THE TERTIARY STRUCTURAL-CHANGES IN BACTERIORHODOPSIN OCCUR BETWEEN M-STATES - X-RAY-DIFFRACTION AND FOURIER-TRANSFORM-INFRARED-SPECTROSCOPY

The tertiary structural changes occurring during the photocycle of bac teriorhodopsin (BR) are assigned by X-ray diffraction to distinct M st ates, M-1, and M-2. Purple membranes (PM) of the mutant Asp96Asn at 15 , 57, 75 and 100% relative humidity (r.h.) were studied in a parallel X-ray diffraction and Fourier transform infrared (FTIR) spectroscopic investigation. Light-dependent conformational changes of BR-Asp96-Asn are observed at high hydration levels (100 and 75% r.h.) but not in pa rtially dehydrated samples (57 and 15% r.h.). The FTIR spectra of cont inuously illuminated samples at low and high hydration, despite some d ifferences, are characteristic of the M intermediate. The changes in d iffraction patterns of samples in the M-2 state are of the same magnit ude as those of wild-type samples trapped with GuaHCl in the M-G state . Additional large changes in the amide bands of the FTIR spectra occu r between M-2 and M-G. This suggests, that the tertiary structural cha nges between M-1 and M-2 are responsible for the switch opening the cy toplasmic half-channel of BR for reprotonation to complete the catalyt ic cycle. These tertiary structural changes seem to be triggered by a charge redistribution which might be a common feature of retinal prote ins also in signal transduction.