Cryo-EM structures of engineered active bc1-cbb3 type CIII2CIV 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, CIII2), 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 (CIII2CIV, 5.2 angstrom resolution) and three conformers of native CIII2 (3.3 angstrom 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. Respiratory chains generate the proton motive force used for ATP synthesis. Cryo-EM structures of functional respiratory CIII2CIV supercomplex and native CIII2 from Rhodobacter capsulatus provide insight into CIII2CIV assembly and respiratory electron transport pathways in Gram-negative bacteria.

Automated summary

Respiratory electron transport complexes in Gram-negative bacteria can be organized as individual entities or combined into large supercomplexes, with specific cytochromes mediating electron transfer. Recent studies have provided insights into the characteristics of membrane-confined and membrane-external respiratory electron transport pathways in these bacteria.