Abstract
In Escherichia coli, three different types of proteins engage the SecY translocon of the inner bacterial membrane for translocation or insertion: 1) polytopic membrane proteins that prior to their insertion into the membrane are targeted to the translocon using the bacterial signal recognition particle (SRP) and its receptor; 2) secretory proteins that are targeted to and translocated across the SecY translocon in a SecA- and SecB-dependent reaction; and 3) membrane proteins with large periplasmic domains, requiring SRP for targeting and SecA for the translocation of the periplasmic moiety. In addition to its role as a targeting device for membrane proteins, a function of the bacterial SRP in the export of SecB-independent secretory proteins has also been postulated. In particular, beta-lactamase, a hydrolytic enzyme responsible for cleavage of the beta-lactam ring containing antibiotics, is considered to be recognized and targeted by SRP. To examine the role of the SRP pathway in beta-lactamase targeting and export, we performed a detailed in vitro analysis. Chemical cross-linking and membrane binding assays did not reveal any significant interaction between SRP and beta-lactamase nascent chains. More importantly, membrane vesicles prepared from mutants lacking a functional SRP pathway did block the integration of SRP-dependent membrane proteins but supported the export of beta-lactamase in the same way as that of the SRP-independent protein OmpA. These data demonstrate that in contrast to previous results, the bacterial SRP is not involved in the export of beta-lactamase and further suggest that secretory proteins of Gram-negative bacteria in general are not substrates of SRP.
Highlights
To target newly synthesized proteins to the SecYEG translocon of the inner bacterial membrane, Escherichia coli employs two different protein targeting routes
Germany In Escherichia coli, three different types of proteins engage the SecY translocon of the inner bacterial membrane for translocation or insertion: 1) polytopic membrane proteins that prior to their insertion into the membrane are targeted to the translocon using the bacterial signal recognition particle (SRP) and its receptor; 2) secretory proteins that are targeted to and translocated across the SecY translocon in a SecA- and SecBdependent reaction; and 3) membrane proteins with large periplasmic domains, requiring SRP for targeting and SecA for the translocation of the periplasmic moiety
Inner membrane proteins on the other hand are selectively recognized by the bacterial signal recognition particle (SRP),1 consisting of the protein Ffh and
Summary
RNase-negative araD139 ⌬(argF-lac)U169 rpsL150 relA1 fIB5301 deoC1 ptsF25 rbsR, tig::kan BL 21 recD FϪ hsdS gal OmpTϪ FϪ ompT hsdSB (rBϪ mBϪ) gal dcm lacY1, (DE3) pLysS (CmR) ompT::kan, secY205 secY39 secE⌬19–111, pCM22 ⌬uncB-C::Tn10 ⌬secG::kan polA end thy gyrA pAra14-FtsYЈ araB-ffhϩ AD202, leu::Tn10, secA36. This study This study breaking glycine residue with a helix-promoting leucine residue [19]. This PhoE derivative, is still exported in an SRP-independent manner. SRP binding to cleavable signal sequences of secretory proteins is further influenced by trigger factor, a ribosome-associated chaperone [15, 17, 18]. In the absence of trigger factor, SRP can be cross-linked to the OmpA signal sequence [18]. No detailed in vitro analyses have been performed to really study the involvement of SRP in the targeting pathway of -lactamase. Our analyses using protease protection assays, flotation gradient experiments, and chemical cross-linking demonstrate that targeting and export of -lactamase proceed independently of the SRP pathway
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
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
