Heliorhodopsin is a family of rhodopsins discovered in 2018 by Alina Pushkarev in the laboratory of Professor Oded Beja. 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. ...

compared with the other rhodopsins, with the N-terminus facing the inside of the cell and the C-terminus 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 reti ...

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

prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Conne ...

s and even some

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

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

diderms Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall s ...

, where there is a proper double membrane around the

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

. 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 formed by two protein monomers, or single proteins, which are usually non-covalently bound. Many macromolecules, such as proteins or nucleic acids, form dimers. The word ''dimer'' has ...

, 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