Many organisms sense light using rhodopsins, photoreceptive proteins containing a retinal chromophore. Rozenberg et al report the discovery, structure and biophysical characterization of bestrhodopsins, a microbial rhodopsin subfamily from marine unicellular algae, in which one rhodopsin domain of eight transmembrane helices or, more often, two such domains in tandem, are C-terminally fused to a bestrophin channel. Cryo-EM analysis of a rhodopsin-rhodopsin-bestrophin fusion revealed that it forms a pentameric megacomplex (~700 kDa) with five rhodopsin pseudodimers surrounding the channel in the center. Bestrhodopsins are metastable and undergo photoconversion between red- and green-absorbing or green- and UVA-absorbing forms in the different variants. The retinal chromophore, in a unique binding pocket, photoisomerizes from all-trans to 11-cis form. Heterologously expressed bestrhodopsin behaves as a light-modulated anion channel.

Rozenberg, A., Kaczmarczyk, I., Matzov, D. et al. Rhodopsin-bestrophin fusion proteins from unicellular algae form gigantic pentameric ion channels. Nat Struct Mol Biol (2022). doi: 10.1038/s41594-022-00783-x.