Characterization of an OmpA-like outer membrane protein of the acidophilic iron-oxidizing bacterium, Acidithiobacillus ferrooxidans.

An OmpA family protein (FopA) previously reported as one of the major outer membrane proteins of an acidophilic iron-oxidizing bacterium Acidithiobacillus ferrooxidans was characterized with emphasis on the modification by heat and the interaction with peptidoglycan. A 30-kDa band corresponding to the FopA protein was detected in outer membrane proteins extracted at 75°C or heated to 100°C for 10 min prior to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). However, the band was not detected in outer membrane proteins extracted at ≤40°C and without boiling prior to electrophoresis. By Western blot analysis using the polyclonal antibody against the recombinant FopA, FopA was detected as bands with apparent molecular masses of 30 and 90 kDa, suggesting that FopA existed as an oligomeric form in the outer membrane of A. ferrooxidans. Although the fopA gene with a sequence encoding the signal peptide was successfully expressed in the outer membrane of Escherichia coli, the recombinant FopA existed as a monomer in the outer membrane of E. coli. FopA was detected in peptidoglycan-associated proteins from A. ferrooxidans. The recombinant FopA also showed the peptidoglycan-binding activity.