Evidence for a controlling role of water in producing the native bacteriorhodopsin structure

The experiments reported in this paper, based on reconstitution of bacteriorhodopsin (bR) from apomembrane at varying environmental conditions, demonstrate that the presence of water is a controlling factor in generating a native wild-type bR conformation. If water is lacking during this reconstitution process, then a non-native bR structure is formed that exhibits altered M formation and decay kinetics, as well as different behavior following extensive dehydration. It is shown that mutants affecting the ability of bR to form appropriate structures of water in specific protein cavities also affect the ability to generate a native bR conformation. The results suggest that aspartic acid 96 plays a major role in anchoring the appropriate water structure conformation associated with bR. It is also demonstrated that the glutamic acid 204 residue is pivotal in controlling the protein/water affinity. This water affinity can be further controlled by modifying the charge environment of the protein with altered pH. These data, based on kinetic absorption spectroscopy and Fourier transform infrared spectroscopy, highlight the central role of water in this protein.