Structural characterization of the L-to-M transition of the bacteriorhodopsin photocycle.
Structural intermediates occurring in the photocycle of wild-type bacteriorhodopsin are trapped by illuminating hydrated, glucose-embedded purple membrane at 170 K, 220 K, 230 K, and 240 K. We characterize light-induced changes in protein conformation by electron diffraction difference Fourier maps, and relate these to previous work on photocycle intermediates by infrared (FTIR) spectroscopy. Samples illuminated at 170 K are confirmed by FTIR spectroscopy to be in the L state; a difference Fourier projection map shows no structural change within the 0.35-nm resolution limit of our data. Difference maps obtained with samples illuminated at 220 K, 230 K, and 240 K, respectively, reveal a progressively larger structural response in helix F when the protein is still in the M state, as judged by the FTIR spectra. Consistent with previous structural studies, an adjustment in the position or in the degree of ordering of helix G accompanies this motion. The model of the photocycle emerging from this and previous studies is that bacteriorhodopsin experiences minimal change in protein structure until a proton is transferred from the Schiff base to Asp85. The M intermediate then undergoes a conformational evolution that opens a hydrated "half-channel," allowing the subsequent reprotonation of the Schiff base by Asp96.
Hendrickson
F M
FM
Life Sciences Division, Donner Laboratory, University of California, Berkeley 94720, USA. fmh@xtalu.lbl.gov
Burkard
F
F
Glaeser
R M
RM
D001224:Aspartic Acid / Q000737:chemistry / Q000528:radiation effects; D001436:Bacteriorhodopsins / Q000737:chemistry* / Q000528:radiation effects*; D055592:Biophysical Phenomena; D001703:Biophysics; D019617:Halobacterium salinarum / Q000737:chemistry / Q000528:radiation effects; D008027:Light; D010777:Photochemistry; D011487:Protein Conformation / Q000528:radiation effects; D017433:Protein Structure, Secondary; D011522:Protons; D012545:Schiff Bases / Q000737:chemistry / Q000528:radiation effects; D017550:Spectroscopy, Fourier Transform Infrared
D011522:Protons; D012545:Schiff Bases; D001224:Aspartic Acid; D001436:Bacteriorhodopsins