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Sci Rep. 2019 Feb 4;9(1):1208. doi: 10.1038/s41598-018-37988-4.

Cu Transport by the Extended Family of CcoA-like Transporters (CalT) in Proteobacteria.

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Department of Biology, University of Pennsylvania, Philadelphia, PA, 19104, USA.
School of Life Science, Xiamen University, Xiamen, 361102, China.
Biology Department, Brookhaven National Laboratory, Upton, NY, 11973, USA.
bioMT-Institute for Biomolecular Targeting, Geisel School of Medicine at Dartmouth, Hanover, NH, 03755, USA.
Microbiology and Mycology Department, Insituto de Ciencias Biomédicas, University of Chile, Santiago, Chile.
Institut für Biochemie und Molekularbiologie, ZBMZ, Faculty of Medicine, Stefan-Meier-Strasse 17, Albert-Ludwigs-Universität Freiburg, 79104, Freiburg, Germany.
Department of Biology, University of Pennsylvania, Philadelphia, PA, 19104, USA.
IPREM, UMR CNRS 5254, and Université de Pau et des Pays de l'Adour, BP1155, Pau, France.


Comparative genomic studies of the bacterial MFS-type copper importer CcoA, required for cbb3-type cytochrome c oxidase (cbb3-Cox) biogenesis, revealed a widespread CcoA-like transporters (CalT) family, containing the conserved CcoA Cu-binding MxxxM and HxxxM motifs. Surprisingly, this family also included the RfnT-like proteins, earlier suggested to transport riboflavin. However, presence of the Cu-binding motifs in these proteins raised the possibility that they might be Cu transporters. To test this hypothesis, the genomic context of the corresponding genes was examined, and three of such genes from Ochrobactrum anthropi, Rhodopseudomonas palustris and Agrobacterium tumefaciens were expressed in Escherichia coli (ΔribB) and Rhodobacter capsulatus (ΔccoA) mutants. Copper and riboflavin uptake abilities of these strains were compared with those expressing R. capsulatus CcoA and Rhizobium leguminosarum RibN as bona fide copper and riboflavin importers, respectively. Overall data demonstrated that the "RfnT-like" CalT proteins are unable to efficiently transport riboflavin, but they import copper like CcoA. Nevertheless, even though expressed and membrane-localized in a R. capsulatus mutant lacking CcoA, these transporters were unable to accumulate Cu or complement for cbb3-Cox defect. This lack of functional exchangeability between the different subfamilies of CalT homologs suggests that MFS-type bacterial copper importers might be species-specific.

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