Structural and functional roles of the N- and C-terminal extended modules in channelrhodopsin-1

Channelrhodopsins have become a focus of interest because of their ability to control neural activity by light, used in a technology called optogenetics. The channelrhodopsin in the eukaryote Chlamydomonas reinhardtii (CrChR-1) is a light-gated cation channel responsible for motility changes upon photo-illumination and a member of the membrane-embedded retinal protein family. Recent crystal structure analysis revealed that CrChR-1 has unique extended modules both at its N- and C-termini compared to other microbial retinal proteins. This study reports the first successful expression of a ChR-1 variant in Escherichia coli as a holoprotein: the ChR-1 variant lacking both the N- and C-termini (CrChR-1-82-308). However, compared to ChR-1 having the extended modules (CrChR-1-1-357), truncation of the termini greatly altered the absorption maximum and photochemical properties, including the pK_a values of its charged residues around the chromophore, the reaction rates in the photocycle and the photo-induced ion channeling activity. The results of some experiments regarding ion transport activity suggest that CrChR-1-82-308 has a proton channeling activity even in the dark. On the basis of these results, we discuss the structural and functional roles of the N- and C-terminal extended modules in CrChR-1.