G
authors.initials: G
authors.lastname: Schatz
mesh_terms: D001426:Bacterial Proteins; D001692:Biological Transport; D018834:Chaperonin 60 / Q000378:metabolism; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism*; D007770:L-Lactate Dehydrogenase / Q000378:metabolism; D042844:L-Lactate Dehydrogenase (Cytochrome); D008928:Mitochondria / Q000378:metabolism*; D011485:Protein Binding; D011489:Protein Denaturation; D017510:Protein Folding; D011498:Protein Precursors / Q000378:metabolism*; D021382:Protein Sorting Signals; D012260:Ribonucleases / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D013762:Tetrahydrofolate Dehydrogenase / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D018834:Chaperonin 60; D018840:HSP70 Heat-Shock Proteins; D011498:Protein Precursors; D021382:Protein Sorting Signals; D007770:L-Lactate Dehydrogenase; D042844:L-Lactate Dehydrogenase (Cytochrome); D013762:Tetrahydrofolate Dehydrogenase; D012260:Ribonucleases; C019760:Bacillus amyloliquefaciens ribonuclease