Lo. Essen et al., LIPID PATCHES IN MEMBRANE-PROTEIN OLIGOMERS - CRYSTAL-STRUCTURE OF THE BACTERIORHODOPSIN-LIPID COMPLEX, Proceedings of the National Academy of Sciences of the United Statesof America, 95(20), 1998, pp. 11673-11678
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.