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 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]