2017年6月6日
Evolutionary steps involving counterion displacement in a tunicate opsin
Proceedings of the National Academy of Sciences of the United States of America
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- 巻
- 114
- 号
- 23
- 開始ページ
- 6028
- 終了ページ
- 6033
- 記述言語
- 掲載種別
- 研究論文(学術雑誌)
- DOI
- 10.1073/pnas.1701088114
- 出版者・発行元
- Proceedings of the National Academy of Sciences
Ci-opsin1 is a visible light-sensitive opsin present in the larval ocellus of an ascidian, <italic>Ciona intestinalis</italic>. This invertebrate opsin belongs to the vertebrate visual and nonvisual opsin groups in the opsin phylogenetic tree. Ci-opsin1 contains candidate counterions (glutamic acid residues) at positions 113 and 181; the former is a newly acquired position in the vertebrate visual opsin lineage, whereas the latter is an ancestral position widely conserved among invertebrate opsins. Here, we show that Glu113 and Glu181 in Ci-opsin1 act synergistically as counterions, which imparts molecular properties to Ci-opsin1 intermediate between those of vertebrate- and invertebrate-type opsins. Synergy between the counterions in Ci-opsin1 was demonstrated by E113Q and E181Q mutants that exhibit a pH-dependent spectral shift, whereas only the E113Q mutation in vertebrate rhodopsin yields this spectral shift. On absorbing light, Ci-opsin1 forms an equilibrium between two intermediates with protonated and deprotonated Schiff bases, namely the MI-like and MII-like intermediates, respectively. Adding G protein caused the equilibrium to shift toward the MI-like intermediate, indicating that Ci-opsin1 has a protonated Schiff base in its active state, like invertebrate-type opsins. Ci-opsin1’s G protein activation efficiency is between the efficiencies of vertebrate- and invertebrate-type opsins. Interestingly, the E113Y and E181S mutations change the molecular properties of Ci-opsin1 into those resembling invertebrate-type or bistable opsins and vertebrate ancient/vertebrate ancient-long or monostable opsins, respectively. These results strongly suggest that acquisition of counterion Glu113 changed the molecular properties of visual opsin in a vertebrate/tunicate common ancestor as a crucial step in the evolution of vertebrate visual opsins.
- リンク情報
- ID情報
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- DOI : 10.1073/pnas.1701088114
- ISSN : 0027-8424
- eISSN : 1091-6490
- Web of Science ID : WOS:000402703800066