Novel Transporter Required for Biogenesis of cbb 3 -Type Cytochrome c Oxidase in Rhodobacter capsulatus

Abstract

ABSTRACTThe acquisition, delivery, and incorporation of metals into their respective metalloproteins are important cellular processes. These processes are tightly controlled in order to prevent exposure of cells to free-metal concentrations that could yield oxidative damage. Copper (Cu) is one such metal that is required as a cofactor in a variety of proteins. However, when present in excessive amounts, Cu is toxic due to its oxidative capability. Cytochrome c oxidases (Coxs) are among the metalloproteins whose assembly and activity require the presence of Cu in their catalytic subunits. In this study, we focused on the acquisition of Cu for incorporation into the heme-Cu binuclear center of the cbb3-type Cox (cbb3-Cox) in the facultative phototroph Rhodobacter capsulatus. Genetic screens identified a cbb3-Cox defective mutant that requires Cu2+ supplementation to produce an active cbb3-Cox. Complementation of this mutant using wild-type genomic libraries unveiled a novel gene (ccoA) required for cbb3-Cox biogenesis. In the absence of CcoA, the cellular Cu content decreases and cbb3-Cox assembly and activity become defective. CcoA shows homology to major facilitator superfamily (MFS)-type transporter proteins. Members of this family are known to transport small solutes or drugs, but so far, no MFS protein has been implicated in cbb3-Cox biogenesis. These findings provide novel insights into the maturation and assembly of membrane-integral metalloproteins and on a hitherto-unknown function(s) of MFS-type transporters in bacterial Cu acquisition.