Cryo-EM structures of engineered active bc-cbb type CIIICIV super-complexes and electronic communication between the complexes.

Respiratory electron transport complexes are organized as individual entities or combined as large supercomplexes (SC). Gram-negative bacteria deploy a mitochondrial-like cytochrome (cyt) bc (Complex III, CIII), and may have specific cbb-type cyt c oxidases (Complex IV, CIV) instead of the canonical aa-type CIV. Electron transfer between these complexes is mediated by soluble (c) and membrane-anchored (c) cyts. Here, we report the structure of an engineered bc-cbb type SC (CIIICIV, 5.2 Å resolution) and three conformers of native CIII (3.3 Å resolution). The SC is active in vivo and in vitro, contains all catalytic subunits and cofactors, and two extra transmembrane helices attributed to cyt c and the assembly factor CcoH. The cyt c is integral to SC, its cyt domain is mobile and it conveys electrons to CIV differently than cyt c. The successful production of a native-like functional SC and determination of its structure illustrate the characteristics of membrane-confined and membrane-external respiratory electron transport pathways in Gram-negative bacteria.