title: C- to N-terminal translocation of preproteins into mitochondria.
abstract: Nuclear-encoded mitochondrial matrix proteins in most cases contain N-terminal targeting signals and are imported in a linear N- to C-terminal (N-->C) fashion. We asked whether import can also occur in a C- to N-terminal direction (C-->N). We placed targeting signals at the C-terminus of passenger proteins. Import did occur in this 'backwards' fashion. It paralleled that of the 'normal' N-->C mechanism in terms of efficiency, rate, energetic requirements and ability to mediate unfolding and refolding during and following import of protein containing a folded domain. Furthermore, this reaction was mediated by the TIM17-23 machinery. The import pathway taken by certain inner-membrane proteins contains elements of such a C-->N translocation pathway, as they are targeted to mitochondria by internal targeting signals. These internal targeting signals appear to form loop structures together with neighbouring transmembrane segments, and penetrate the inner membrane in a membrane-potential-dependent manner. The dimeric TIM17-23 complex, together with mt-Hsp70, acts on both sides of the loop structure to facilitate their translocation into the matrix. On one side of the loop import occurs in the common N-->C direction, whereas the translocation of the other side involves the novel C-->N import direction. We conclude therefore that the mitochondrial import machinery displays no preference for the directionality of the import process.
authors.affiliation: Institut für Physiologische Chemie, Goethestrasse 33, 80336 München, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Fölsch
authors.forename: B
authors.initials: B
authors.lastname: Gaume
authors.forename: M
authors.initials: M
authors.lastname: Brunner
authors.forename: W
authors.initials: W
authors.lastname: Neupert
authors.forename: R A
authors.initials: RA
authors.lastname: Stuart
mesh_terms: D001483:Base Sequence; D001692:Biological Transport; D017931:DNA Primers; D003576:Electron Transport Complex IV / Q000378:metabolism; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism*; D011498:Protein Precursors / Q000737:chemistry / Q000378:metabolism*; D021382:Protein Sorting Signals / Q000737:chemistry / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics; D013762:Tetrahydrofolate Dehydrogenase / Q000378:metabolism
chemical_list: D017931:DNA Primers; D018840:HSP70 Heat-Shock Proteins; D011498:Protein Precursors; D021382:Protein Sorting Signals; D013762:Tetrahydrofolate Dehydrogenase; D003576:Electron Transport Complex IV