Heliorhodopsin

Heliorhodopsin is a family of rhodopsins discovered in 2018 by Alina Pushkarev in the laboratory of Professor Oded Beja.^[1] The new family of heliorhodopsins has a distinct protein sequence from known Type 1 (microbial) and Type 2 (animal) rhodopsins. Heliorhodopsins also exhibit the reverse orientation in the membrane compared with the other rhodopsins, with the N-terminus facing the inside of the cell and the C-terminus outside the cell.^[1]

Heliorhodopsins use all-trans retinal as a chromophore, and do not have any ion pumping activity across the membrane. Heliorhodopsins are distributed globally and exist in eukaryotes, prokaryotes and even some viruses.^[1] Despite the wide distribution, Heliorhodopsins are never present in true gram-negative bacteria, where there is a proper double membrane around the microorganism. It has been suggested that the function of Heliorhodopsin requires a direct interaction with the environment.^[2]

Crystal structures of Heliorhodopsins suggest they form a homodimer, contain a fenestration leading toward the retinal molecule and have a large extracellular loop facing the outside of the cell.^[3][4][5]