Updated project metadata. The purple α-proteobacterium Rhodobacter sphaeroides is a long-established model organism owing to its ability to switch between aerobic, anaerobic and phototrophic metabolism. This organism is also of great interest because of its phylogenetic relationship with the bacterial progenitor of the mitochondrion in eukaryotes. A component of the electron transport chain in both bacteria and mitochondria is the cytochrome bc1 (cytbc1) complex, a quinol:cytochrome c oxidoreductase that translocates protons across energy-transducing membranes. The resulting proton motive force is then utilized to drive ATP synthesis. Cytbc1 comprises three core subunits together with, in some organisms, one or more additional subunits. In Rhodobacter sphaeroides, the one additional subunit (subunit IV) is not fully characterized in terms of both structure and function. In this project, proteomic analysis is used to confirm the presence of all four subunits in a cytbc1 preparation isolated in SMA nanodiscs for structural analysis by cryo-EM.