In vitro studies with purified components reveal signal recognition particle (SRP) and SecA/SecB as constituents of two independent protein-targeting pathways of Escherichia coli.
The molecular requirements for the translocation of secretory proteins across, and the integration of membrane proteins into, the plasma membrane of Escherichia coli were compared. This was achieved in a novel cell-free system from E. coli which, by extensive subfractionation, was simultaneously rendered deficient in SecA/SecB and the signal recognition particle (SRP) components, Ffh (P48), 4. 5S RNA, and FtsY. The integration of two membrane proteins into inside-out plasma membrane vesicles of E. coli required all three SRP components and could not be driven by SecA, SecB, and DeltamicroH+. In contrast, these were the only components required for the translocation of secretory proteins into membrane vesicles, a process in which the SRP components were completely inactive. Our results, while confirming previous in vivo studies, provide the first in vitro evidence for the dependence of the integration of polytopic inner membrane proteins on SRP in E. coli. Furthermore, they suggest that SRP and SecA/SecB have different substrate specificities resulting in two separate targeting mechanisms for membrane and secretory proteins in E. coli. Both targeting pathways intersect at the translocation pore because they are equally affected by a blocked translocation channel.
Institut für Biochemie und Molekularbiologie, Universität Freiburg, D-79104 Freiburg, Germany.
H G
HG
Koch
T
T
Hengelage
C
C
Neumann-Haefelin
J
J
MacFarlane
H K
HK
Hoffschulte
K L
KL
Schimz
B
B
Mechler
M
M
Müller
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins; D008565:Membrane Proteins / Q000378:metabolism*; D026901:Membrane Transport Proteins; D008967:Molecular Biology / Q000379:methods; D009004:Monosaccharide Transport Proteins; D015388:Organelles / Q000378:metabolism; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System / Q000378:metabolism; D012313:RNA / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism; D000069816:SEC Translocation Channels; D000081416:SecA Proteins; D018271:Signal Recognition Particle / Q000378:metabolism*
D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; C082931:Ffh protein, E coli; C087406:FtsY protein, Bacteria; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D018160:Receptors, Cytoplasmic and Nuclear; D000069816:SEC Translocation Channels; C057842:SecB protein, Bacteria; D018271:Signal Recognition Particle; D012313:RNA; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System; C022358:mannitol PTS permease, E coli; D000251:Adenosine Triphosphatases; D000081416:SecA Proteins