Interaction of Bacillus subtilis CsaA with SecA and precursor proteins

CsaA from the Gram-positive bacterium Bacillus subtilis has been identified previously as a suppressor of the growth and protein-export defect of Esch erichia coli secA(Ts) mutants. CsaA has chaperone-like activities in vivo a nd in vitro. To examine the role of CsaA in protein export in B. subtilis, expression of the csaA gene was repressed. While export of most proteins re mained unaffected, export of at least two proteins was significantly reduce d upon CsaA depletion. CsaA co-immunoprecipitates and co-purifies with the SecA proteins of E. coli and B. subtilis, and binds the B. subtilis preprot ein prePhoB. Purified CsaA stimulates the translocation of prePhoB into E. coli membrane vesicles bearing the B. subtilis translocase, whereas it inte rferes with the SecB-mediated translocation of proOmpA into membrane vesicl es of E. coli. The specific interaction with the SecA translocation ATPase and preproteins suggests that CsaA acts as a chaperone that promotes the ex port of a subset of preproteins in B. subtilis.