Abstract
Inner membrane proteins (IMPs) of Escherichia coli use different pathways for membrane targeting and integration. YidC plays an essential but poorly defined role in the integration and folding of IMPs both in conjunction with the Sec translocon and as a Sec-independent insertase. Depletion of YidC only marginally affects the insertion of Sec-dependent IMPs, whereas it blocks the insertion of a subset of Sec-independent IMPs. Substrates of this latter "YidC-only" pathway include the relatively small IMPs M13 procoat, Pf3 coat protein, and subunit c of the F(1)F(0) ATPase. Recently, it has been shown that the steady state level of the larger and more complex CyoA subunit of the cytochrome o oxidase is also severely affected upon depletion of YidC. In the present study we have analyzed the biogenesis of the integral lipoprotein CyoA. Collectively, our data suggest that the first transmembrane segment of CyoA rather than the signal sequence recruits the signal recognition particle for membrane targeting. Membrane integration and assembly appear to occur in two distinct sequential steps. YidC is sufficient to catalyze insertion of the N-terminal domain consisting of the signal sequence, transmembrane segment 1, and the small periplasmic domain in between. Translocation of the large C-terminal periplasmic domain requires the Sec translocon and SecA, suggesting that for this particular IMP the Sec translocon might operate downstream of YidC.
Highlights
In Escherichia coli, IMPs4 are targeted and inserted into the inner membrane via different pathways [1]
The E. coli signal recognition particle (SRP), which consists of a signal sequence binding protein (Ffh for Fifty-four homologue) and a 4.5 S RNA, samples nascent chains at the ribosomal exit site for the presence of hydrophobic targeting signals that are primarily present in nascent Inner membrane proteins (IMPs)
CyoA is an unusual polytopic IMP in the sense that it is synthesized with an N-terminal, lipoprotein type signal sequence that is cleaved from the precursor protein [23]
Summary
In the present study we have analyzed the targeting and insertion of CyoA using complementary in vivo protease mapping experiments and an in vitro photocross-linking technique. Our data demonstrate that the SRP interacts with the first TM of CyoA (rather than with its signal sequence) to mediate targeting of CyoA to the Sec/YidC insertion site in the inner membrane. YidC is required and sufficient for membrane assembly of the N-terminal part of the protein, whereas translocation of the large C-terminal periplasmic domain requires the Sec translocon, including SecA. This suggests that for membrane assembly of CyoA, the Sec translocon operates downstream of YidC
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
