Abstract
Channelrhodopsin-2 (ChR2) is a light-activated and non-selective cationic channel protein that can be easily expressed in specific neurons to control neuronal activity by light. Although ChR2 has been extensively used as an optogenetic tool in neuroscience research, the molecular mechanism of cation channel formation following retinal photoisomerization in ChR2 is not well understood. In this paper, studies of the closed and opened state ChR2 structures are presented. The formation of the cationic channel is elucidated in atomic detail using molecular dynamics simulations on the all-trans-retinal (ChR2-trans) configuration of ChR2 and its isomerization products, 13-cis-retinal (ChR2-cis) configuration, respectively. Photoisomerization of the retinal-chromophore causes the destruction of interactions among the crucial residues (e.g., E90, E82, N258, and R268) around the channel and the extended H-bond network mediated by numerous water molecules, which opens the pore. Steering molecular dynamics (SMD) simulations show that the electrostatic interactions at the binding sites in intracellular gate (ICG) and central gate (CG) can influence the transmembrane transport of Na+ in ChR2-cis obviously. Potential of mean force (PMF) constructed by SMD and umbrella sampling also found the existing energy wells at these two binding sites during the transportation of Na+. These wells partly hinder the penetration of Na+ into cytoplasm through the ion channel. This investigation provides a theoretical insight on the formation mechanism of ion channels and the mechanism of ion permeation.
Highlights
It was found that channelrhodopsins (ChRs) could be expressed in neurons of living animals to mediate accurate and reliable control of action potential triggering in response to light pulses, without the need for exogenous retinal in vertebrate systems [1,2,3,4,5,6]
The crystal structure of the ChR2 reported in 2018 [35] indicates considerable differences to C1C2 chimera: (1) the ion channel between TM1, TM2, TM3, and TM7 in ChR2 is respectively divided into four cavities by extracellular gate (ECG), central gate (CG), and intracellular gate (ICG)
In the pores of the channel, R120, E90, and R268 are the cores of ECG, CG, and ICG respectively
Summary
It was found that channelrhodopsins (ChRs) could be expressed in neurons of living animals to mediate accurate and reliable control of action potential triggering in response to light pulses, without the need for exogenous retinal in vertebrate systems [1,2,3,4,5,6]. A brief pulse of light triggers an influx of protons and sodium ions which lead to a photocurrent entering the cell [22]. Many efforts have been made to modify ChRs to be more light-sensitive and more selective to sodium than to protons [23]. One of the early modifications of ChRs involved replacement of histidine 134 with arginine, which reduces the inactivation of photocurrent and leads to higher ion selectivity and lower proton conductivity [3,24]
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