Functional significance of a protein conformation change at the cytoplasmic end of helix F during the bacteriorhodopsin photocycle

The second half of the photocycle of the light-driven proton pump bacteriorhodopsin includes proton transfers between D96 and the retinal Schiff base (the M to N reaction) and between the cytoplasmic surface and D96 (decay of the N intermediate). The inhibitory effects of decreased water activity and increased hydrostatic pressure have suggested that a conformational change resulting in greater hydration of the cytoplasmic region is required for proton transfer from D96 to the Schiff base, and have raised the possibility that the reversal of this process might be required for the subsequent reprotonation of D96 from the cytoplasmic surface. Tilt of the cytoplasmic end of helix F has been suggested by electron diffraction of the M intermediate. Introduction of bulky groups, such as various maleimide labels, to engineered cysteines at the cytoplasmic ends of helices A, B, C, E, and G produce only minor perturbation of the decays of M and N, but major changes in these reactions when the label is linked to helix F. In these samples the reprotonation of the Schiff base is accelerated and the reprotonation of D96 is strongly retarded. Cross-linking with benzophenone introduced at this location, but not at the others, causes the opposite change: the reprotonation of the Schiff base is greatly slowed while the reprotonation of D96 is accelerated. We conclude that, consistent with the structure from diffraction, the proton transfers in the second half of the photocycle are facilitated by motion of the cytoplasmic end of helix F, first away from the center of the protein and then back.