Evolutionary steps involving counterion displacement in a tunicate opsin

Proc Natl Acad Sci U S A. 2017 Jun 6;114(23):6028-6033. doi: 10.1073/pnas.1701088114. Epub 2017 May 22.


Ci-opsin1 is a visible light-sensitive opsin present in the larval ocellus of an ascidian, Ciona intestinalis 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.

Keywords: G protein-coupled receptors; ascidian; counterion; molecular evolution; opsin.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Biological Evolution
  • Ciona intestinalis / physiology
  • Evolution, Molecular
  • GTP-Binding Proteins / metabolism
  • Glutamic Acid / metabolism
  • Opsins / chemistry*
  • Opsins / metabolism*
  • Opsins / physiology*
  • Phylogeny
  • Receptors, G-Protein-Coupled / metabolism
  • Rhodopsin / metabolism
  • Rod Opsins / metabolism
  • Urochordata / physiology


  • Opsins
  • Receptors, G-Protein-Coupled
  • Rod Opsins
  • Glutamic Acid
  • Rhodopsin
  • GTP-Binding Proteins