On the two forms of bacteriorhodopsin.

In our previous work [(1993) FEBS Lett. 313, 248-250; (1993) Biochem. Int. 30, 461-469] M-intermediate formation of wild-type bacteriorhodopsin was shown to involve two components differing in time constants (tau 1 = 60-70 microseconds and tau 2 = 220-250 microseconds), which were suggested to reflect two independent pathways of M-intermediate formation. The contribution of the fast M was 4-times higher than the slow one. Our present research on M-intermediate formation in the D115N bacteriorhodopsin mutant revealed the same components but at a contribution ratio of 1:1. Upon lowering the pH, the slow phase of M-formation vanished at a pK of 6.2, and in the pH region 3.0-5.5 only the M-intermediate with a rise time of 60 microseconds was present. A 5-6 h incubation of D115N bacteriorhodopsin at pH 10.6 resulted in the irreversible transformation of 50% of the protein into a form with a difference absorbance maximum at 460 nm. This form was stable at pH 7.5 and had no photocycle, including M-intermediate formation. The remaining bacteriorhodopsin contained 100% fast M-intermediate. The disappearance of the 250-microseconds phase concomitant with bR460 formation indicates that at neutral pH bacteriorhodopsin exists as two spectroscopically indistinguishable forms.