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Abstract

RECENT evidence shows that membrane-bound proteins can be crystallized successfully in the presence of detergent, which seems to facilitate the ordered packing of the proteins by binding to their hydrophobic surfaces in micellar manner. This approach has enabled the molecular structures of two bacterial photosynthetic reaction centres to be solved at high resolution by X-ray crystallography, each of which has provided insights into the mechanism of photo-activated electron transport across the cell membrane. The detergent, however, although present in high concentration in the crystals, is not seen in these high-resolution structures because of disordering. To determine the structural motifs formed by the detergent that are involved in crystal packing, we have therefore generated a low-resolution structure using neutron diffraction with contrast variation. We find that the detergent is concentrated in rings which fill all the available space around the membrane-spanning α-helices of the reaction-centre protein subunits L, M and H. These rings are interconnected throughout the crystal lattice by short cylindrical detergent bridges such that zig-zag chains are formed parallel to the c direction. The average structure of the detergent therefore is spatially complementary to the structure of the reaction-centre complex and provides a model for the interaction between the lipid bilayer and the complex in vivo.