Crystal structure of schizorhodopsin reveals mechanism of inward proton pumping.

Abstract

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.