A Constructive Study Based on Rhodopsin to Explore the Origin of Extreme Redshift and Nontypical Isomerization of Bestrhodopsin.

Bestrhodopsins are recently discovered microbial rhodopsins comprising one or two photosensitive rhodopsin domains and an ion channel. Their rhodopsin domains exhibit extremely red-shifted absorption spectra and a nontypical all--to-11- photoisomerization of the retinal chromophore. To determine the origin of these characteristics, we reconstituted a bestrhodopsin-like retinal-binding pocket in a prototypical microbial rhodopsin, rhodopsin (GR). A triple mutation, D121E/T125D/A256M, in GR induced a 70-nm redshift of its absorption maximum and a pH-dependent spectral shift mirroring -RRB, the best-characterized bestrhodopsin. The D121E/T125D/A256M substitutions also changed the isomerization position on the retinal chromophore from the typical C13=C14 to the C9=C10 bond, whereas an additional mutation, V126A, was found to be critical for efficient photoreaction. Thus, the present study identified four amino acid residues from bestrhodopsin that partially confer unique bestrhodopsin-like spectroscopic and photochemical properties on GR, providing insights into the mechanisms determining the photoisomerization pattern among rhodopsins.