Cryo-EM Structures of Respiratory bc 1 - cbb 3 type CIII 2 CIV 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 cytochrome (Complex III, CIII), and may have bacteria-specific -type cytochrome oxidases (Complex IV, CIV) in addition to, or instead of, the canonical -type CIV. Electron transfer between these complexes is mediated by two different carriers: the soluble cytochrome which is similar to mitochondrial cytochrome and the membrane-anchored cytochrome which is unique to bacteria. Here, we report the first cryo-EM structure of a respiratory - type SC (CIIICIV, 5.2Å resolution) and several conformers of native CIII (3.3Å resolution) from the Gram-negative bacterium . The SC contains all catalytic subunits and cofactors of CIII and CIV, as well as two extra transmembrane helices attributed to cytochrome and the assembly factor CcoH. Remarkably, some of the native CIII are structural heterodimers with different conformations of their [2Fe-2S] cluster-bearing domains. The unresolved cytochrome domain of suggests that it is mobile, and it interacts with CIIICIV differently than cytochrome . Distance requirements for electron transfer suggest that cytochrome and cytochrome donate electrons to heme and heme of CIV, respectively. For the first time, the CIIICIV 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.