Structure-function Studies on Bacteriorhodopsin. V. Effects of Amino Acid Substitutions in the Putative Helix F

To test structural and mechanistic proposals about bacteriorhodopsin, a series of analogues with single amino acid substitutions has been studied. Mutants in the proposed helix F of bacteriorhodopsin were chosen for investigation because of the probable interaction of this part of the protein with the retinal chromophore. Seven mutants of the bacteriorhodopsin gene were constructed by site-directed mutagenesis, and the gene products were expressed in Escherichia coli. The resulting mutant proteins were purified and assayed for their ability to interact with retinal in phospholipid/detergent micelles to form a bacteriorhodopsin-like chromophore. Four mutants, Ser-183----Ala, Tyr-185----Phe, Ser-193----Ala, and Glu-194----Gln, bound retinal to give pigments with absorption maxima approximately the same as the wild type. Three mutant opsins bound retinal to give chromophores that were blue-shifted relative to the wild type. Two Trp----Phe substitutions at positions 182 and 189 gave absorption maxima of 480 and 524 nm, respectively, and the mutant Pro-186----Leu gave a pigment with an absorption maximum of 470 nm. However, none of the amino acid substitutions eliminated the ability of the mutant bacteriorhodopsin to pump protons in response to illumination.