A plasmid-encoded surface protein found in chronic-disease isolates of Coxiella burnetii

The cbbE' gene codes for the E' protein of Coxiella burnetii and was detected in genomic DNA from all known human isolates of the biotzere strain but not in DNA from the other five strains of C. burnetii. The biotzere strain is strictly associated with chronic disease in humans. Extrinsic iodination of biotzere strain cells radiolabeled a 55-kDa protein which comigrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with the E' protein synthesized in vitro from recombinants containing the cbbE' gene. The ¹²⁵I-labeled 55-kDa protein was immunoprecipitated with polyclonal anti-E' antiserum, confirming its identity as E'. Predicted secondary structure of the E' polypeptide shows six regions of beta-sheet structure and an alpha-helix near the C terminus with adequate lengths to span a membrane. The predicted hydropathy profile of E' is similar to profiles of known outer membrane proteins and corroborates the biochemical data, indicating that the protein is located in the outer membrane of C. burnetii.