Characterization and crystal packing of three-dimensional bacteriorhodopsin crystals.

The three-dimensional crystals of the integral membrane protein bacteriorhodopsin have been characterized by X-ray diffraction and freeze-fracture electron microscopy: the needle-like form A crystals belong to space group P 1 (pseudohexagonal) with seven molecules per crystallographic unit cell forming one turn of a non-crystallographic helix. The probable arrangement of the bacteriorhodopsin molecules is derived from freeze-fracture electron micrographs and chromophore orientation. Membrane-like structures are not present. The same helices of bacteriorhodopsin molecules found in crystal form A also make up the cube-like crystal form B. They are now arranged in all three mutually perpendicular directions. These cubes are always highly disordered, since the unit cell length corresponds to 6.7 molecules of the 7-fold helix. Very often, conversion of bacteriorhodopsin from the three-dimensional crystals into filamentous material occurs.