{Reference Type}: Journal Article
{Title}: Structural basis for ion selectivity in potassium-selective channelrhodopsins.
{Author}: Tajima S;Kim YS;Fukuda M;Jo Y;Wang PY;Paggi JM;Inoue M;Byrne EFX;Kishi KE;Nakamura S;Ramakrishnan C;Takaramoto S;Nagata T;Konno M;Sugiura M;Katayama K;Matsui TE;Yamashita K;Kim S;Ikeda H;Kim J;Kandori H;Dror RO;Inoue K;Deisseroth K;Kato HE;
{Journal}: Cell
{Volume}: 186
{Issue}: 20
{Year}: 2023 09 28
{Factor}: 66.85
{DOI}: 10.1016/j.cell.2023.08.009
{Abstract}: 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.