Detergent structure in crystals of a bacterial photosynthetic reaction centre

RECENT evidence shows that membrane-bound proteins can be crystallized successfully in the presence of detergent^1-3, which seems to facilitate the ordered packing of the proteins by binding to their hydrophobic surfaces in micellar manner^4,5. This approach has enabled the molecular structures of two bacterial photosynthetic reaction centres to be solved at high resolution by X-ray crystallography^6-9, 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.