The molecular determinants of channelrhodopsin‐2 ion conductance

Abstract

Channelrhodopsin‐2 (ChR2) is a microbial‐type rhodopsin found in the green algae Chlamydomonas reinhardtii. Under physiological conditions, ChR2 is an inwardly rectifying cation channel that permeates a wide range of mono‐ and divalent cations. Although this protein shares a high sequence homology with other microbial‐type rhodopsins, which are ion pumps, ChR2 is an ion channel. A sequence alignment of ChR2 with bacteriorhodopsin, a proton pump, reveals that ChR2 lacks specific motifs and residues, such as serine and threonine, known to contribute to non‐covalent interactions within transmembrane domains. We hypothesized that reintroduction of the eight transmembrane serine residues present in bacteriorhodopsin, but not in ChR2, will restrict the conformational flexibility and reduce the pore diameter of ChR2. Furthermore, we used cysteine scanning mutagenesis combined with methanethiosulfonate labeling to examine the permeation pathway of channelrhodopsin‐2. An analysis of our experimental results provide important insight into the cation permeation pathway of channelrhodopsin‐2.