3.E.1.6.7
Na⁺ or H⁺ pumping bacteriorhodopsin, NaR, Kr2 or KR2 (Krokinobacter rhodopsin 2), of 280 aas and 7 TMSs.  It uses light to pump protons or sodium ions from the cell depending on the ionic composition of the medium. In cells suspended in a KCl solution, NaR functions as a light-driven proton pump, whereas in a NaCl solution, it exhibits light-driven sodium ion pumping, a novel activity within the rhodopsin family (da Silva et al. 2015).  A cation switch controls its conformations, and specific interactions of Na⁺ with the half-channels open an appropriate path for ion translocation (da Silva et al. 2015). Several high resolution x-ray structures have been solved (4XTO, Kato et al. 2015). Putative Na⁺ binding sites have been identified, and it was shown how protonation and conformational changes gate the ion through these sites toward the extracellular side (Suomivuori et al. 2017). Evidence for homology of this and other microbial rhodopsin with GPCR receptors including mamalian rhodopsins has been presented (Yee et al. 2013; Shalaeva et al. 2015). The 3-d structures have been determined by different groups (3X3B_A,B, 4XT0_A-E; 5JRF_A, and 6RF4_A-E). A covalent bond between Lys-255 and the polypeptide chain is responsible for stable retinal chromophore binding and sodium-pumping activity of KR2,but not for transport activity (Ochiai et al. 2023). Light-driven Na⁺-pumping rhodopsin (NaR) has been reviewed with an overview of structural and functional studies encompassing ground/intermediate-state structures and photocycle kinetics (Yang and Chen 2023). The review focuses on (1) unraveling the translocation pathway of Na⁺; (2) examining the role of structural changes within the photocycle, particularly in the O state, in facilitating Na⁺ transport; and (3) investigating the timing of Na⁺ uptake/release.