Conformational Change of Helix G in the Bacteriorhodopsin Photocycle: Investigation with Heavy Atom Labeling and X-Ray Diffraction

Abstract

According to the current structural model of bacteriorhodopsin, Ile 222 is located at the cytoplasmic end of helix G. We labeled the single cysteine of the site-directed mutant Ile 222 → Cys with p-chloromercuribenzoic acid and determined the position of the labeled mercury by x-ray diffraction in the unphotolyzed state, and in the M N photointermediate accumulated in the presence of guanidine hydrochloride at pH 9.5. According to the difference Fourier maps between the M N intermediate and the unphotolyzed state, the structural change in the M N intermediate was not affected by mercury labeling. The difference Fourier map between the labeled and the unlabeled I222C gave the position of the mercury label. This information was obtained for both the unphotolyzed state and the M N intermediate. We found that the position of the mercury at residue 222 is shifted by 2.1 ± 0.8 Å in the M N intermediate. This agrees with earlier results that suggested a structural change in the G helix. The movement of the mercury label is so large that it must originate from a cooperative conformational change in the helix G at its cytoplasmic end, rather than from displacement of residue 222. Because Ile 222 is located at the same level on the z coordinate as Asp 96, the structural change in the G helix could have the functional role of perturbing the environment and therefore the pK a of this functionally important aspartate.