Multi-modal ion-transport function of potassium channelrhodopsins
Light-gated K+ channelrhodopsins (KCRs) are recognized as an ideal neuronal silencing tool in optogenetics. KCRs contain functionally important residues for a light-driven H+ pump bacteriorhodopsin, but their roles have been unclear. Utilizing electrophysiological methods, we demonstrate that KCRs function as a light-driven H+ pump in the absence of K+. Transport experiments using pH electrodes further elucidated the function of the H+-pump, which operates even in the presence of K+. The hypothesis that the H+ pump exerts a blocking effect on K+ transport in KCRs has been postulated. Furthermore, we ascertained that there was conversion of KCRs from the H+ pump into the H+ channel by light, yet this conversion was never reversed. However, the addition of K+ resulted in the formation of the K+ channel. KCRs exhibit three distinct functional states, H+ pump, H+ channel, and K+ channel. These multifaceted functions are modulated by environmental ions and light.