Imaging bacteriorhodopsin lattices in purple membranes with atomic force microscopy

Abstract

Surface images of the membrane protein bacteriorhodopsin have been obtained using atomic force microscopy. Trigonal lattices formed by trimers of bacteriorhodopsin are observed, but details of the protein surface structure have not yet been resolved. Grey scale oscilloscope images show that the surface features along one of the three equivalent axes in the plane of the membrane are more prominent than along the other two, resulting in parallel rows. This may result from anisotropic properties of the atomic force probe, substrate structure and membrane deformation, or differences of symmetry between the membrane interior and the membrane surface. Parallel rows have been obtained at several angles to the line scan direction, but not parallel to the scan. The measured spacings of the rows are in reasonably good agreement with the lattice constants of purple membranes determined by diffraction methods that are much more accurate.