pCsaA from the Gram-positive bacterium iBacillus subtilis/i has been identified previously as a suppressor of the growth and protein-export defect of iEscherichia coli secA/i(Ts) mutants. CsaA has chaperone-like activities iin vivo/i and iin vitro/i. To examine the role of CsaA in protein export in iB. subtilis/i, expression of the icsaA/i gene was repressed. While export of most proteins remained unaffected, export of at least two proteins was significantly reduced upon CsaA depletion. CsaA co-immunoprecipitates and co-purifies with the SecA proteins of iE. coli/i and iB. subtilis/i, and binds the iB. subtilis/i preprotein prePhoB. Purified CsaA stimulates the translocation of prePhoB into iE. coli/i membrane vesicles bearing the iB. subtilis/i translocase, whereas it interferes with the SecB-mediated translocation of proOmpA into membrane vesicles of iE. coli/i. The specific interaction with the SecA translocation ATPase and preproteins suggests that CsaA acts as a chaperone that promotes the export of a subset of preproteins in iB. subtilis/i./p
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