Similarity of bacteriorhodopsin structural changes triggered by chromophore removal and light-driven proton transport.

Bacteriorhodopsin (bR) is the light-driven proton pump found in the purple membrane of Halobacterium salinarium. A series of conformational changes occur during the bR photocycle which involve alterations in buried-helical structure as well as in the protonation state of Asp residues which are part of the proton transport pathway. Here we report evidence that similar conformational changes occur upon removal of the retinylidene chromophore of bacteriorhodopsin to form the apoprotein bacterioopsin (bO). This suggests a simple ligand-binding model of proton transport in bacteriorhodopsin which may have relevance to other transport and signal transducing membrane proteins including the visual photoreceptor rhodopsin.