The role of subunit NuoL for proton translocation by the respiratory complex I

The NADH:ubiquinone oxidoreductase, respira- tory complex I, couples the transfer ofelectrons from NADHto ubiquinone with a translocation of protons across the mem- brane. The complex consists of a peripheral arm catalyzing the electron transfer reaction and a membrane arm involved in protontranslocation.TherecentlypublishedX-raystructuresof the complex revealed the presence of a unique 110 A "hor- izontal" helix aligning the membrane arm. On the basis of this finding, it was proposed that the energy released by the redox reaction is transmitted to the membrane arm via a conforma- tional change in the horizontal helix. The helix corresponds to the C-terminal part of the most distal subunit NuoL. To investigate its role in proton translocation, we characterized the electron transfer and proton translocation activity of complex I variants lacking either NuoL or parts of the C-terminal domain. Our data suggest that the H þ /2estoichiometry of the ΔNuoL variantis2,indicatingadifferentstoichiometryforprotontranslocationasproposedfromstructuraldata.Inaddition,thesameH þ / estoichiometry is obtained with the variant lacking the C-terminal transmembraneous helix of NuoL, indicating its role in energy transmission.