The Mechanism of Channel Opening of Anion Channelrhodopsin GtACR1: A Molecular Dynamics Simulation

Guillardia theta anion channelrhodopsin 1 (GtACR1) is a widely used inhibitor of optogenetics with unique conductance mechanisms and photochemistry. However, the molecular mechanism of light-gated anion conduction is poorly understood without a crystal structure for the intermediate state. In this study, we built the dark-state model based on the crystal structure of retinal and isomerized the model by twisting the C12-C13=C14-C15 dihedral step by step using molecular dynamics simulation. The conformational changes revealed the all-trans to 13-cis photoisomerization of the retinal chromophore cannot open the channel. There is no water influx, and a pre-opened K-like intermediate after photoisomerization of retinal is formed. During the opening of the ion channel, proton transfer occurs between E68 and D234. Steered molecular dynamics (SMD) and umbrella sampling indicated that the E68 and D234 were the key residues for chloride-ion conducting. We propose a revised channel opening pathway model of GtACR1 after analyzing (de)protonation of E68 and D234. Reprotonation of D234 will result in two different early L intermediates, named L-1-like and L1'-like, which correspond to the L1 and L1' intermediates reported in a recent study. Simulation results showed that L1-like may convert by parallel paths into L1'-like and L2-like states. This model provides conformational details for the intermediate as well.