Characterization of RS29, a blue-green proteorhodopsin variant from the Red Sea.

Using structural modeling comparisons and mutagenesis, amino acid residue 105 was found to function as a spectral tuning switch in marine proteorhodopsins (PR). Changes at this position account for most of the spectral difference between blue-absorbing PRs (B-PRs), and green-absorbing PRs (G-PRs). Here we analyzed a Red Sea variant (RS29) from a new family of PRs that is composed of G-PR type variants that possess glutamine instead of leucine at position 105 like in B-PRs. The absorption spectrum as well as photocycle of RS29 variant were measured and compared to point-mutated 'position 105' PRs. Unexpectedly, the absorption maximum of RS29 was 515 nm, a smaller blue shift compared to the 498 nm maximum of G-PR_L105Q. We found that two additional residues at positions 65 and 70 each contribute a small red shift to the absorption spectrum of G-PR and therefore appear to account for the intermediate absorption maximum of RS29 by their opposing influences on the spectrum. Our results show that in addition to the retinal pocket position 105 determinant, other residues predicted to be outside the retinal pocket fine-tune the absorption spectra of marine PRs. The RS29 photochemical reaction cycle was found to be 2 orders of magnitude slower than that of G-PR with a t(1/2) of >600 ms. This result raises the possibility of regulatory (i.e. sensory) rather than energy harvesting functions for some members of the PR family.