Structure of the <i>bc</i>₁-<i>cbb</i>₃ respiratory supercomplex from <i>Pseudomonas aeruginosa</i>.

Abstract

Energy conversion by electron transport chains occurs through the sequential transfer of electrons between protein complexes and intermediate electron carriers, creating the proton motive force that enables ATP synthesis and membrane transport. These protein complexes can also form higher order assemblies known as respiratory supercomplexes (SCs). The electron transport chain of the opportunistic pathogen <i>Pseudomonas aeruginosa</i> is closely linked with its ability to invade host tissue, tolerate harsh conditions, and resist antibiotics but is poorly characterized. Here, we determine the structure of a <i>P. aeruginosa</i> SC that forms between the quinol:cytochrome <i>c</i> oxidoreductase (cytochrome <i>bc</i>₁) and one of the organism's terminal oxidases, cytochrome <i>cbb</i>₃, which is found only in some bacteria. Remarkably, the SC structure also includes two intermediate electron carriers: a diheme cytochrome <i>c</i>₄ and a single heme cytochrome <i>c</i>₅. Together, these proteins allow electron transfer from ubiquinol in cytochrome <i>bc</i>₁ to oxygen in cytochrome <i>cbb</i>₃. We also present evidence that different isoforms of cytochrome <i>cbb</i>₃ can participate in formation of this SC without changing the overall SC architecture. Incorporating these different subunit isoforms into the SC would allow the bacterium to adapt to different environmental conditions. Bioinformatic analysis focusing on structural motifs in the SC suggests that cytochrome <i>bc</i>₁-<i>cbb</i>₃ SCs also exist in other bacterial pathogens.