Kalium channelrhodopsins are natural light-gated potassium channels that mediate optogenetic inhibition.

Channelrhodopsins are used widely for optical control of neurons, in which they generate photoinduced proton, sodium or chloride influx. Potassium (K) is central to neuron electrophysiology, yet no natural K-selective light-gated channel has been identified. Here, we report kalium channelrhodopsins (KCRs) from Hyphochytrium catenoides. Previously known gated potassium channels are mainly ligand- or voltage-gated and share a conserved K-selectivity filter. KCRs differ in that they are light-gated and have independently evolved an alternative K selectivity mechanism. The KCRs are potent, highly selective of K over Na, and open in less than 1 ms following photoactivation. The permeability ratio P/P of 23 makes H. catenoides KCR1 (HcKCR1) a powerful hyperpolarizing tool to suppress excitable cell firing upon illumination, demonstrated here in mouse cortical neurons. HcKCR1 enables optogenetic control of K gradients, which is promising for the study and potential treatment of potassium channelopathies such as epilepsy, Parkinson's disease and long-QT syndrome and other cardiac arrhythmias.