Crystal structure of schizorhodopsin reveals mechanism of inward proton pumping

Akimitsu Higuchi, Wataru Shihoya, Masae Konno, Tatsuya Ikuta, Hideki Kandori, Keiichi Inoue, Osamu Nureki

Research output: Contribution to journalArticlepeer-review

13 Citations (Scopus)


Schizorhodopsins (SzRs), a new rhodopsin family identified in Asgard archaea, are phylogenetically located at an intermediate position between type-1 microbial rhodopsins and heliorhodopsins. SzRs work as light-driven inward H+ pumps as xenorhodopsins in bacteria. Although E81 plays an essential role in inward H+ release, the H+ is not metastably trapped in such a putative H+ acceptor, unlike the other H+ pumps. It remains elusive why SzR exhibits different kinetic behaviors in H+ release. Here, we report the crystal structure of SzR AM_5_00977 at 2.1 Å resolution. The SzR structure superimposes well on that of bacteriorhodopsin rather than heliorhodopsin, suggesting that SzRs are classified with type-1 rhodopsins. The structure-based mutagenesis study demonstrated that the residues N100 and V103 around the β-ionone ring are essential for color tuning in SzRs. The cytoplasmic parts of transmembrane helices 2, 6, and 7 are shorter than those in the other microbial rhodopsins, and thus E81 is located near the cytosol and easily exposed to the solvent by light-induced structural change. We propose a model of untrapped inward H+ release; H+ is released through the water-mediated transport network from the retinal Schiff base to the cytosol by the side of E81. Moreover, most residues on the H+ transport pathway are not conserved between SzRs and xenorhodopsins, suggesting that they have entirely different inward H+ release mechanisms.

Original languageEnglish
Article numbere2016328118
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number14
Publication statusPublished - 2021 Apr 6
Externally publishedYes


  • Proton pump
  • Rhodopsin
  • X-ray crystallography

ASJC Scopus subject areas

  • General


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