SRP-dependent co-translational targeting and SecA-dependent translocation analyzed as individual steps in the export of a bacterial protein.
Recently it has been recognized that the signal recognition particle (SRP) of Escherichia coli represents a specific targeting device for hydrophobic inner membrane proteins. It has remained unclear, however, whether the bacterial SRP functions in concert with SecA, which is required for the translocation of secretory proteins across the inner membrane. Here, we have analyzed a hybrid protein constructed by fusing the signal anchor sequence of an SRP-dependent inner membrane protein (MtlA) to the mature part of an exclusively SecA-requiring secretory protein (OmpA). We show that the signal anchor sequence of MtlA confers the novel properties onto nascent chains of OmpA of being co-translationally recognized and targeted to SecY by SRP. Once targeted to SecY, ribosome-associated nascent chains of the hybrid protein, however, remain untranslocated unless SecA is present. These results indicate that SRP and SecA cooperate in a sequential, non-overlapping manner in the topogenesis of those membrane proteins which, in addition to a signal anchor sequence, harbor a substantial hydrophilic domain to be translocated into the periplasm.
Neumann-Haefelin
C
C
Institut für Biochemie und Molekularbiologie, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany.
Schäfer
U
U
Müller
M
M
Koch
H G
HG
D000251:Adenosine Triphosphatases / Q000235:genetics / Q000378:metabolism*; D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism; D001426:Bacterial Proteins / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D008954:Models, Biological; D009004:Monosaccharide Transport Proteins; D009154:Mutation; D019696:Peptidylprolyl Isomerase / Q000378:metabolism; D019897:Periplasm / Q000378:metabolism; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System / Q000378:metabolism; D010957:Plasmids / Q000378:metabolism; D014176:Protein Biosynthesis; D021381:Protein Transport; D006180:Proton-Translocating ATPases / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism; D000069816:SEC Translocation Channels; D000081416:SecA Proteins; D018271:Signal Recognition Particle / Q000378:metabolism*
D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; C082931:Ffh protein, E coli; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D011993:Recombinant Fusion Proteins; D000069816:SEC Translocation Channels; C057842:SecB protein, Bacteria; C065493:SecE protein, E coli; C057841:SecY protein, E coli; D018271:Signal Recognition Particle; C078167:OMPA outer membrane proteins; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System; C022358:mannitol PTS permease, E coli; D000251:Adenosine Triphosphatases; D006180:Proton-Translocating ATPases; C403324:trigger factor, E coli; D019696:Peptidylprolyl Isomerase; D000081416:SecA Proteins