Reconstitution of a proton pump.

The retinal-protein complex bacteriorhodopsin mediates lightxnergy conversion in halobacteria. This chromoprotein is embedded in the cell membrane and forms insular regions. These can be isolated by membrane-fractionation procedures, and they consist of a lipid matrix containing bacteriorhodopsin molecules in a hexagonal crystalline arrangement called the purple membrane. Light-absorption by bacteriorhodopsin induces cyclic changes of the molecule accompanied by a vectorial release and uptake of protons. By means of this photochemical cycle an electrochemical proton gradient is created across the cell membrane on illumination, and the gradient energy is used for ATP synthesis inside the cell. A summarizing report on the function of bacteriorhodopsin and this new type of photophosphorylation mediated by it has been given (Oesterhelt, 1976).