The light-sensitive channelrhodopsin protein consists of two single molecules (light and dark grey). It forms an ion channel that can be closed in three places. When closed (left), three 'gates' (black bars) prevent ions from flowing through the channel pore (green tube). In the open state (right), a conformational change causes the opening of the channel pore. Channelrhodopsin also pumps positive hydrogen ions (protons) into the molecule's interior (orange arrow) -- a property that is important for the opening of the channel pore. [less]
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