title: Differential translocation of protein precursors across SecY-deficient membranes of Escherichia coli: SecY is not obligatorily required for translocation of certain secretory proteins in vitro.
abstract: SecY, a component of the protein translocation system in Escherichia coli, was depleted at a nonpermissive temperature in a strain which had a temperature-sensitive polar effect on the expression of its secY. Membrane vesicles prepared from these cells, when grown at the nonpermissive temperature, contained about 5% SecY and similarly low levels of SecG. As expected, translocation of alkaline phosphatase precursors across these SecY-deficient membranes was severely impaired and appeared to be directly related to the decrease of SecY amounts. However, despite such a dramatic reduction in SecY and SecG levels, these membranes exhibited 50 to 70% of the wild-type translocation activity, including the processing of the signal peptide, of OmpA precursor (proOmpA). This translocation activity in SecY-deficient membranes was still SecA and ATP dependent and was not unique to proOmpA, as lipoprotein and lambda receptor protein precursors were also transported efficiently. Membranes that were reconstituted from these SecY-depleted membranes contained undetectable amounts of SecY yet were also shown to possess substantial translocation activity for proOmpA. These results indicate that the requirement of SecY for translocation is not obligatory for all secretory proteins and may depend on the nature of precursors. Consequently, it is unlikely that SecY is the essential core channel through which all precursors traverse across membranes; rather, SecY probably contributes to efficiency and specificity.
authors.affiliation: Department of Biology, Georgia State University, Atlanta 30303, USA.
authors.forename: Y B
authors.initials: YB
authors.lastname: Yang
authors.forename: J
authors.initials: J
authors.lastname: Lian
authors.forename: P C
authors.initials: PC
authors.lastname: Tai
mesh_terms: D000469:Alkaline Phosphatase / Q000378:metabolism; D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism; D001426:Bacterial Proteins / Q000378:metabolism*; D001692:Biological Transport; D002352:Carrier Proteins; D002462:Cell Membrane / Q000378:metabolism*; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins; D007473:Ion Channels / Q000378:metabolism; D008074:Lipoproteins; D008836:Micrococcal Nuclease / Q000378:metabolism; D018272:Porins; D011498:Protein Precursors / Q000378:metabolism*; D011991:Receptors, Virus / Q000378:metabolism; D000069816:SEC Translocation Channels
chemical_list: D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D007473:Ion Channels; D008074:Lipoproteins; C099932:Lpp protein, E coli; D018272:Porins; D011498:Protein Precursors; D011991:Receptors, Virus; D000069816:SEC Translocation Channels; C057841:SecY protein, E coli; C016532:maltoporins; C092767:pro-OmpA nuclease A; D000469:Alkaline Phosphatase; D008836:Micrococcal Nuclease