LIPID PATCHES IN MEMBRANE-PROTEIN OLIGOMERS - CRYSTAL-STRUCTURE OF THE BACTERIORHODOPSIN-LIPID COMPLEX

Heterogenous nucleation on small molecule crystals causes a monoclinic crystal form of bacteriorhodopsin (BR) in which trimers of this membr ane protein pack differently than in native purple membranes. Analysis of single crystals by nano-electrospray ionization-mass spectrometry demonstrated a preservation of the purple membrane lipid composition i n these BR crystals. The 2.9-Angstrom x-ray structure shows a lipid-me diated stabilization of BR trimers where the glycolipid S-TGA-1 binds into the central compartment of BR trimers, The BR trimer/lipid comple x provides an example of local membrane thinning as the lipid head-gro up boundary of the central lipid patch is shifted by 5 Angstrom toward the membrane center, Nonbiased electron density maps reveal structura l differences to previously reported BR structures, especially for the cytosolic EF loop and the proton exit pathway. The terminal proton re lease complex now comprises an E194-E204 dyad as a diffuse proton buff er.