Effect of Mutation of M145F/F146M on the Photocycle of Photoreceptors Bacteriorhodopsin and Archaerhodopsin 4

Both archaerhodopsin 4 (aR4) and bacteriorhodopsin (bR) belong to the Halobacterium family, share 59% homology and function as proton pumps to transfer protons from the cytoplasmic side to the extracellular side to convert light energy to chemical energy through ATP synthesis. Although aR4 has a similar photocycle as bR, it has an opposite temporal order of proton uptake and release at neutral pH. Methionine-145 (M145), one of the key residues locating within the retinal binding pocket in bR, has significant influence on the bR photocycle. Phenylalanine-146 (F146) is the corresponding residue within the retinal binding pocket in aR4, and is the only different binding residue between the two proteins. Effects of M145F and F146M mutations on the photocycle of bR and aR4 were studied by UV-VIS spectroscopy, light-induced kinetic change spectroscopy, proton pumping analysis and low temperature transmission FTIR spectroscopy. Loss of L state and decrease of the proton pumping capability in M145F mutation were observed in the bR photocycle, whereas neither a significant affect to the photocycle nor any a change to proton release and uptake order by F146M mutation were observed in aR4, which clear indicated that the function roles of the two residues are not exactly the same in the two proton pumps.