Structure of the bacteriorhodopsin D85ND96N double mutant showing substantial structural changes and a highly twinned, disordered lattice

Bacteriorhodopsin (bR) containing the double mutation D85N, D96N undergoes light-induced colour changes but is much less active in proton transport. It is known to have a substantially perturbed structure [Kataoka et al., J. Mol. Biol. 243 (1994) 621], but still forms two-dimensional crystals in the halobacterial cell membrane. Using electron microscopy and diffraction, the crystal lattice has been found to be less stable than in wild-type and more highly twinned with much smaller areas of coherent crystallinity. It can be studied only in the fully frozen, hydrated state. Comparison of the mutant and wild type structure after compensation for twinning and disorder shows extensive structural changes distributed throughout the bR molecule. Some of the changes, in the region of helices F and G, parallel those observed during light-induced proton pumping in wild type and some other mutants that are active in proton transport, but additional structural changes unique to the double mutant are also found.