The ScoI homologue SenC is a copper binding protein that interacts directly with the cbb3-type cytochrome oxidase in Rhodobacter capsulatus.

Sco proteins are widespread assembly factors for the Cu(A) centre of aa3-type cytochrome oxidases in eukaryotic and prokaryotic organisms. However, Sco homologues are also found in bacteria like Rhodobacter capsulatus which lack aa3-type cytochrome oxidases and instead use a cbb3-type cytochrome oxidase (cbb3 Cox) without a Cu(A) centre as a terminal oxidase. In the current study, we have analyzed the role of Sco (SenC) during cbb3 Cox assembly in R. capsulatus. In agreement with earlier works, we found a strong cbb3 Cox defect in the absence of SenC that impairs the steady-state stability of the CcoN, CcoO and CcoP core subunits, without the accumulation of detectable assembly intermediates. In vivo cross-linking results demonstrate that SenC is in close proximity to the CcoP and CcoH subunits of cbb3 Cox, suggesting that SenC interacts directly with cbb3 Cox during its assembly. SenC binds copper and the cbb3 Cox assembly defect in the absence of SenC can be rescued by the addition of least 0.5µM Cu. Neither copper nor SenC influenced the transcription of the ccoNOQP operon encoding for cbb3 Cox. Transcription of senC itself was also not influenced by Cu unless the putative Cu-export ATPase CcoI was absent. As CcoI is specifically required for the cbb3 Cox assembly, these data provide a direct link between Cu delivery to cbb3 Cox and SenC function.

Biogenesis of cbb(3)-type cytochrome c oxidase in Rhodobacter capsulatus.

The cbb(3)-type cytochrome c oxidases (cbb(3)-Cox) constitute the second most abundant cytochrome c oxidase (Cox) group after the mitochondrial-like aa(3)-type Cox. They are present in bacteria only, and are considered to represent a primordial innovation in the domain of Eubacteria due to their phylogenetic distribution and their similarity to nitric oxide (NO) reductases. They are crucial for the onset of many anaerobic biological processes, such as anoxygenic photosynthesis or nitrogen fixation. In addition, they are prevalent in many pathogenic bacteria, and important for colonizing low oxygen tissues. Studies related to cbb(3)-Cox provide a fascinating paradigm for the biogenesis of sophisticated oligomeric membrane proteins. Complex subunit maturation and assembly machineries, producing the c-type cytochromes and the binuclear heme b(3)-Cu(B) center, have to be coordinated precisely both temporally and spatially to yield a functional cbb(3)-Cox enzyme. In this review we summarize our current knowledge on the structure, regulation and assembly of cbb(3)-Cox, and provide a highly tentative model for cbb(3)-Cox assembly and formation of its heme b(3)-Cu(B) binuclear center. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.