Heliorhodopsin compared with Type 1 and Type 2 rhodopsins. Before the discovery of the Heliorhodopsin, all know rhodopsins were known to have the N terminus outside the cell membrane. The plus signs represent positively charged amino acids.
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-transretinal 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 diderms, 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 structure of a monomer of heliorhodopsin from Thermoplasmatales archaeon SG8-52-1, based on [1].
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]
