SecB-binding does not maintain the translocation-competent state of prePhoE

The role of SecB protein in the export of the precursor of outer membrane protein PhoE and mutant forms of this precursor was studied in vitro. When synthesized in the absence of SecB, translocation-competent prePhoE was observed post-translationally, but addition of SecB was required for efficient translocation into inner membrane vesicles. The translocation competency of in vitro synthesized prePhoE diminished with a similar half-life during incubations in the presence or absence of SecB. The loss of translocation competency of prePhoE, synthesized in the presence of SecB, was not due to dissociation of prePhoE-SecB complexes as could be demonstrated in co-immunoprecipitation experiments with anti-SecB serum. Apparently, SecB does not maintain the translocation-competent conformation of prePhoE, but is mainly required for efficient targeting of this precursor to the export apparatus.