Exciton circular dichroism in channelrhodopsin

Area of Science:

• Optogenetics and biophysics
• Molecular spectroscopy
• Computational chemistry

Background:

• Channelrhodopsins (ChRs) are crucial for optogenetics, enabling neuronal photostimulation.
• Understanding the spectral properties of ChRs is key to their application.
• C1C2, a chimera of ChR1 and ChR2, serves as a model system for studying ChR structure-function relationships.

Purpose of the Study:

• To investigate the absorption and circular dichroism (CD) spectra of the C1C2 channelrhodopsin chimera.
• To theoretically model the C1C2 chromophore using various computational approaches.
• To elucidate the role of protein environment and dimerization in ChR spectral properties.

Main Methods:

• Experimental measurement of absorption and CD spectra.
• Time-dependent density functional theory (TDDFT) calculations for chromophore models.
• Hybrid quantum mechanical/molecular mechanical (QM/MM) modeling of the binding pocket.
• DeVoe polarizability theory and exciton calculations for dimer analysis.
• Patch-clamp electrophysiology for mutant analysis.

Main Results:

• The visible absorption spectrum of C1C2 exhibits vibronic fine structure.
• Experimental CD spectra show exciton coupling indicative of dimerization.
• TDDFT and exciton calculations reproduced experimental CD spectra, with the isolated retPSB model showing best agreement.
• A truncated N-terminal mutant displayed a CD spectrum comparable to wild-type, confirming its dimeric nature.
• N-terminal truncation affects protein stabilization and kinetics, not dimerization or channel activity.

Conclusions:

• The study provides a detailed spectral characterization of the C1C2 channelrhodopsin chimera.
• Computational models, particularly the isolated retPSB model, can effectively predict ChR spectral properties.
• The N-terminal domain of ChRs is not essential for dimerization but plays a role in protein function and stability.