Surface structures of native bacteriorhodopsin depend on the molecular packing arrangement in the membrane

Bacteriorhodopsin is the one of the best-studied models of an ion pump. Fiv e atomic models are now available, yet their comparison reveals differences of some loops connecting the seven transmembrane alpha-helices. In an atte mpt to resolve this enigma, topographs were recorded in aqueous solution wi th the atomic force microscope (AFM) to reveal the most native surface stru cture of bacteriorhodopsin molecules in the purple membrane. Individual pep tide loops were observed with a lateral resolution of between 4.5 Angstrom and 5.8 Angstrom, and a vertical resolution of about 1 Angstrom. The AFM im ages demonstrate for the first time, that the shape, the position, and the flexibility of individual polypeptide loops depend on the packing arrangeme nt of bacteriorhodopsin molecules in the lipid bilayer. (C) 1999 Academic P ress.