Membrane protein biogenesis in Ffh- or FtsY-depleted Escherichia coli.
BACKGROUND
The Escherichia coli version of the mammalian signal recognition particle (SRP) system is required for biogenesis of membrane proteins and contains two essential proteins: the SRP subunit Ffh and the SRP-receptor FtsY. Scattered in vivo studies have raised the possibility that expression of membrane proteins is inhibited in cells depleted of FtsY, whereas Ffh-depletion only affects their assembly. These differential results are surprising in light of the proposed model that FtsY and Ffh play a role in the same pathway of ribosome targeting to the membrane. Therefore, we decided to evaluate these unexpected results systematically.


RESULTS
We characterized the following aspects of membrane protein biogenesis under conditions of either FtsY- or Ffh-depletion: (i) Protein expression, stability and localization; (ii) mRNA levels; (iii) folding and activity. With FtsY, we show that it is specifically required for expression of membrane proteins. Since no changes in mRNA levels or membrane protein stability were detected in cells depleted of FtsY, we propose that its depletion may lead to specific inhibition of translation of membrane proteins. Surprisingly, although FtsY and Ffh function in the same pathway, depletion of Ffh did not affect membrane protein expression or localization.


CONCLUSIONS
Our results suggest that indeed, while FtsY-depletion affects earlier steps in the pathway (possibly translation), Ffh-depletion disrupts membrane protein biogenesis later during the targeting pathway by preventing their functional assembly in the membrane.
Yosef
Ido
I
Department of Biological Chemistry, Weizmann Institute of Science, Rehovot, Israel.
Bochkareva
Elena S
ES
Adler
Julia
J
Bibi
Eitan
E
D000469:Alkaline Phosphatase / Q000235:genetics / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D015153:Blotting, Western; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D014176:Protein Biosynthesis; D021381:Protein Transport; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012270:Ribosomes / Q000378:metabolism; D018271:Signal Recognition Particle / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction
D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; C082931:Ffh protein, E coli; C087406:FtsY protein, Bacteria; C118438:Mdfa protein, E coli; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D018160:Receptors, Cytoplasmic and Nuclear; D018271:Signal Recognition Particle; D000469:Alkaline Phosphatase; C114089:phoA protein, E coli