DIRECT OBSERVATION OF PROTEIN SECONDARY STRUCTURE IN GAS VESICLES BY ATOMIC-FORCE MICROSCOPY

The protein that forms the gas vesicle in the cyanobacterium Anabaena flos-aquae has been imaged by atomic force microscopy (AFM) under liqu id at room temperature. The protein constitutes ''ribs'' which, stacke d together, form the hollow cylindrical tube and conical end caps of t he gas vesicle. By operating the microscope in deflection mode, it has been possible to achieve sub-nanometer resolution of the rib structur e. The lateral spacing of the ribs was found to be 4.6 +/- 0.1 nm. At higher resolution the ribs are observed to consist of pairs of lines a t an angle of similar to 55 degrees to the rib axis, with a repeat dis tance between each line of 0.57 +/- 0.05 nm along the rib axis. These observed dimensions and periodicities are consistent with those determ ined from previous x-ray diffraction studies, indicating that the prot ein is arranged in beta-chains crossing the rib at an angle of 55 degr ees to the rib axis. The AFM results confirm the x-ray data and repres ent the first direct images of a beta-sheet protein secondary structur e using this technique. The orientation of the GvpA protein component of the structure and the extent of this protein across the ribs have b een established for the first time.