Dungan Sarah Z, Chang Belinda S W
Department Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, Canada M5S 3B2.
Department Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, Canada M5S 3B2
Proc Biol Sci. 2017 Mar 15;284(1850). doi: 10.1098/rspb.2016.2743.
Like many aquatic vertebrates, whales have blue-shifting spectral tuning substitutions in the dim-light visual pigment, rhodopsin, that are thought to increase photosensitivity in underwater environments. We have discovered that known spectral tuning substitutions also have surprising epistatic effects on another function of rhodopsin, the kinetic rates associated with light-activated intermediates. By using absorbance spectroscopy and fluorescence-based retinal release assays on heterologously expressed rhodopsin, we assessed both spectral and kinetic differences between cetaceans (killer whale) and terrestrial outgroups (hippo, bovine). Mutation experiments revealed that killer whale rhodopsin is unusually resilient to pleiotropic effects on retinal release from key blue-shifting substitutions (D83N and A292S), largely due to a surprisingly specific epistatic interaction between D83N and the background residue, S299. Ancestral sequence reconstruction indicated that S299 is an ancestral residue that predates the evolution of blue-shifting substitutions at the origins of Cetacea. Based on these results, we hypothesize that intramolecular epistasis helped to conserve rhodopsin's kinetic properties while enabling blue-shifting spectral tuning substitutions as cetaceans adapted to aquatic environments. Trade-offs between different aspects of molecular function are rarely considered in protein evolution, but in cetacean and other vertebrate rhodopsins, may underlie multiple evolutionary scenarios for the selection of specific amino acid substitutions.
与许多水生脊椎动物一样，鲸鱼在暗光视觉色素视紫红质中具有蓝移光谱调谐替代，这被认为可提高水下环境中的光敏感性。我们发现，已知的光谱调谐替代对视紫红质的另一个功能——与光激活中间体相关的动力学速率，也有惊人的上位效应。通过对异源表达的视紫红质进行吸光光谱分析和基于荧光的视黄醛释放测定，我们评估了鲸类动物（虎鲸）和陆生外类群（河马、牛）之间的光谱和动力学差异。突变实验表明，虎鲸视紫红质对关键蓝移替代（D83N和A292S）对视黄醛释放的多效性影响具有异常的抗性，这主要是由于D83N与背景残基S299之间存在惊人的特异性上位相互作用。祖先序列重建表明，S299是一个祖先残基，早于鲸目起源时蓝移替代的进化。基于这些结果，我们推测，分子内上位作用有助于保留视紫红质的动力学特性，同时在鲸类动物适应水生环境时实现蓝移光谱调谐替代。在蛋白质进化中，很少考虑分子功能不同方面之间的权衡，但在鲸类和其他脊椎动物的视紫红质中，可能是选择特定氨基酸替代的多种进化场景的基础。
