Characterization and crystal packing of three-dimensional bacteriorhodopsin crystals

Abstract

The three-dimensional crystals of the integral membrane protein bacteriorhodopsinhave been characterized by X-ray diffraction and freeze-fractureelectron microscopy: the needle-like form A crystals belong to spacegroup P 1 (pseudohexagonal) with seven molecules per crystallographicunit cell forming one turn of a non-crystallographic helix. The probablearrangement of the bacteriorhodopsin molecules is derived from freeze-fractureelectron micrographs and chromophore orientation. Membrane-like structuresare not present. The same helices of bacteriorhodopsin moleculesfound 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 lengthcorresponds to 6.7 molecules of the 7-fold helix. Very often, conversionof bacteriorhodopsin from the three-dimensional crystals into filamentousmaterial occurs.