SecD and SecF are required for the proton electrochemical gradient stimulation of preprotein translocation.
Mutations in secD and secF show impaired protein translocation across the inner membrane of Escherichia coli. We investigated the effect of SecD and SecF (SecD/F) depletion on preprotein translocation into inverted inner membrane vesicles (IMVs). Both IMVs and cells which were depleted of SecD/F were defective in their ability to maintain a proton electrochemical gradient. The translocation of pre-maltose binding protein (preMBP), which is strongly delta microH+ dependent, showed a 5-fold decreased rate with IMVs lacking SecD/F. In contrast, proteolytic processing of preMBP to MBP by leader peptidase was similar in IMVs containing and lacking SecD/F, consistent with earlier findings that only ATP-dependent translocation is required for the initiation of translocation. In the absence of a delta microH+, with ATP as the sole energy source, preMBP translocation into IMVs which contained or were depleted of SecD/F was identical. Translocation of the precursor of outer membrane protein A (proOmpA) in the presence of subsaturating ATP also required a generated delta microH+ and, under these conditions, proOmpA translocation required SecD/F. With saturating concentrations of ATP, where delta microH+ has little effect on in vitro proOmpA translocation, SecD/F also had little effect on translocation. These results explain why SecD/F effects are precursor protein dependent in vitro.
Arkowitz
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Molecular Biology Institute, University of California, Los Angeles 90024-1570.
Wickner
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D018528:ATP-Binding Cassette Transporters; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000378:metabolism*; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D004563:Electrochemistry; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins; D008320:Maltose / Q000378:metabolism*; D058619:Maltose-Binding Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins; D018272:Porins / Q000378:metabolism; D011498:Protein Precursors / Q000378:metabolism*; D011510:Proteolipids / Q000378:metabolism; D011522:Protons
D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D058619:Maltose-Binding Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D018272:Porins; D011498:Protein Precursors; D011510:Proteolipids; D011522:Protons; C499394:SecD protein, E coli; C489583:SecF protein, E coli; C461599:maltose transport system, E coli; C020485:proteoliposomes; C075767:secD protein, Bacteria; C075768:secF protein, Bacteria; D008320:Maltose