Elucidating the H Coupled Zn Transport Mechanism of ZIP4; Implications in Acrodermatitis Enteropathica.

Cellular Zn homeostasis is tightly regulated and primarily mediated by designated Zn transport proteins, namely zinc transporters (ZnTs; SLC30) that shuttle Zn efflux, and ZRT-IRT-like proteins (ZIPs; SLC39) that mediate Zn influx. While the functional determinants of ZnT-mediated Zn efflux are elucidated, those of ZIP transporters are lesser understood. Previous work has suggested three distinct molecular mechanisms: (I) HCO3 or (II) H coupled Zn transport, or (III) a pH regulated electrodiffusional mode of transport. Here, using live-cell fluorescent imaging of Zn and H, in cells expressing ZIP4, we set out to interrogate its function. Intracellular pH changes or the presence of HCO3 failed to induce Zn influx. In contrast, extracellular acidification stimulated ZIP4 dependent Zn uptake. Furthermore, Zn uptake was coupled to enhanced H influx in cells expressing ZIP4, thus indicating that ZIP4 is not acting as a pH regulated channel but rather as an H powered Zn co-transporter. We further illustrate how this functional mechanism is affected by genetic variants in that in turn lead to Acrodermatitis enteropathica, a rare condition of Zn deficiency.