Whereas pumps move ions actively, channels move ions passively. Fudim et al. generated a high-resolution crystal structure for Coccomyxa subellipsoidea rhodopsin, a light-activated proton pump. These data enabled the authors to identify a critical interaction between Arg83 and Tyr14 in a transmembrane domain and generate a point mutant of this rhodopsin that behaved as a light-gated proton channel. These results provide greater insight into the molecular determinants that distinguish proton pumps from channels.

Fudim, R., Szczepek, M., Vierock, J., Vogt, A., Schmidt, A., Kleinau, G., Fischer, P., Bartl, F., Scheerer, P., Hegemann, P. (2019). Design of a light-gated proton channel based on the crystal structure of Coccomyxa rhodopsin. Science Signaling 12, eaav4203, doi: 10.1126/scisignal.aav4203.