Electron crystallographic studies onE.coli- expressed variants of bacteriorhodopsin

Bacterorhodopsin (BR) is an integral membrane protein present in the purple membrane (PM) ofHalobacterium halobiumwhere it is arranged in a highly ordered 2-dimensional hexagonal P3 lattice. BR contains a single retinylidene chromophore bound via a protonated Schiff base to Lys-216 and functions as a light-driven proton pump. Upon absorption of a photon, BR cycles through a series of photointermediates resulting in a vectorial pumping of a proton from the intracellular side to the extracellular side. Site-directed mutagenesis has identified residues whose substitution affects the chromophore environment in BR and those that are involved in the proton transport. Specifically Asp-85, Asp-212, and Arg-82 are involved in the proton release pathway, whereas Asp-96 is involved in Schiff base reprotonation.