Bacteriorhodopsin: the mechanism of 2D-array formation and the structure of retinal in the protein.

Bacteriorhodopsin, the light driven proton pump of the extreme halophilic bacterium H. salinarium, is an integral membrane protein (M(r) ca. 26000) which forms 2D arrays in the purple membrane of the bacterium. It is this feature which has permitted the use of electron diffraction methods to resolve the protein structure to some degree of atomic detail, although the prosthetic group has not been fully resolved. However, the features which induce the protein to form these arrays have not been previously clarified. We have now shown that the protein array formation is driven by specific interaction of the protein with the charged phospholipid, phosphatidyl glycerol phosphate (or the sulphate derivative), a major (ca. 60%) lipid of the bacterial host membrane. In addition, in an effort to provide further structural information about the chromophore, retinal, of this protein, the orientation of the individual methyl groups of retinal have been determined from solid state deuterium NMR studies of the deuterated chromophore when in the protein binding site. This approach to structural resolution of the prosthetic group is ab initio, agrees with other studies on the chromophore and resolves new features of the bound retinal to a high degree (+/- 2 degrees) of precision. Here, these two studies on this integral membrane protein will be reviewed.