Fig. 4: Ion permeation pathway comparison of exoKCR1 and endoKCR1.
From: Channelrhodopsins with distinct chromophores and binding patterns
a, b Cartoon model of exoKCR1 in gray (a) and endoKCR1 in purple (b) with ion and water permeation pathway. The potential potassium ion and water are shown as purple and red spheres, respectively. The side chain of residues along the pore is labeled and color-coded. The retinal in exoKCR1 is colored in green and the retinal in endoKCR1 is colored in blue. The lumen is divided into five layers (L1–L5) according to the putative distribution and dehydration of potential potassium ion built in the pore domain: the intracellular hydrated layer (L5, E248-R244-D116), which contains the hydrated potassium; the dehydrated layer (L4), containing two water molecules (F195-W199) and two putative continuous dehydrated potassium ion; the extracellular monohydrated layer (L3, K233-D229-D105-K84), which contains a putative monohydrate potassium ion; the selectivity filter layer (L2, W102-Y222), which contains at least one partial hydrated potassium ion through π-cation interactions. the extracellular hydrated layer (L1, W100), which contains the bulky hydrated potassium ion. The exoKCR1 exhibit the continuous potassium permeation pathway while the endoKCR1 exhibit the discontinuous potassium permeation pathway. c–f The dehydrated layer of exoKCR1 (c–d) and endoKCR1 (e–f) were presented by Coot. No density is observed in exoKCR1, whereas two distinct putative water densities interact with F195 and W199, and two potassium/water densities occupy the lumen.
