Cryo-EM Structures of Respiratorybc1-cbb3type CIII2CIV Supercomplex and Electronic Communication Between the Complexes

The respiratory electron transport complexes convey electrons from nutrients to oxygen and generate a proton-motive force used for energy (ATP) production in cells. These enzymes are conserved among organisms, and organized as individual complexes or combined forming large super-complexes (SC). Bacterial electron transport pathways are more branched than those of mitochondria and contain multiple variants of such complexes depending on their growth modes. The Gram-negative species deploy a mitochondrial-like cytochromebc1(Complex III, CIII2), and may have bacteria-specificcbb3-type cytochromecoxidases (Complex IV, CIV) in addition to, or instead of, the canonicalaa3-type CIV. Electron transfer between these complexes is mediated by two different carriers: the soluble cytochromec2which is similar to mitochondrial cytochromecand the membrane-anchored cytochromecywhich is unique to bacteria. Here, we report the first cryo-EM structure of a respiratorybc1-cbb3type SC (CIII2CIV, 5.2Å resolution) and several conformers of native CIII2(3.3Å resolution) from the Gram-negative bacteriumRhodobacter capsulatus. The SC contains all catalytic subunits and cofactors of CIII2and CIV, as well as two extra transmembrane helices attributed to cytochromecyand the assembly factor CcoH. Remarkably, some of the native CIII2are structural heterodimers with different conformations of their [2Fe-2S] cluster-bearing domains. The unresolved cytochromecdomain ofcysuggests that it is mobile, and it interacts with CIII2CIV differently than cytochromec2. Distance requirements for electron transfer suggest that cytochromecyand cytochromec2donate electrons to hemecp1and hemecp2of CIV, respectively. For the first time, the CIII2CIV architecture and its electronic connections establish the structural features of two separate respiratory electron transport pathways (membrane-confined and membrane-external) between its partners in Gram-negative bacteria.