Institute for Biology, Humboldt-Universität zu Berlin, Germany.
Biophys J. 2010 Mar 3;98(5):753-61. doi: 10.1016/j.bpj.2009.10.052.
Channelrhodopsins 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 (Ca(2+) and/or Mg(2+)), as compared to monovalent cations (Na(+) and/or K(+)). Compared to monovalent cation conductance, the H(+) conductance, alpha, is approximately 3 x 10(6) and the divalent cation conductance, beta, is approximately 0.01 in the early conducting state. In the stationary mixture of the early and late states, alpha is larger and beta smaller, both by a factor of approximately 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.
通道视紫红质是一种光门控离子通道，介导了如衣藻等趋光性绿藻的视觉反应。在神经科学中，通道视紫红质被广泛用于转染细胞的光触发动作电位。所有已知的通道视紫红质都优先传导 H(+)。先前的研究表明通道视紫红质光循环中有早期和晚期的导通状态。在这里，我们表明对于在非洲爪蟾卵母细胞和 HEK 细胞中表达的通道视紫红质-2，这两个开放状态具有不同的离子选择性，这会导致在光脉冲期间通道视紫红质-2反转电压的变化。我们应用了酶动力学算法，将各种离子条件下的反转电压转换为 H(+)和二价阳离子（Ca(2+)和/或 Mg(2+)）与单价阳离子（Na(+)和/或 K(+)）的电导比。与单价阳离子电导相比，早期导通状态下的 H(+)电导 alpha 约为 3 x 10(6)，二价阳离子电导 beta 约为 0.01。在早期和晚期状态的稳态混合物中，alpha 增大而 beta 减小，均约为 2 倍。结果表明，在光脉冲的开始，衣藻中光感知的离子基础相对非特异性，但在更长时间的光照下，对质子的选择性更高。
