2.A.4.4.5
Golgi/secretory granule Zn²⁺ uptake (into Golgi or granules) permease, ZnT7 (Ishihara et al., 2006). Bui et al. 2023 presented the 2.2-3.1 Å-resolution cryo-EM structures of the human Zn²⁺/H⁺ antiporter ZnT7 in Zn²⁺-bound and unbound forms. Cryo-EM analyses show that hZnT7 exists as a dimer via tight interactions in both the cytosolic and transmembrane (TM) domains of two protomers, each of which contains a single Zn²⁺-binding site in its TM domain. hZnT7 undergoes a TM-helix rearrangement to create a negatively charged cytosolic cavity for Zn²⁺ entry in the inward-facing conformation and widens the luminal cavity for Zn²⁺ release in the outward-facing conformation. An exceptionally long cytosolic histidine-rich loop, characteristic of hZnT7, binds two Zn²⁺ ions, seemingly facilitating Zn²⁺ recruitment to the TM metal transport pathway. These structures allow mechanisms of hZnT7-mediated Zn²⁺ uptake into the Golgi to be proposed (Bui et al. 2023).