title: Export of beta-lactamase is independent of the signal recognition particle.
abstract: In Escherichia coli, three different types of proteins engage the SecY translocon of the inner bacterial membrane for translocation or insertion: 1) polytopic membrane proteins that prior to their insertion into the membrane are targeted to the translocon using the bacterial signal recognition particle (SRP) and its receptor; 2) secretory proteins that are targeted to and translocated across the SecY translocon in a SecA- and SecB-dependent reaction; and 3) membrane proteins with large periplasmic domains, requiring SRP for targeting and SecA for the translocation of the periplasmic moiety. In addition to its role as a targeting device for membrane proteins, a function of the bacterial SRP in the export of SecB-independent secretory proteins has also been postulated. In particular, beta-lactamase, a hydrolytic enzyme responsible for cleavage of the beta-lactam ring containing antibiotics, is considered to be recognized and targeted by SRP. To examine the role of the SRP pathway in beta-lactamase targeting and export, we performed a detailed in vitro analysis. Chemical cross-linking and membrane binding assays did not reveal any significant interaction between SRP and beta-lactamase nascent chains. More importantly, membrane vesicles prepared from mutants lacking a functional SRP pathway did block the integration of SRP-dependent membrane proteins but supported the export of beta-lactamase in the same way as that of the SRP-independent protein OmpA. These data demonstrate that in contrast to previous results, the bacterial SRP is not involved in the export of beta-lactamase and further suggest that secretory proteins of Gram-negative bacteria in general are not substrates of SRP.
authors.affiliation: Institute for Biochemistry and Molecular Biology, Hermann-Herder-Strasse 7, 79104 Freiburg, Germany.
authors.forename: Daniel
authors.initials: D
authors.lastname: Beha
authors.forename: Sandra
authors.initials: S
authors.lastname: Deitermann
authors.forename: Matthias
authors.initials: M
authors.lastname: Müller
authors.forename: Hans-Georg
authors.initials: HG
authors.lastname: Koch
mesh_terms: D001692:Biological Transport; D002455:Cell Division; D002462:Cell Membrane / Q000378:metabolism; D003432:Cross-Linking Reagents / Q000494:pharmacology; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000235:genetics; D005838:Genotype; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D000069816:SEC Translocation Channels; D018271:Signal Recognition Particle / Q000378:metabolism*; D001618:beta-Lactamases / Q000378:metabolism*
chemical_list: D003432:Cross-Linking Reagents; D029968:Escherichia coli Proteins; D000069816:SEC Translocation Channels; C057841:SecY protein, E coli; D018271:Signal Recognition Particle; D001618:beta-Lactamases