Neutron Diffraction Studies of Bacteriorhodopsin Structure

Abstract

Halobacterium halobium is a microorganism that can survive only in solutions containing more than 12% salt (1). When oxygen supplies become limited, the organism synthesizes a specialized region in the plasma membrane that converts visible light energy into an electrochemical gradient by pumping protons across the plasma membrane out of the cell (2). This specialized membrane can be isolated in a stable form at low ionic strength (3,4) and is known as the purple membrane by virtue of its strong pigmentation. Purple membrane is 75% protein and only 25% lipid. The single protein species whose retinal prosthetic group is responsible for the purple color is called bacteriorhodopsin, by analogy with the vertebrate visual pigments. As shown by x-ray diffraction studies, bacteriorhodopsin exists in a hexagonal lattice in the plane of the membrane bilayer, and the packing arrangement is described by the two-dimensional crystallographic space group p3 (5).