Light-Dark Adaptation of Channelrhodopsin Involves Photoconversion between the all-trans and 13-cis Retinal Isomers.

Channelrhodopsins (ChR) are light-gated ion channels of green algae that are widely used to probe the function of neuronal cells with light. Most ChRs show a substantial reduction in photocurrents during illumination, a process named "light adaptation". The main objective of this spectroscopic study was to elucidate the molecular processes associated with light-dark adaptation.

Photochemical chromophore isomerization in histidine kinase rhodopsin HKR1.

Histidine kinase rhodopsin 1 is a photoreceptor in green algae functioning as a UV-light sensor. It switches between a UV-absorbing state (Rh-UV) and a blue-absorbing state (Rh-Bl) with a protonated retinal Schiff base (RSB) cofactor in a mixture of 13-trans,15-anti and 13-cis,15-syn isomers. The present spectroscopic study now shows that cofactor-protein assembly stabilizes the protonated 13-trans,15-anti RSB isomer.

A photochromic histidine kinase rhodopsin (HKR1) that is bimodally switched by ultraviolet and blue light.

Rhodopsins are light-activated chromoproteins that mediate signaling processes via transducer proteins or promote active or passive ion transport as ion pumps or directly light-activated channels. Here, we provide spectroscopic characterization of a rhodopsin from the Chlamydomonas eyespot. It belongs to a recently discovered but so far uncharacterized family of histidine kinase rhodopsins (HKRs).

The chromophore structure of the long-lived intermediate of the C128T channelrhodopsin-2 variant.

The photocycle of the light-activated channel, channelrhodopsin-2 C128T, has been studied by resonance Raman (RR) spectroscopy focussing on the intermediates P380 and P353 that constitute a side pathway in the recovery of the parent state. The P353 species displays a UV-vis absorption spectrum with a fine-structure reminiscent of the reduced-retro form of bacteriorhodopsin, whereas the respective RR spectra differ substantially. Instead, the RR spectra of the P380/P353 intermediate couple are closely related to that of a free retinal in the all-trans configuration.