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

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... 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, ␤-lactamase, a hydrolytic enzyme responsible for cleavage of the ␤-lactam ring containing antibiotics, is considered to be recognized and targeted by SRP. To examine the role of the SRP pathway in ␤-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 ␤-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 ␤-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 ␤-lactamase and further suggest that secretory proteins of Gram-negative bacteria in general are not substrates of SRP.