Congratulations to this constantly amazing group of students, postdocs, staff & collaborators including the indomitable @yoon_Seok_Kim, with @emekato, @sokosoko_seeya & our whole structure team! A new mechanism for K+ selectivity->new tools for biology. shorturl.at/pqDM1 As physiologists know, K+ selectivity in membrane ion channels is not easy to achieve. Once a solution emerged in the ancient oceans (fourfold symmetry to coordinate K+ ion chains and replace the hydration shell)­, all life on earth, from microbes to metazoa (it seemed), adhered to the same basic motif of fourfold symmetry– suggesting that very few solutions, and perhaps even just the one (with variations on its theme) were evolutionarily accessible. The competing high concentration of sodium ions in physiological systems (and natural environments like oceans)—with radius close to that of potassium ions—poses a challenge. As we reported last year (biorxiv.org/content/10.1101/…), solving the initial KCR structures allowed us to show that channelrhodopsins (the family of microbial light-activated ion channels that we’ve studied for nearly 20 years! and developed for optogenetics) do things completely differently. This finding also allowed us to design, create, and validate new channelrhodopsins for inhibitory optogenetics in mammals (biorxiv.org/content/10.1101/…) – including C110T (for long-lasting inhibitory K+ photocurrents) and H225F (the KALI variants, for enhanced K+ selectivity and more potent optogenetic inhibition)… Nature continues to surprise us!