Abstract
Mitochondrial complex III (CIII2) and complex IV (CIV), which can associate into a higher-order supercomplex (SC III2+IV), play key roles in respiration. However, structures of these plant complexes remain unknown. We present atomic models of CIII2, CIV, and SC III2+IV from Vigna radiata determined by single-particle cryoEM. The structures reveal plant-specific differences in the MPP domain of CIII2 and define the subunit composition of CIV. Conformational heterogeneity analysis of CIII2 revealed long-range, coordinated movements across the complex, as well as the motion of CIII2's iron-sulfur head domain. The CIV structure suggests that, in plants, proton translocation does not occur via the H channel. The supercomplex interface differs significantly from that in yeast and bacteria in its interacting subunits, angle of approach and limited interactions in the mitochondrial matrix. These structures challenge long-standing assumptions about the plant complexes and generate new mechanistic hypotheses.
Highlights
The canonical mitochondrial electron transport chain, composed of four integral membrane protein complexes located in the inner mitochondrial membrane (IMM), transfers electrons from NADH and succinate to molecular oxygen
Upon 2D classification of the particles in the micrographs, it became evident that the pooled fractions contained the complex intermediate CI* (Maldonado et al, 2020), and CIII2 and SC III2+IV
CIII2 and SC III2+complex IV (CIV) were purified in silico from the micrographs we had previously used to solve the structure of CI* (Maldonado et al, 2020)
Summary
The canonical mitochondrial electron transport chain (mETC), composed of four integral membrane protein complexes (complexes I–IV; CI–CIV) located in the inner mitochondrial membrane (IMM), transfers electrons from NADH and succinate to molecular oxygen. Complex III (CIII2), called the cytochrome bc complex or ubiquinol-cytochrome c oxidoreductase, is an obligate dimer that transfers electrons from ubiquinol in the IMM (reduced by CI, CII, or alternative NADH dehydrogenases) to soluble cytochrome c in the intermembrane space (IMS) (Nicholls, 2013). This redox reaction is coupled to the pumping four H+ to the IMS.
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