Molecular Basis of Spectral Tuning in the Newt Short Wavelength Sensitive Visual Pigment

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Abstract
Previously we reported the sequence of the member of the short wavelength sensitive 2 (SWS2) family of vertebrate visual pigments from the retina of the Japanese common newt, Cynops pyrrhogaster [Takahashi, Y. et al. (2001) FEBS Lett. 501, 151−155]. Now we have expressed the apopigment and regenerated it with A1 retinal. Its absorption maximum, 474 nm, is greatly red shifted compared to other known SWS2 pigments (418−455 nm). To determine the amino acid residues that control its spectral tuning, we replaced the residues that were near the chromophore and which differed between the newt and the bullfrog (λmax = 430 nm) wild-type SWS2 pigments: Pro91Ser, Ser94Ala, Ile122Met, Cys127Ser, Ser211Cys, Tyr261Phe, and Ala292Ser. Each of these site-directed mutants led to blue shifts of the newt pigment with five of them causing substantial shifts; their sum was about equal to the difference between the absorption maximum of the bullfrog and newt pigments, 44 nm. The 32 nm shift of the absorption maximum of the multiple seven-residue mutant to 442 nm is fairly close to that of the wild-type bullfrog pigment. Thus, the seven amino acid residues that we replaced are the major cause of the red shift of the newt SWS2 pigment's spectrum. Two of the residues, 91 and 94, have not previously been identified as wavelength regulating sites in visual pigments. One of these, 91, probably regulates color via a new mechanism: altering of a hydrogen bonding network that is connected via a water to the chromophore, in this case its counterion, Glu113.