The water‐exposed C‐terminal sequence of bacteriorhodopsin does not affect H+ pumping

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Abstract
The fast kinetics of photocycle and H+‐pumping activity of papain‐treated bacteriorhodopsin deprived of 17 C‐terminal amino acid residues has been investigated. As demonstrated by a single‐turnover study, the formation and decomposition of the M412 intermediate as well as the generation of the photoelectric potential are similar in the native and in the papain‐treated protein. On the other hand, acidification of the medium caused by the deprotonation of bacteriorhodopsin due to M412 formation is much smaller in the C‐tail‐deprived protein. Short‐term sonication or addition of a small amount of detergent completely abolishes this effect. As a result, papain‐treated bacteriorhodopsin exhibits the same acidification as the native one. It is concluded that a decrease in the light‐induced pH response of the C‐tail‐deprived bacteriorhodopsin is caused by the aggregation of purple sheets rather than by a special role of the C‐terminal sequence in H+ pumping.