1.    History

        Structure deduced by Mitchell in early 1960's from naturally occuring 2D crystals oriented in cytoplasmic membrane of saline-loving archaebacterium Halobacterium salinarum (also present in H. halobium) using cryo-electron microscopy.

        Initial research on structure and mechanism led to revelation of its key role in proton transport mechanism.  Difficulties in purifying from membrane while maintaining photochemical activity led to development of cubic lipidic phase purification process by Landau
and Rosenbusch in the mid 1970's.

        Between then and now, continual research has been done generating around twenty coordinate files in PDB.  Research on this protein has led to breakthroughs in fields including
cryo-electron microscopy, x-ray crystallography, and solid-state NMR.

2.     Basic structural information

        Bacteriorhodopsin (BR) is a relatively small 26 kDa protein containing seven alpha helices and spanning the cytoplasmic membrane with short interhelical loop regions containing small beta sheets.  A retinal cofactor is positioned in the center of the helices, and its photoisomerization from all-trans to 13-cis provides the necessary energy to drive a directional proton pump from cytoplasm to extracellular space.

3.    Photocycle, Mechanism, etc.

        The entire process of photoisomerization and proton transport across the cell membrane involves a series of intermediate substates called K, L, M (thought to consist of at least 2 subsubstates), N, and O.  Recent research has focused upon the cryo-trapping of these intermediate states, in both wild-type and mutant forms, in order to elucidate with increasing resolution the biomechanical and bioenergetic specifics of the whole photo cycle.   CHIME Graphics

4.    Why do we care?

        We care because this little molecule has revealed a huge amount of information concerning a fundamental biochemical process (i.e. proton transport against a gradient) and become a model for larger membrane proteins while playing a part in the improvement of structural, bioenergetic and biophysical techniques.