Lipid location in deoxycholate-treated purple membrane at 2.6 Å

A high resolution projection at 2.6 A of deoxycholate-treated purple membrane using only images has been obtained with a 200 keV FEG microscope operated at liquid helium temperature. Examination of this high quality map has allowed the following conclusions to be made: Comparison between the internal structure of the trimers of the native and the deoxycholate-treated crystal forms shows that almost every detail of the structure at high resolution is identical. The cell dimension change from 62.4 A to 57.9 A is accompanied by a loss of about half the normal lipids and a 2 degrees anticlockwise rotation of the trimer as a rigid body. Three of the lipids per bacteriorhodopsin molecule remain in identical positions relative to the trimer. In addition, from the projection map together with a packing analysis using the atomic model for bacteriorhodopsin, space for three further lipids has been identified making a total of six lipids per bacteriorhodopsin molecule in this crystal form. Finally, the surprisingly small rotation of the trimer between the two crystal forms with completely different Van der Waals contacts suggest that the crystals are held together by strong, long-range electrostatic interactions.