Inhibition of the M1→M2 (Mclosed→Mopen) transition in the D96N mutant photocycle and its relation to the corresponding transition in wild-type bacteriorhodopsin

Glutaraldehyde, lutetium ions and glycerol inhibit the blue shift of the difference spectra maximum of the M intermediate in the D96N mutant. The M formed has a spectrum indistinguishable from the M intermediate in wild-type bacteriorhodopsin. It has been concluded that the M open form previously described by us is identical to the M2 and M n intermediates postulated by Zimanyi et al. ( Photochem. Photobiol. (1992) 56, 1049–1055) and Sasaki et al. ( J. Biol. Chem. (1992) 267, 20782–20786), respectively. It is supposed that its formation is accompanied by the appearance of the cytoplasmic proton half-channel. M open in the wild-type protein is present in a very low amount due to the shift of the M closed ↔M open equilibrium towards the M closed . The inhibitors used do not prevent the multiphase pattern of the M formation in either mutant or wild-type proteins.