Elucidating the H(+) Coupled Zn(2+) Transport Mechanism of ZIP4; Implications in Acrodermatitis Enteropathica

Cellular Zn(2+) homeostasis is tightly regulated and primarily mediated by designated Zn(2+) transport proteins, namely zinc transporters (ZnTs; SLC30) that shuttle Zn(2+) efflux, and ZRT-IRT-like proteins (ZIPs; SLC39) that mediate Zn(2+) influx. While the functional determinants of ZnT-mediated Zn(2+) 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(2+) transport, or (III) a pH regulated electrodiffusional mode of transport. Here, using live-cell fluorescent imaging of Zn(2+) 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(2+) influx. In contrast, extracellular acidification stimulated ZIP4 dependent Zn(2+) uptake. Furthermore, Zn(2+) 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(2+) co-transporter. We further illustrate how this functional mechanism is affected by genetic variants in SLC39A4 that in turn lead to Acrodermatitis enteropathica, a rare condition of Zn(2+) deficiency.