Histidine pairing at the metal transport site of mammalian ZnT transporters controls Zn²⁺ over Cd²⁺ selectivity

Zinc and cadmium are similar metal ions, but though Zn²⁺ is an essential nutrient, Cd²⁺ is a toxic and common pollutant linked to multiple disorders. Faster body turnover and ubiquitous distribution of Zn ²⁺ vs. Cd²⁺ suggest that a mammalian metal transporter distinguishes between these metal ions. We show that the mammalian metal transporters, ZnTs, mediate cytosolic and vesicular Zn²⁺ transport, but reject Cd²⁺, thus constituting the first mammalian metal transporter with a refined selectivity against Cd²⁺. Remarkably, the bacterial ZnT ortholog, YiiP, does not discriminate between Zn²⁺ and Cd²⁺. A phylogenetic comparison between the tetrahedral metal transport motif of YiiP and ZnTs identifies a histidine at the mammalian site that is critical for metal selectivity. Residue swapping at this position abolished metal selectivity of ZnTs, and fully reconstituted selective Zn ²⁺ transport of YiiP. Finally, we show that metal selectivity evolves through a reduction in binding but not the translocation of Cd²⁺ by the transporter. Thus, our results identify a unique class of mammalian transporters and the structural motif required to discriminate between Zn ²⁺ and Cd²⁺, and show that metal selectivity is tuned by a coordination-based mechanism that raises the thermodynamic barrier to Cd ²⁺ binding.