Cryo-EM structures of the zinc transporters ZnT3 and ZnT4 provide insights into their transport mechanisms.

Zinc transporters (ZnTs) act as H⁺/Zn²⁺ antiporters, crucial for zinc homeostasis. Brain-specific ZnT3 expressed in synaptic vesicles transports Zn²⁺ from the cytosol into vesicles and is essential for neurotransmission, with ZnT3 dysfunction associated with neurological disorders. Ubiquitously expressed ZnT4 localized to lysosomes facilitates the Zn²⁺ efflux from the cytosol to lysosomes, mitigating the cell injury risk. Despite their importance, the structures and Zn²⁺ transport mechanisms remain unclear. We characterized the three-dimensional structures of human ZnT3 (inward-facing) and ZnT4 (outward-facing) using cryo-electron microscopy. By combining these structures, we assessed the conformational changes that could occur within the transmembrane domain during Zn²⁺ transport. Our results provide a structural basis for a more comprehensive understanding of the H⁺/Zn²⁺ exchange mechanisms exhibited by ZnTs.