Proton pump caught in the act.

On page 255 of this issue, Luecke et al. (1) reveal the structure of bacteriorhodopsin, an active transport protein, frozen in midstroke. Bacteriorhodopsin is a membrane protein that uses a photon of visible light as an energy source to transport a proton across the membrane against an electrochemical gradient, resulting in proton release on one side of the membrane and proton uptake on the opposite side. In order to take up a proton on one side and release it on the other, the affinity for the transported substance (a proton in this instance) must be altered along its path during the transport. Furthermore, a switch mechanism must assure that the pump is unidirectional. Some of the underlying molecular mechanisms are now revealed by the 2.0 Å resolution x-ray crystallographic structure of bacteriorhodopsin trapped in an intermediate state (1).