EVIDENCE FOR CHARGE-CONTROLLED CONFORMATIONAL-CHANGES IN THE PHOTOCYCLE OF BACTERIORHODOPSIN

The existence of two different M-state structures in the photocycle of the bacteriorhodopsin mutant ASP38ARG was proved. At pH 6.7 (0 to -6 degrees C) a spectroscopic M intermediate (M,) that does not differ si gnificantly in its tertiary structure from the light-adapted ground st ate accumulates under illumination. At pH > 9 another state (M-2), cha racterized by additional pronounced changes in the Fourier transform i nfrared difference spectrum in the region of the amide I and II bands, accumulates. The M-2 intermediate trapped at pH 9.6 displays the same changes in the x-ray diffraction intensities under continuous illumin ation as previously described for x-ray experiments with the mutant AS P96ASN. These observations indicate that in this mutant the altered ch arge distribution at neutral pH controls the tertiary structural chang es that seem to be necessary for proton translocation.