LIPID LOCATION IN DEOXYCHOLATE-TREATED PURPLE MEMBRANE AT 2.6 ANGSTROM

A high resolution projection at 2.6 Angstrom of deoxycholate-treated p urple 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: Com parison between the internal structure of the trimers of the native an d the deoxycholate-treated crystal forms shows that almost every detai l of the structure at high resolution is identical. The cell dimension change from 62.4 Angstrom to 57.9 Angstrom is accompanied by a loss o f about half the normal lipids and a 2 degrees anticlockwise rotation of the trimer as a rigid body. Three of the lipids per bacteriorhodops in molecule remain in identical positions relative to the trimer. In a ddition, from the projection map together with a packing analysis usin g the atomic model for bacteriorhodopsin, space for three further lipi ds has been identified making a total of six lipids per bacteriorhodop sin molecule in this crystal form. Finally, the surprisingly small rot ation of the trimer between the two crystal forms with completely diff erent Van der Waals contacts suggest that the crystals are held togeth er by strong, long-range electrostatic interactions. (C) 1995 Academic Press Limited