Mutant of a Light-Driven Sodium Ion Pump Can Transport Cesium Ions

Krokinobacter eikastus rhodopsin 2 (KR2) is a light-driven Na⁺ pump found in marine bacterium. KR2 pumps Li⁺ and Na⁺, but it becomes an H⁺ pump in the presence of K⁺, Rb⁺, and Cs⁺. Site-directed mutagenesis of the cytoplasmic surface successfully converted KR2 into a light-driven K⁺ pump, suggesting that ion selectivity is determined at the cytoplasmic surface. Here we extended this research and successfully created a light-driven Cs⁺ pump. KR2 N61L/G263F pumps Cs⁺ as well as other monovalent cations in the presence of a protonophore. Ion-transport activities correlated with the additive volume of the residues at 61 and 263. The result suggests that an ion-selectivity filter is affected by these two residues and functions by strict exclusion of K⁺ and larger cations in the wild type (N61/G263). In contrast, introduction of large residues possibly destroys local structures of the ion-selectivity filter, leading to the permeation of K⁺ (P61/W263) and Cs⁺ (L61/F263).