Evolution of nonspectral rhodopsin function at high altitudes

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 114; no. 28; pp. 7385 - 7390
• Main Authors: Castiglione, Gianni M., Hauser, Frances E., Liao, Brian S., Lujan, Nathan K., Van Nynatten, Alexander, Morrow, James M., Schott, Ryan K., Bhattacharyya, Nihar, Dungan, Sarah Z., Chang, Belinda S. W.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 11.07.2017
• Subjects: Adaptation; Altitude; Amino acids; Biological evolution; Biological Sciences; Catfish; Chemical bonds; Cold; Evolution; Experiments; G protein-coupled receptors; High altitude; High altitude environments; Hydrogen bonding; Hypoxia; Mutagenesis; Photopigments; Positive selection; Proteins; Receptors; Rhodopsin; Site-directed mutagenesis; Stability; Terrestrial environments; Visual perception; G protein-coupled receptor; Andean catfishes; protein evolution; visual pigment; in vitro expression
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1705765114

High-altitude environments present a range of biochemical and physiological challenges for organisms through decreases in oxygen, pressure, and temperature relative to lowland habitats. Protein-level adaptations to hypoxic high-altitude conditions have been identified in multiple terrestrial endotherms; however, comparable adaptations in aquatic ectotherms, such as fishes, have not been as extensively characterized. In enzyme proteins, cold adaptation is attained through functional trade-offs between stability and activity, often mediated by substitutions outside the active site. Little is known whether signaling proteins [e.g., G protein-coupled receptors (GPCRs)] exhibit natural variation in response to cold temperatures. Rhodopsin (RH1), the temperature-sensitive visual pigment mediating dim-light vision, offers an opportunity to enhance our understanding of thermal adaptation in a model GPCR. Here, we investigate the evolution of rhodopsin function in an Andean mountain catfish system spanning a range of elevations. Using molecular evolutionary analyses and site-directed mutagenesis experiments, we provide evidence for cold adaptation in RH1. We find that unique amino acid substitutions occur at sites under positive selection in high-altitude catfishes, located at opposite ends of the RH1 intramolecular hydrogen-bonding network. Natural high-altitude variants introduced into these sites via mutagenesis have limited effects on spectral tuning, yet decrease the stability of dark-state and light-activated rhodopsin, accelerating the decay of ligand-bound forms. As found in cold-adapted enzymes, this phenotype likely compensates for a cold-induced decrease in kinetic rates—properties of rhodopsin that mediate rod sensitivity and visual performance. Our results support a role for natural variation in enhancing the performance of GPCRs in response to cold temperatures.