Cryo-EM structures of engineered active bc(1)-cbb(3) type CIII(2)CIV 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(1) (Complex III, CIII(2)), and may have specific cbb(3)-type cyt c oxidases (Complex IV, CIV) instead of the canonical aa(3)-type CIV. Electron transfer between these complexes is mediated by soluble (c(2)) and membrane-anchored (c(y)) cyts. Here, we report the structure of an engineered bc(1)-cbb(3) type SC (CIII(2)CIV, 5.2 Å resolution) and three conformers of native CIII(2) (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(y) and the assembly factor CcoH. The cyt c(y) is integral to SC, its cyt domain is mobile and it conveys electrons to CIV differently than cyt c(2). 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.