Cryo-EM structure of the four-subunit cytochrome complex in styrene maleic acid nanodiscs.

Cytochrome complexes are ubiquinol:cytochrome oxidoreductases, and as such, they are centrally important components of respiratory and photosynthetic electron transfer chains in many species of bacteria and in mitochondria. The minimal complex has three catalytic components, which are cytochrome , cytochrome , and the Rieske iron-sulfur subunit, but the function of mitochondrial cytochrome complexes is modified by up to eight supernumerary subunits. The cytochrome complex from the purple phototrophic bacterium   has a single supernumerary subunit called subunit IV, which is absent from current structures of the complex. In this work we use the styrene-maleic acid copolymer to purify the cytochrome complex in native lipid nanodiscs, which retains the labile subunit IV, annular lipids, and natively bound quinones. The catalytic activity of the four-subunit cytochrome complex is threefold higher than that of the complex lacking subunit IV. To understand the role of subunit IV, we determined the structure of the four-subunit complex at 2.9 Å using single particle cryogenic electron microscopy. The structure shows the position of the transmembrane domain of subunit IV, which lies across the transmembrane helices of the Rieske and cytochrome subunits. We observe a quinone at the Q quinone-binding site and show that occupancy of this site is linked to conformational changes in the Rieske head domain during catalysis. Twelve lipids were structurally resolved, making contacts with the Rieske and cytochrome subunits, with some spanning both of the two monomers that make up the dimeric complex.