Heliorhodopsin is a family of

rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...

s discovered in 2018 by Alina Pushkarev in the laboratory of Professor

Oded Beja Oded Béjà () is a professor in the Technion-Israel Institute of Technology, in the field of marine microbiology and metagenomics. Academic work Oded Béjà is best known for discovering the first bacterial rhodopsin naming it proteorhodop ...

. 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 A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...

compared with the other rhodopsins, with the

N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...

facing the inside of the cell and the

C-terminus The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, carboxy tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein Proteins are large biomolecules and macromolecules that comp ...

outside the cell. Heliorhodopsins use ''all-trans''

retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...

as a

chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...

, and do not have any ion pumping activity across the membrane. Heliorhodopsins are distributed globally and exist in

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

prokaryote A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...

s and even some

virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...

es. Despite the wide distribution, Heliorhodopsins are never present in true

gram-negative bacteria Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelo ...

, where there is a proper double membrane around the

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

. It has been suggested that the function of Heliorhodopsin requires a direct interaction with the environment. Heliorhodopsin animation

Crystal structures of Heliorhodopsins suggest they form a

homodimer In biochemistry, a protein dimer is a macromolecular complex or protein multimer, multimer formed by two protein monomers, or single proteins, which are usually Non-covalent interaction, non-covalently bound. Many macromolecules, such as proteins ...

, contain a fenestration leading toward the

molecule and have a large

extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

loop facing the outside of the cell.{{Cite journal, last1=Lu, first1=Yang, last2=Zhou, first2=X. Edward, last3=Gao, first3=Xiang, last4=Wang, first4=Na, last5=Xia, first5=Ruixue, last6=Xu, first6=Zhenmei, last7=Leng, first7=Yu, last8=Shi, first8=Yuying, last9=Wang, first9=Guangfu, last10=Melcher, first10=Karsten, last11=Xu, first11=H. Eric, date=January 2020, title=Crystal structure of heliorhodopsin 48C12, journal=Cell Research, language=en, volume=30, issue=1, pages=88–90, doi=10.1038/s41422-019-0266-0 , pmid=31879417 , pmc=6951262, issn=1001-0602, doi-access=free

References

Rhodopsins