%0 Journal Article
%T Structural basis for ion selectivity in potassium-selective channelrhodopsins.
%A Tajima S
%A Kim YS
%A Fukuda M
%A Jo Y
%A Wang PY
%A Paggi JM
%A Inoue M
%A Byrne EFX
%A Kishi KE
%A Nakamura S
%A Ramakrishnan C
%A Takaramoto S
%A Nagata T
%A Konno M
%A Sugiura M
%A Katayama K
%A Matsui TE
%A Yamashita K
%A Kim S
%A Ikeda H
%A Kim J
%A Kandori H
%A Dror RO
%A Inoue K
%A Deisseroth K
%A Kato HE
%J Cell
%V 186
%N 20
%D 2023 09 28
%M 37652010
%F 66.85
%R 10.1016/j.cell.2023.08.009
%X KCR channelrhodopsins (K+-selective light-gated ion channels) have received attention as potential inhibitory optogenetic tools but more broadly pose a fundamental mystery regarding how their K+ selectivity is achieved. Here, we present 2.5-2.7 Å cryo-electron microscopy structures of HcKCR1 and HcKCR2 and of a structure-guided mutant with enhanced K+ selectivity. Structural, electrophysiological, computational, spectroscopic, and biochemical analyses reveal a distinctive mechanism for K+ selectivity; rather than forming the symmetrical filter of canonical K+ channels achieving both selectivity and dehydration, instead, three extracellular-vestibule residues within each monomer form a flexible asymmetric selectivity gate, while a distinct dehydration pathway extends intracellularly. Structural comparisons reveal a retinal-binding pocket that induces retinal rotation (accounting for HcKCR1/HcKCR2 spectral differences), and design of corresponding KCR variants with increased K+ selectivity (KALI-1/KALI-2) provides key advantages for optogenetic inhibition in vitro and in vivo. Thus, discovery of a mechanism for ion-channel K+ selectivity also provides a framework for next-generation optogenetics.