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

In Escherichia coil, signal recognition particle (SRP)-dependent targeting of inner membrane proteins has been described. In vitro cross-linking studi es have demonstrated that short nascent chains exposing a highly hydrophobi c targeting signal interact with the SRP. This SRP, assisted by its recepto r, 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-mediated membrane insertion in which purified r ibosome-nascent chain-SRP complexes are targeted to the purified SecYEG com plex contained in proteoliposomes in a process that requires the SRP-recept or FtsY and GTP. We found that in this system SecA and ATP are dispensable for both the transfer of the nascent inner membrane protein FtsQ to SecY an d its stable membrane insertion. Release of the SRP from nascent FtsQ also occurred in the absence of SecYEG complex indicating a functional interacti on of FtsY with lipids. These data suggest that SRP/FtsY and SecB/SecA cons titute distinct targeting routes.