The signal recognition particle-targeting pathway does not necessarily deliver proteins to the Sec-translocase in Escherichia coli

ProW is an Escherichia coil inner membrane protein that consists of a 100-r esidue-long periplasmic N-terminal tail (N-tail) followed by seven closely spaced transmembrane segments. N-tail translocation presumably proceeds in a C-to-N-terminal direction and represents a poorly understood aspect of me mbrane protein biogenesis. Here, using an in vivo depletion approach, we sh ow that N-tail translocation in a ProW derivative comprising the N-tail and the first transmembrane segment fused to the globular P2 domain of leader peptidase depends both on the bacterial signal recognition particle (SRP) a nd the Sec-translocase. Surprisingly, however, a deletion construct with on ly one transmembrane segment downstream of the N-tail can assemble properly even under severe depletion of SecE, a central component of the Sec-transl ocase, but not under SRP-depletion conditions. To our knowledge, this is th e first demonstration that the SRP-targeting pathway does not necessarily d eliver SRP-dependent inner membrane proteins to the Sec-translocase. The da ta further suggest that N-tail translocation can proceed in the absence of a functional Sec-translocase.