A light-driven proton-pumping protein (248 residues, 26 kD), similar to rhodopsin, found in "purple patches" in the cytoplasmic membrane of the bacterium Halobacterium halobium. It is composed of 7 transmembrane helices, and contains the light-absorbing chromophore, retinal. Light absorption maxima: 568nm (light-adapted) ; 558nm (dark-adapted). Each photon results in the movement of 2 protons from cytoplasmic to extracellular sides of the membrane. The resulting proton gradient is used (amongst other things) to drive synthesis of ATP by chemiosmosis.
Dictionary of molecular biology. 2004.