DEER Spectroscopy of Channelrhodopsin-2 Helix B Movements in Trapped Photocycle Intermediates

Abstract

The light-gated dimeric cation channel channelrhodopsin-2 (ChR2) has been established as one of the most important optogenetic tools. During its functional cycle, ChR2 undergoes conformational changes, the most prominent ones include a movement of transmembrane helix B. In the present work, we assign this movement to a trapped photocycle intermediate using DEER spectroscopy combined with sample illumination inside the microwave resonator, allowing trapping and relaxation of defined ChR2 intermediates at different temperatures between 180 and 278 K. Intradimer distances measured between spin-labeled positions 79 located in helix B of ChR2 in the dark state and upon light activation and relaxation at 180 K were similar. In contrast, light activation at 180 K and 30 min relaxation at between 230 and 255 K results in significant changes of the distance distribution. We show that the light-induced movement of helix B is correlated with the presence of the P480 state of ChR2. We hypothesize that conformational changes occurring in this area are key elements responsible for desensitizing the channel for cation conduction.