The Role of the NDQ Motif in Sodium‐Pumping Rhodopsins

Abstract

AbstractSodium‐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.