Role of Subunit NuoL for Proton Translocation by Respiratory Complex I

The NADH:ubiquinone oxidoreductase, respiratory complex I, couples the transfer of electrons from NADH to ubiquinone with a translocation of protons across the membrane. The complex consists of a peripheral arm catalyzing the electron transfer reaction and a membrane arm involved in proton translocation. The recently published X-ray structures of the complex revealed the presence of a unique 110 angstrom horizontal 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 conformational 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+/2e(-) stoichiometry of the Delta NuoL variant is 2, indicating a different stoichiometry for proton translocation as proposed from structural data. In addition, the same H+/e(-) stoichiometry is obtained with the variant lacking the C-terminal transmembraneous helix of NuoL, indicating its role in energy transmission.