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 liquid at room temperature. The protein constitutes "ribs" which, stacked together, form the hollow cylindrical tube and conical end caps of the gas vesicle. By operating the microscope in deflection mode, it has been possible to achieve sub-nanometer resolution of the rib structure. 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 at an angle of approximately 55 degrees to the rib axis, with a repeat distance between each line of 0.57 +/- 0.05 nm along the rib axis. These observed dimensions and periodicities are consistent with those determined from previous x-ray diffraction studies, indicating that the protein is arranged in beta-chains crossing the rib at an angle of 55 degrees to the rib axis. The AFM results confirm the x-ray data and represent the first direct images of a beta-sheet protein secondary structure using this technique. The orientation of the GvpA protein component of the structure and the extent of this protein across the ribs have been established for the first time.