Two Open States with Progressive Proton Selectivities in the Branched Channelrhodopsin-2 Photocycle


AbstractChannelrhodopsins are light-gated ion channels that mediate vision in phototactic green algae like Chlamydomonas. In neurosciences, channelrhodopsins are widely used to light-trigger action potentials in transfected cells. All known channelrhodopsins preferentially conduct H+. Previous studies have indicated the existence of an early and a late conducting state within the channelrhodopsin photocycle. Here, we show that for channelrhodopsin-2 expressed in Xenopus oocytes and HEK cells, the two open states have different ion selectivities that cause changes in the channelrhodopsin-2 reversal voltage during a light pulse. An enzyme kinetic algorithm was applied to convert the reversal voltages in various ionic conditions to conductance ratios for H+ and divalent cations (Ca2+ and/or Mg2+), as compared to monovalent cations (Na+ and/or K+). Compared to monovalent cation conductance, the H+ conductance, α, is ∼3 × 106 and the divalent cation conductance, β, is ∼0.01 in the early conducting state. In the stationary mixture of the early and late states, α is larger and β smaller, both by a factor of ∼2. The results suggest that the ionic basis of light perception in Chlamydomonas is relatively nonspecific in the beginning of a light pulse but becomes more selective for protons during longer light exposures

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Last time updated on 6/5/2019

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