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

Abstract

ABSTRACT The 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 cbb 3 -type Cox ( cbb 3 -Cox) in the facultative phototroph Rhodobacter capsulatus . Genetic screens identified a cbb 3 -Cox defective mutant that requires Cu 2+ supplementation to produce an active cbb 3 -Cox. Complementation of this mutant using wild-type genomic libraries unveiled a novel gene ( ccoA ) required for cbb 3 -Cox biogenesis. In the absence of CcoA, the cellular Cu content decreases and cbb 3 -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 cbb 3 -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. IMPORTANCE Biogenesis of energy-transducing membrane-integral enzymes, like the heme copper-containing cytochrome c oxidases, and the acquisition of transition metals, like copper, as their catalytic cofactors are vital processes for all cells. These widespread and well-controlled processes are poorly understood in all organisms, including bacteria. Defects in these processes lead to severe mitochondrial diseases in humans and poor crop yields in plants. In this study, using the facultative phototroph Rhodobacter capsulatus as a model organism, we report on the discovery of a novel major facilitator superfamily (MFS)-type transporter (CcoA) that affects cellular copper content and cbb 3 -type cytochrome c oxidase production in bacteria.