Abstract
Sodium-pumping rhodopsins (NaRs) are light-driven outward Na+ pumps. NaRs have a conserved Asn, Asp, and Gln motif (NDQ) in the third transmembrane helix (helix C). The NDQ motif is thus expected to play a crucial role in the operation of the Na+ pump. Herein, we studied the photocycles of the NDQ-motif mutants of Krokinobacter rhodopsin 2 (KR2), the first discovered NaR, by flash photolysis, to obtain insight into the mechanism of Na+ transport. For example, the KR2 N112A mutant did not accumulate the transient red-shifted Na+-bound state, suggesting that Asn112 is vital for the binding of Na+ ions. Additionally, Q123A and Q123V mutants showed significantly slower Na+ uptake and recovery of the initial state. Overall, the Gln123 residue was found to contribute to the optimization of the kinetics of sodium-ion uptake and release. These results demonstrate that the cooperative operation of the three residues of the NDQ motif are important in the operation of the Na+ pump.
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