The cyanobacterial rhodopsin "SyHR" has a unique ability to import chloride ions (Cl⁻) and sulfate ions (SO₄²⁻) into the cell in response to light. In this study, we systematically substituted each of the twelve basic amino acid residues in SyHR with alternative amino acids to investigate their functional roles. The results revealed that Arg71 is essential for SO₄²⁻ binding, while His167 plays a critical role in the internal transport of anions. Notably, replacing Arg71 with a neutral amino acid significantly reduced SO₄²⁻ binding and transport activity. Similarly, substituting His167 with a hydrophobic residue inhibited the formation of the O intermediate in the photocycle, thereby halting ion transport. Furthermore, theoretical modeling predicted the three-dimensional structure of the SO₄²⁻ binding site, offering new insights into the binding mechanism. This study provides important contributions to our understanding of the molecular mechanism of light-driven ion pumps.

https://www.sciencedirect.com/science/article/pii/S0021925825001838?via%3Dihub