3.E.1.1.18
Glycylhalorhodopsin is an anion pumping microbial rhodopsin.  It is 260 aas long with 7 TMSs.  It takes up anions including chloride and sulfate. The function of many microbial rhodopsins is determined by three residues in the third transmembrane helix called motif residues. Ishizuka et al. 2024 reported a group of microbial rhodopsins with a novel Thr–Thr–Gly (TTG) motif. They function as light-driven inward anion pumps similar to halorhodopsins. Based on the characteristic glycine residue in their motif and light-driven anion-pumping function, these rhodopsins are called glycylhalorhodopsins (GHRs). X-ray crystallographic analysis found large cavities on the cytoplasmic side of these proteins, which are produced by the small side-chain volume of the glycine residue in the motif. The opened structure of GHR on the cytoplasmic side is related to the anion releasing process to the cytoplasm during the photoreaction compared to canonical halorhodopsin from Natronomonas pharaonis (NpHR) (Ishizuka et al. 2024).