Histidine pairing at the metal transport site of mammalian ZnT transporters controls Zn 2+ over Cd 2+ selectivity

Zinc and cadmium are similar metal ions, but though Zn 2+ is an essential nutrient, Cd 2+ is a toxic and common pollutant linked to multiple disorders. Faster body turnover and ubiquitous distribution of Zn 2+ vs. Cd 2+ suggest that a mammalian metal transporter distinguishes between these metal ions. We show that the mammalian metal transporters, ZnTs, mediate cytosolic and vesicular Zn 2+ transport, but reject Cd 2+ , thus constituting the first mammalian metal transporter with a refined selectivity against Cd 2+ . Remarkably, the bacterial ZnT ortholog, YiiP, does not discriminate between Zn 2+ and Cd 2+ . 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 Cd 2+ by the transporter. Thus, our results identify a unique class of mammalian transporters and the structural motif required to discriminate between Zn 2+ and Cd 2+ , and show that metal selectivity is tuned by a coordination-based mechanism that raises the thermodynamic barrier to Cd 2+ binding.