Kinetic Analysis of H+-Na+ Selectivity in a Light-Driven Na+-Pumping Rhodopsin

Krokinobacter eikastus rhodopsin 2 (KR2) is a recently identified lightdriven Na+ pump from a marine bacterium. KR2 pumps Na+ in NaC solution but pumps H+ in the absence of Na+ and Li+. The Na+ transport mechanism in KR2 has been extensively studied, whereas understanding of the Fr transport mechanism is very limited. Here we studied ion uptake mechanisms and H+-Na+ selectivity using flash photolysis. The results show that decay of the blue-shifted M intermediate is dependent on both [Na+] and [H+], indicating that KR2 competitively uptakes Na+ or H+ upon M decay. Comprehensive concentration dependence of Na+ and H+ revealed that the rate constant of H+ uptake (k(H)) was much larger than that of Na+ uptake (k(Na)) with a ratio (k(H)/k(Na)) of >10(3). Therefore, KR2 pumps only H+ when Na+ and H+ concentrations are similar. On the contrary, KR2 pumps Na+ exclusively under physiological conditions in which [Na+] is much greater than [H+].