Discrimination between SRP- and SecA/SecB-dependent substrates involves selective recognition of nascent chains by SRP and trigger factor.
Besides SecA and SecB, Escherichia coli cells possess a signal recognition particle (SRP) to target exported proteins to the SecY translocon. Using chemical and site-specific cross-linking in vitro, we show that SRP recognizes the first signal anchor sequence of a polytopic membrane protein (MtlA) resulting in cotranslational targeting of MtlA to SecY and phospholipids of the plasma membrane. In contrast, a possible interaction of SRP with the secretory protein pOmpA is prevented by the association of trigger factor with nascent pOmpA. Trigger factor also prevents SecA from binding to the first 125 amino acids of pOmpA when they are still associated with the ribosome. Under no experimental conditions was SecA found to interact with MtlA. Likewise, virtually no binding of trigger factor to ribosome-bound MtlA occurs even in the complete absence of SRP. Collectively, our results indicate that at the stage of nascent polypeptides, polytopic membrane proteins are selected by SRP for co-translational membrane targeting, whereas secretory proteins are directed into the SecA/SecB-mediated post-translational targeting pathway by means of their preferential recognition by trigger factor.
Beck
K
K
Institut für Biochemie und Molekularbiologie und, Universität Freiburg, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany.
Wu
L F
LF
Brunner
J
J
Müller
M
M
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism; D001426:Bacterial Proteins / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D008970:Molecular Weight; D009004:Monosaccharide Transport Proteins; D019696:Peptidylprolyl Isomerase / Q000378:metabolism; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System / Q000378:metabolism; D014176:Protein Biosynthesis; 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; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D000069816:SEC Translocation Channels; C057842:SecB protein, Bacteria; 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; D019696:Peptidylprolyl Isomerase; D000081416:SecA Proteins