Two-dimensional Crystallization of the Light-harvesting Complex from Rhodospirillum rubrum

Homogeneous detergent-solubilized B873 light-harvesting complexes from a carotenoid-less mutant of the purple non-sulfur bacterium, Rhodospirillum rubrum G9, were reassembled spontaneously into two-dimensional (2D) hexagonal arrays during extensive and controlled dialysis. As the complexes contain only 1 to 2 mol phospholipid per mol αβ dimer, the arrays formed by a self assembly process are primary due to protein-protein interactions. The hexagonal lattices were analyzed by negative stain electron microscopy and digital image processing. They exhibited a unit cell size of 12·3 nm, in close agreement with the particle diameter of the active photo-unit in native chromatophore membranes. The unit cell contains a central 15 nm stain-filled depression, embraced by a ring with an outer diameter of 10 nm.