Evolutionary steps involving counterion displacement in a tunicate opsin.

Area of Science:

• Biochemistry
• Evolutionary Biology
• Molecular Biology

Background:

• Ci-opsin1 is a light-sensitive opsin found in the ascidian *Ciona intestinalis*, phylogenetically linking invertebrate and vertebrate opsins.
• Opsins utilize counterions, such as glutamic acid residues, to modulate their molecular and spectral properties.

Purpose of the Study:

• To investigate the synergistic roles of Glu113 and Glu181 in Ci-opsin1's molecular properties.
• To understand the evolutionary significance of counterion acquisition in opsin evolution.

Main Methods:

• Site-directed mutagenesis (E113Q, E181Q, E113Y, E181S) to alter counterion residues.
• Spectroscopic analysis to determine spectral shifts and intermediate formation.
• G protein activation assays to assess signaling efficiency.

Main Results:

• Glu113 and Glu181 act synergistically, creating intermediate molecular properties between vertebrate and invertebrate opsins.
• Ci-opsin1 forms pH-dependent intermediates and shifts equilibrium towards a protonated Schiff base upon G protein interaction, similar to invertebrate opsins.
• Mutations at positions 113 and 181 modulated Ci-opsin1's properties towards invertebrate- or vertebrate-like opsins.

Conclusions:

• The synergistic action of Glu113 and Glu181 in Ci-opsin1 fine-tunes its molecular characteristics.
• Acquisition of Glu113 was a critical evolutionary event in the development of vertebrate visual opsins from a common ancestor.