A Novel Color Switch of Microbial Rhodopsin.

Area of Science:

• Biophysics
• Molecular Biology
• Optogenetics

Background:

• Animal and microbial rhodopsins function via light absorption by the retinal chromophore.
• Rhodopsin color tuning is determined by the energy gap between the ground (S0) and excited (S1) states of the chromophore.
• Known microbial rhodopsin color switches are limited, found in C-helix (TM3), G-helix (TM7), and F-helix (TM6).

Purpose of the Study:

• To identify and characterize novel color switches in microbial rhodopsins.
• To investigate the role of specific amino acid residues in spectral shifts.
• To explore the potential applications of new color switches in optogenetics.

Main Methods:

• Investigated color-determining residues in microbial rhodopsins.
• Analyzed amino acid substitutions and their effects on spectral properties (λmax).
• Examined the location and chemical properties of identified residues relative to the chromophore.

Main Results:

• A novel color switch, termed the N/LI switch, was identified in the D-helix (TM4).
• This switch involves Asn (red-shifting) and Leu/Ile (blue-shifting) amino acids.
• The N/LI switch aligns with the general rule of color tuning based on amino acid polarity and is located near the β-ionone ring.

Conclusions:

• The N/LI switch represents a new mechanism for microbial rhodopsin color tuning.
• This finding expands the understanding of chromophore-protein interactions in rhodopsins.
• The presence of blue-shifting amino acids like Leu and Ile in this switch suggests utility for optogenetic applications.