title: Step-size analyses of the mitochondrial Hsp70 import motor reveal the Brownian ratchet in operation.
abstract: Newly synthesized mitochondrial precursor proteins have to become unfolded by the mitochondrial Hsp70 (mtHsp70) import motor to cross the mitochondrial membranes. To assess the mechanism of unfolding of precursor proteins by mtHsp70, we designed a system to measure step sizes of the mtHsp70 import motor, which are distances at which the motor system moves along polypeptide chains during a single turnover of ATP. We made a series of fusion proteins consisting of a mitochondrial presequence containing the first mtHsp70 binding site, a spacer sequence containing an Hsp70 avoidance segment followed by the second mtHsp70 binding site, and different folded mature domains. Analyses of the dependence of the import rates of those fusion proteins on the lengths of Hsp70 avoidance segments allowed us to estimate the step sizes, which differ for different mature domains and different lengths of the spacers. These results suggest that the mtHsp70 import motor functions at least as a molecular Brownian ratchet to unfold mitochondrial precursor proteins.
authors.affiliation: Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan.
authors.forename: Koji
authors.initials: K
authors.lastname: Yamano
authors.forename: Mika
authors.initials: M
authors.lastname: Kuroyanagi-Hasegawa
authors.forename: Masatoshi
authors.initials: M
authors.lastname: Esaki
authors.forename: Mihoko
authors.initials: M
authors.lastname: Yokota
authors.forename: Toshiya
authors.initials: T
authors.lastname: Endo
mesh_terms: D000255:Adenosine Triphosphate / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D000818:Animals; D018840:HSP70 Heat-Shock Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008928:Mitochondria / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D024101:Mitochondrial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017510:Protein Folding; D011498:Protein Precursors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D021381:Protein Transport / Q000502:physiology; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
chemical_list: D018840:HSP70 Heat-Shock Proteins; D024101:Mitochondrial Proteins; D011498:Protein Precursors; D011993:Recombinant Fusion Proteins; D000255:Adenosine Triphosphate