The Architecture of Respiratory Supercomplexes

Mitochondrial electron transport chain complexes are organized into supercomplexes responsible for carrying out cellular respiration and energy production. The majority of complex I in heart mitochondria is found bound with a complex III dimer and complex IV (CICIII2CIV, the respirasome) or with a complex III dimer alone (CICIII2). Here I will present three architectures of mammalian (ovine) supercomplexes determined by cryo-electron microscopy. We identify two distinct arrangements of the respirasome—a major ‘tight’ form and a minor ‘loose’ form (resolved at the resolution of 5.8 A and 6.7 A, respectively), which may represent different stages in supercomplex assembly or disassembly. We have also determined an architecture of supercomplex CICIII2 at 7.8 A resolution. All observed density can be attributed to the known 80 subunits of the individual complexes, including 132 transmembrane helices. The individual complexes form tight interactions that vary between the architectures, with complex IV subunit COX7a switching contact from complex III to complex I. The arrangement of active sites within the supercomplex may help control reactive oxygen species production. These are the first complete architectures of the dominant, physiologically relevant state of complex I in the electron transport chain.