Energy Transducing Roles of Antiporter-like Subunits in Escherichia coli NDH-1 with Main Focus on Subunit NuoN (ND2)

The proton-translocating NADH-quinone oxidoreductase (complex I/NDH-1) contains a peripheral and a membrane domain. Three antiporter-like subunits in the membrane domain, NuoL, NuoM, and NuoN (ND5, ND4 and ND2, respectively), are structurally similar. We analyzed the role of NuoN in Escherichia coli NDH-1. The lysine residue at position 395 in NuoN (NLys395) is conserved in NuoL (LLys399) but is replaced by glutamic acid (MGlu407) in NuoM. Our mutation study on NLys395 suggests that this residue participates in the proton translocation. Furthermore, we found that MGlu407 is also essential and most likely interacts with conserved LArg175. Glutamic acids, NGlu133, MGlu144, and LGlu144, are corresponding residues. Unlike mutants of MGlu144 and LGlu144, mutation of NGlu133 scarcely affected the energy-transducing activities. However, a double mutant of NGlu133 and nearby KGlu72 showed significant inhibition of these activities. This suggests that NGlu133 bears a functional role similar to LGlu144 and MGlu144 but its mutation can be partially compensated by the nearby carboxyl residue. Conserved prolines located at loops of discontinuous transmembrane helices of NuoL, NuoM, and NuoN were shown to play a similar role in the energy-transducing activity. It seems likely that NuoL, NuoM, and NuoN pump protons by a similar mechanism. Our data also revealed that NLys158 is one of the key interaction points with helix HL in NuoL. A truncation study indicated that the C-terminal amphipathic segments of NTM14 interacts with the Mβ sheet located on the opposite side of helix HL. Taken together, the mechanism of H+ translocation in NDH-1 is discussed.