Structures of channelrhodopsin paralogs in peptidiscs explain their contrasting K+ and Na+ selectivities.

Kalium channelrhodopsin 1 from Hyphochytrium catenoides (HcKCR1) is a light-gated channel used for optogenetic silencing of mammalian neurons. It selects K⁺ over Na⁺ in the absence of the canonical tetrameric K⁺ selectivity filter found universally in voltage- and ligand-gated channels. The genome of H. catenoides also encodes a highly homologous cation channelrhodopsin (HcCCR), a Na⁺ channel with >100-fold larger Na⁺ to K⁺ permeability ratio. Here, we use cryo-electron microscopy to determine atomic structures of these two channels embedded in peptidiscs to elucidate structural foundations of their dramatically different cation selectivity. Together with structure-guided mutagenesis, we show that K⁺ versus Na⁺ selectivity is determined at two distinct sites on the putative ion conduction pathway: in a patch of critical residues in the intracellular segment (Leu69/Phe69, Ile73/Ser73 and Asp116) and within a cluster of aromatic residues in the extracellular segment (primarily, Trp102 and Tyr222). The two filters are on the opposite sides of the photoactive site involved in channel gating. Recently discovered kalium channelrhodopsins (KCRs) are optogenetic tools for neuronal silencing. Here, authors report cryo-electron microscopy structures of KCR1 from Hyphochytriumcatenoides and a highly homologous but sodium-selective channel from the same organism. [ABSTRACT FROM AUTHOR]