SecA is not required for signal recognition particle-mediated targeting and initial membrane insertion of a nascent inner membrane protein

In Escherichia coli, signal recognition particle (SRP)-dependent targeting of inner membrane proteins has been described. In vitro cross-linking studies have dem-onstrated that short nascent chains exposing a highly hydrophobic targeting signal interact with the SRP. This SRP, assisted by its receptor, FtsY, mediates the transfer to a common translocation site in the inner membrane that contains SecA, SecG, and SecY. Here we describe a further in vitro reconstitution of SRP-medi-ated membrane insertion in which purified ribosome-nascent chain-SRP complexes are targeted to the puri-fied SecYEG complex contained in proteoliposomes in a process that requires the SRP-receptor FtsY and GTP. We found that in this system SecA and ATP are dispen-sable for both the transfer of the nascent inner mem