Analysis of the conformational heterogeneity of the Rieske iron-sulfur protein in complex III2 by cryo-EM

Movement of the Rieske domain of the iron-sulfur protein is essential for intramolecular electron transfer within complex III2 (CIII2) of the respiratory chain as it bridges a gap in the cofactor chain towards the electron acceptor cytochrome c. We present cryo-EM structures of CIII2 from Yarrowia lipolytica at resolutions up to 2.0 A degrees under different conditions, with different redox states of the cofactors of the high-potential chain. All possible permutations of three primary positions were observed, indicating that the two halves of the dimeric complex act independently. Addition of the substrate analogue decylubiquinone to CIII2 with a reduced high-potential chain increased the occupancy of the Qo site. The extent of Rieske domain interactions through hydrogen bonds to the cytochrome b and cytochrome c(1) subunits varied depending on the redox state and substrate. In the absence of quinols, the reduced Rieske domain interacted more closely with cytochrome b and cytochrome c(1) than in the oxidized state. Upon addition of the inhibitor antimycin A, the heterogeneity of the cd(1)-helix and ef-loop increased, which may be indicative of a long-range effect on the Rieske domain.