Copper is an essential metal notably found in respiration complexes for its redox properties. It is also toxic hence its cellular trafficking is tightly controlled. Bacteria have developed a number of defense systems against copper excess, but its acquisition pathways remain poorly characterized. Ubiquitous in Gram-negative bacteria, TonB-dependent transporters (TBDTs) are outer membrane β-barrel proteins that mediate the proton motive force-dependent import of various nutrients to the periplasm. Here, we characterized a TBDT that imports copper in the whooping cough agent Bordetella pertussis, CrtABp (formerly BfrG), which is a prototype of a new subfamily of TBDTs. Our data indicate that CrtABp is dedicated to the import of copper for heme-copper respiratory oxidoreductases. We revealed that CrtABp imports chelated rather than free copper, solved the crystal structure of CrtABp and identified a conserved ligand binding site. By combining bacterial growth experiments, biophysical approaches and AlphaFold3 modeling we sketched out the features of copper-ligand complexes for CrtABp. In contrast with ferrisiderophore-specific TBDTs, no high-affinity chalkophore ligand of CrtABp could be identified, implying two non-mutually exclusive models. In the host, CrtABp might use a xenometallophore produced by another species present in the same niche to acquire copper. In vitro however, CrtA appears not to have high-affinity ligands but to import copper chelated by small molecules notably harboring carboxylate groups, which might represent a paradigm of 'scavenger' TBDTs with low ligand selectivity. We identified an essential, invariant histidine residue that might serve as a selectivity filter for copper-chelate complexes.
Hachmi et al. (Thu,) studied this question.