Aspartic acid substitutions affect proton translocation by bacteriorhodopsin.

Abstract

We have substituted each of the aspartic acid residues in bacteriorhodopsin to determine their possible role in proton translocation by this protein. The aspartic acid residues were replaced by asparagines; in addition, Asp-85, -96, -115, and -212 were changed to glutamic acid and Asp-212 was also replaced by alanine. The mutant bacteriorhodopsin genes were expressed in Escherichia coli and the proteins were purified. The mutant proteins all regenerated bacteriohodopsin-like chromophores when treated with a detergent-phospholipid mixture and retinal. However, the rates of regeneration of the chromophores and their .lambda.max varied widely. No support was obtained for the external point charge model for the opsin shift. The Asp-85 .fwdarw. Asn mutant showed no detectable proton pumping, the Asp-96 .fwdarw. Asn and Asp-212 .fwdarw. Glu mutants showed < 10% and the Asp-115 .fwdarw. Glu mutant showed .apprxeq. 30% of the normal proton pumping. The implications of these findings for possible mechanisms of proton translocation by bacteriorhodopsin are discussed.