The C-terminally truncated NuoL subunit (ND5 homologue) of the Na+-dependent complex I from Escherichia coli transports Na+

The NADH: quinone oxidoreductase ( complex I) from Escherichia coli acts as a primary Na+ pump. Expression of a C-terminally truncated version of the hydrophobic NuoL subunit (ND5 homologue) from E. coli complex I resulted in Na+-dependent growth inhibition of the E. coli host cells. Membrane vesicles containing the truncated NuoL subunit ( NuoL(N)) exhibited 2-4-fold higher Na+ uptake activity than control vesicles without NuoL(N). Respiratory proton transport into inverted vesicles containing NuoL(N) decreased upon addition of Na+, but was not affected by K+, indicating a Na+-dependent increase of proton permeability of membranes in the presence of NuoL(N). The His-tagged NuoL(N) protein was solubilized, enriched by affinity chromatography, and reconstituted into proteoliposomes. Reconstituted His(6)-NuoL(N) facilitated the uptake of Na+ into the proteoliposomes along a concentration gradient. This Na+ uptake was prevented by EIPA (5-(N-ethyl-N-isopropyl)amiloride), which acts as inhibitor against Na+/H+ antiporters.