Structure of a mitochondrial supercomplex formed by respiratory-chain complexes I and III

Mitochondria are central to the efficient provision of energy for eukaryotic cells. The oxidative-phosphorylation system of mitochondria consists of a series of five major membrane complexes: NADH–ubiquinone oxidoreductase (commonly known as complex I), succinate–ubiquinone oxidoreductase (complex II), ubiquinol–cytochrome c oxidoreductase (cytochrome bc1 complex or complex III), cytochrome c–O2 oxidoreductase (complex IV), and F1F0–ATP synthase (complex V). Several lines of evidence have recently suggested that complexes I and III–V might interact to form supercomplexes. However, because of their fragility, the structures of these supercomplexes are still unknown. A stable supercomplex consisting of complex I and dimeric complex III was purified from plant mitochondria. Structural characterization by single-particle EM indicates a specific type of interaction between monomeric complex I and dimeric complex III in a 1:1 ratio. We present a model for how complexes I and III are spatially organized within the I+III2 supercomplex.