2.A.5.5.1
Broad specificity heavy metal divalent cation uptake transporter, ZupT (Fe²⁺, Co²⁺, Mn²⁺, Cd²⁺ and Zn²⁺ are transported) (Grass et al., 2005). Point mutations change the specificity and kinetics of metal uptake (Taudte and Grass, 2010). Important for virulence in Salmonella (Karlinsey et al., 2010). ZupT has an asymmetric binuclear metal center in the transmembrane domain; one metal-binding site, M1, binds zinc, cadmium, and iron, while the other, M2, binds iron only and with higher affinity than M1. Using site-specific mutagenesis and transport activity measurements in whole cells and proteoliposomes, Roberts et al. 2021 showed that zinc is transported from M1, while iron is transported from M2. The two sites share a common bridging ligand, a conserved glutamate residue. M1 and M2 have ligands from highly conserved motifs in transmembrane domains 4 and 5. Additionally, M2 has a ligand from transmembrane domain 6, a glutamate residue, which is conserved in the gufA subfamily of ZIP transporters, including ZupT and the human ZIP11. Unlike cadmium, iron transport from M2 does not inhibit the zinc transport activity but slightly stimulates it. This stimulated activity is mediated through the bridging carboxylate ligand. The binuclear zinc-iron binding center in ZupT has likely evolved to enable the transport of essential metals from two different sites without competition; a similar mechanism of metal transport is likely to be found in the gufA subfamily of ZIP transporter proteins (Roberts et al. 2021).