Structural and mechanistic study of ion transport in Channelrhodopsin-2

ended

Channelrhodopsins are type-I rhodopsin proteins found in green algae that function as sensory photoreceptors and turn into ion channels under illumination. Upon light absorbance, the retinal moiety induces a conformational change on the protein that opens a channel through which ions can pass. Neurons expressing channelrhodopsin-2 (ChR2) can be depolarized rapidly and reversibly by illumination, hence allowing control of the activation/inactivation of neurons in specific locations of the brain. For this reason, ChR2 has been used widely in optogenetics to study neuronal circuits and disorders in the brain, and to restore light sensitivity and visual capabilities in damaged retinas. However, in contrast to closely related bacteriorhodopsins or halorhodopsins, very little is known about their structure, light cycle and mechanism of action. The current structural evidences of ChR2 is limited to 1) the 6 Å projection map obtained by cryo-electron microscopy that contains a mixture of light (open channel) and dark (closed channel) states; and 2) the 2.3 Å X-ray structure of the dark state of a ChR1/ChR2 chimera. In the present proposal, we aim at elucidating the structure, properties and mechanism of action of the transient species of ChR2 during its photochemical cycle by means of theoretical methods and in close collaboration with the experimental biophysics groups of the host institute. The mechanisms of ion transport and channel opening will be simulated by enhanced sampling and free energy methods. Specific quantum-mechanics/molecular-mechanics (QM/MM) force matching force field will be generated ad hoc for the retinal moiety in the ChR2 environment. The model structures generated for the closed, open, and desensitized states will be validated 1) by comparison of the QM/MM spectroscopic properties of the model with experimental observations; and 2) by comparison to electron microscopy structures using a Bayesian analysis method recently developed in the host group.