THEORETICAL STUDY OF COLOR CONTROL MECHANISM IN RETINAL PROTEINS. I. ROLE OF THE TRYPTOPHAN RESIDUE, TYROSINE RESIDUE and WATER MOLECULE

Additive and epistatic effects influence spectral tuning in molluscan retinochrome opsin

The relationship between genotype and phenotype is nontrivial due to often complex molecular pathways that make it difficult to unambiguously relate phenotypes to specific genotypes. Photopigments, an opsin apoprotein bound to a light-absorbing chromophore, present an opportunity to directly relate the amino acid sequence to an absorbance peak phenotype (λmax). We examined this relationship by conducting a series of site-directed mutagenesis experiments of retinochrome, a non-visual opsin, from two closely related species: the common bay scallop, Argopecten irradians, and the king scallop, Pecten maximus. Using protein folding models, we identified three amino acid sites of likely functional importance and expressed mutated retinochrome proteins in vitro. Our results show that the mutation of amino acids lining the opsin binding pocket are responsible for fine spectral tuning, or small changes in the λmax of these light sensitive proteins Mutations resulted in a blue or red shift as predicted, but with dissimilar magnitudes. Shifts ranged from a 16 nm blue shift to a 12 nm red shift from the wild-type λmax. These mutations do not show an additive effect, but rather suggests the presence of epistatic interactions. This work highlights the importance of binding pocket shape in the evolution of spectral tuning and builds on our ability to relate genotypic changes to phenotypes in an emerging model for opsin functional analysis.

Molecular genetic basis of adaptive selection: examples from color vision in vertebrates

A central unanswered question in phototransduction is how photosensitive molecules, visual pigments, regulate their absorption spectra. In nature, there exist various types of visual pigments that are adapted to diverse photic environments. To elucidate the molecular mechanisms involved in the adaptive selection of these pigments, we have to identify amino acid changes of pigments that are potentially important in changing the wavelength of maximal absorption (lambda max) and then determine the effects of these mutations on the shift in lambda max. The desired mutants can be constructed using site-directed mutagenesis, expressed in tissue culture cells, and the functional effect of virtually any such mutant can be rigorously determined. The availability of these cell/molecular methods makes vision an ideal model system in studying adaptive mechanisms at the molecular level. The identification of potentially important amino acid changes using evolutionary biological means is an indispensable step in elucidating the molecular mechanisms that underlie the spectral tuning of visual pigments.