A novel spectral tuning in the short wavelength-sensitive (SWS1 and SWS2) pigments of bluefin killifish (Lucania goodei)

Abstract

The molecular bases of spectral tuning in the UV-, violet-, and blue-sensitive pigments are not well understood. Using the in vitro assay, here we show that the SWS1, SWS2-A, and SWS2-B pigments of bluefin killifish (Lucania goodei) have the wavelengths of maximal absorption (lambda(max)'s) of 354, 448, and 397 nm, respectively. The spectral difference between the SWS2-A and SWS2-B pigments is largest among those of all currently known pairs of SWS2 pigments within a species. The SWS1 pigment contains no amino acid replacement at the currently known 25 critical sites and seems to have inherited its UV-sensitivity directly from the vertebrate ancestor. Mutagenesis analyses show that the amino acid differences at sites 44, 46, 94, 97, 109, 116, 118, 265, and 292 of the SWS2-A and SWS2-B pigments explain 80% of their spectral difference. Moreover, the larger the individual effects of amino acid changes on the lambda(max)-shift are, the larger the synergistic effects tend to be generated, revealing a novel mechanism of spectral tuning of visual pigments.

Fig. 1

The absorption spectra of the bluefin killifish SWS1, SWS2-A, and SWS2-B pigments evaluated by the in vitro assay. The dark-light difference spectra are shown in the inset.

Fig. 2

The phylogenetic tree of some representative fish visual pigments. The numbers after P in the SWS2 pigments refer to λ_max’s evaluated by the in vitro assay. The filled circles at different nodes indicate that their bootstrap values are >0.95; otherwise, the bootstrap values are <0.95. The bar at the bottom indicates evolutionary distance measured as the number of nucleotide substitutions per site.