High-resolution structural insights into the heliorhodopsin family

Proc Natl Acad Sci U S A. 2020 Feb 25;117(8):4131-4141. doi: 10.1073/pnas.1915888117. Epub 2020 Feb 7.


Rhodopsins are the most abundant light-harvesting proteins. A new family of rhodopsins, heliorhodopsins (HeRs), has recently been discovered. Unlike in the known rhodopsins, in HeRs the N termini face the cytoplasm. The function of HeRs remains unknown. We present the structures of the bacterial HeR-48C12 in two states at the resolution of 1.5 Å, which highlight its remarkable difference from all known rhodopsins. The interior of HeR's extracellular part is completely hydrophobic, while the cytoplasmic part comprises a cavity (Schiff base cavity [SBC]) surrounded by charged amino acids and containing a cluster of water molecules, presumably being a primary proton acceptor from the Schiff base. At acidic pH, a planar triangular molecule (acetate) is present in the SBC. Structure-based bioinformatic analysis identified 10 subfamilies of HeRs, suggesting their diverse biological functions. The structures and available data suggest an enzymatic activity of HeR-48C12 subfamily and their possible involvement in fundamental redox biological processes.

Keywords: X-ray crystallography; crystal structure; membrane protein; retinal; rhodopsin.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Computational Biology*
  • Hydrogen-Ion Concentration
  • Models, Molecular
  • Photolysis
  • Protein Conformation
  • Rhodopsins, Microbial / chemistry*


  • Rhodopsins, Microbial

Associated data

  • PDB/6SU3
  • PDB/6SU4