Spectral Tuning in the Mammalian Short-Wavelength Sensitive Cone Pigments

The wild-type mouse ultraviolet (UV) and bovine blue cone visual pigments have absorption maxima of 358 and 438 nm, respectively, while sharing 87% amino acid identity. To determine the molecular basis underlying the 80 nm spectral shift between these pigments, we selected several amino acids in helices II and III for site-directed mutagenesis. These amino acids included: (1) those that differ between mouse UV and bovine blue; (2) the conserved counterion, Glu113; and (3) Ser90, which is involved in wavelength modulation in avian short-wavelength sensitive cone pigments. These studies resulted in the identification of a single amino acid substitution at position 86 responsible for the majority of the spectral shift between the mouse UV and bovine blue cone pigments. This is the first time that this amino acid by itself has been shown to play a major role in the spectral tuning of the SWS1 cone pigments. A single amino acid substitution appears to be the dominant factor by which the majority of mammalian short-wavelength sensitive cone pigments have shifted their absorption maxima from the UV to the visible regions of the spectrum. Studies investigating the role of the conserved counterion Glu113 suggest that the bovine and mouse SWS1 pigments result from a protonated and unprotonated Schiff base chromophore, respectively.