Heliorhodopsins, distantly related to type-1 rhodopsins, are embedded in the membrane with their N termini facing the cell cytoplasm, an orientation that is opposite to that of type-1 or type-2 rhodopsins. Heliorhodopsins show photocycles that are longer than one second, which is suggestive of light-sensory activity. Heliorhodopsin photocycles accompany retinal isomerization and proton transfer, as in type-1 and type-2 rhodopsins, but protons are never released from the protein [1].The structures of several heliorhodopsins have been solved displaying seven transmembrane helices (TM), six loops and short N and C termini [2,3]. Heliorhodopsins share a common fold with the type-1 rhodopsins, however there are clear structural differences, particularly within the loop regions and the large cavity in the cytoplasmic part of heliorhodpsin [2,3]. Heliorhodpsins are present in Archaea, Bacteria, Eukarya, and viruses [3].

1: A distinct abundant group of microbial rhodopsins discovered using functional metagenomics. Pushkarev A, Inoue K, Larom S, Flores-Uribe J, Singh M, Konno M, Tomida S, Ito S, Nakamura R, Tsunoda SP, Philosof A, Sharon I, Yutin N, Koonin EV, Kandori H, Beja O; Nature. 2018;558:595-599. PMID:29925949

2: Crystal structure of heliorhodopsin. Shihoya W, Inoue K, Singh M, Konno M, Hososhima S, Yamashita K, Ikeda K, Higuchi A, Izume T, Okazaki S, Hashimoto M, Mizutori R, Tomida S, Yamauchi Y, Abe-Yoshizumi R, Katayama K, Tsunoda SP, Shibata M, Furutani Y, Pushkarev A, Beja O, Uchihashi T, Kandori H, Nureki O; Nature. 2019;574:132-136. PMID:31554965

3: High-resolution structural insights into the heliorhodopsin family. Kovalev K, Volkov D, Astashkin R, Alekseev A, Gushchin I, Haro-Moreno JM, Chizhov I, Siletsky S, Mamedov M, Rogachev A, Balandin T, Borshchevskiy V, Popov A, Bourenkov G, Bamberg E, Rodriguez-Valera F, Buldt G, Gordeliy V; Proc Natl Acad Sci U S A. 2020;117:4131-4141. PMID:32034096