title: Mobility of the SecA 2-helix-finger is not essential for polypeptide translocation via the SecYEG complex.
abstract: The bacterial ATPase SecA and protein channel complex SecYEG form the core of an essential protein translocation machinery. The nature of the conformational changes induced by each stage of the hydrolytic cycle of ATP and how they are coupled to protein translocation are not well understood. The structure of the SecA-SecYEG complex revealed a 2-helix-finger (2HF) of SecA in an ideal position to contact the substrate protein and push it through the membrane. Surprisingly, immobilization of this finger at the edge of the protein channel had no effect on translocation, whereas its imposition inside the channel blocked transport. This analysis resolves the stoichiometry of the active complex, demonstrating that after the initiation process translocation requires only one copy each of SecA and SecYEG. The results also have important implications on the mechanism of energy transduction and the power stroke driving transport. Evidently, the 2HF is not a highly mobile transducing element of polypeptide translocation.
authors.affiliation: School of Biochemistry, University of Bristol, Bristol BS8 1TD, England, UK.
authors.forename: Sarah
authors.initials: S
authors.lastname: Whitehouse
authors.forename: Vicki A M
authors.initials: VA
authors.lastname: Gold
authors.forename: Alice
authors.initials: A
authors.lastname: Robson
authors.forename: William J
authors.initials: WJ
authors.lastname: Allen
authors.forename: Richard B
authors.initials: RB
authors.lastname: Sessions
authors.forename: Ian
authors.initials: I
authors.lastname: Collinson
mesh_terms: D000251:Adenosine Triphosphatases / Q000737:chemistry / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D003198:Computer Simulation; D004926:Escherichia coli / Q000201:enzymology; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D011487:Protein Conformation; D021381:Protein Transport; D000069816:SEC Translocation Channels; D000081416:SecA Proteins; D020124:Thermotoga maritima / Q000201:enzymology
chemical_list: D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D000069816:SEC Translocation Channels; D000251:Adenosine Triphosphatases; D000081416:SecA Proteins