title: A comparative study of rhodopsin function in the great bowerbird (Ptilonorhynchus nuchalis): Spectral tuning and light-activated kinetics.
abstract: Rhodopsin is the visual pigment responsible for initiating the phototransduction cascade in vertebrate rod photoreceptors. Although well-characterized in a few model systems, comparative studies of rhodopsin function, particularly for nonmammalian vertebrates are comparatively lacking. Bowerbirds are rare among passerines in possessing a key substitution, D83N, at a site that is otherwise highly conserved among G protein-coupled receptors. While this substitution is present in some dim-light adapted vertebrates, often accompanying another unusual substitution, A292S, its functional relevance in birds is uncertain. To investigate functional effects associated with these two substitutions, we use the rhodopsin gene from the great bowerbird (Ptilonorhynchus nuchalis) as a background for site-directed mutagenesis, in vitro expression and functional characterization. We also mutated these sites in two additional rhodopsins that do not naturally possess N83, chicken and bovine, for comparison. Both sites were found to contribute to spectral blue-shifts, but had opposing effects on kinetic rates. Substitutions at site 83 were found to primarily affect the kinetics of light-activated rhodopsin, while substitutions at site 292 had a larger impact on spectral tuning. The contribution of substitutions at site 83 to spectral tuning in particular depended on genetic background, but overall, the effects of substitutions were otherwise surprisingly additive, and the magnitudes of functional shifts were roughly similar across all three genetic backgrounds. By employing a comparative approach with multiple species, our study provides new insight into the joint impact of sites 83 and 292 on rhodopsin structure-function as well as their evolutionary significance for dim-light vision across vertebrates.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Canada.
authors.forename: Ilke
authors.initials: I
authors.lastname: van Hazel
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Canada.
authors.forename: Sarah Z
authors.initials: SZ
authors.lastname: Dungan
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Canada.
authors.forename: Frances E
authors.initials: FE
authors.lastname: Hauser
authors.affiliation: Department of Cell and Systems Biology, University of Toronto, Canada.
authors.forename: James M
authors.initials: JM
authors.lastname: Morrow
authors.affiliation: Centre for Integrative Ecology, Deakin University, Australia.
authors.forename: John A
authors.initials: JA
authors.lastname: Endler
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Canada.|Department of Cell and Systems Biology, University of Toronto, Canada.|Centre for the Analysis of Genome Evolution and Function, University of Toronto, Canada.
authors.forename: Belinda S W
authors.initials: BS
authors.lastname: Chang
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D002417:Cattle; D002645:Chickens / Q000235:genetics / Q000378:metabolism*; D019143:Evolution, Molecular; D007700:Kinetics; D008958:Models, Molecular; D046109:Passeriformes / Q000235:genetics / Q000378:metabolism*; D010802:Phylogeny; D012243:Rhodopsin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D040681:Structural Homology, Protein
chemical_list: D012243:Rhodopsin
keywords: comparative biochemistry; evolution of protein structure and function; evolution of vision; passerine birds; visual pigment