Fig. 3: Cryo-EM structure of the Zn²⁺-unbound hZnT7 heterodimer composed of the inward-facing (IF) and OF protomers (IF-OF form).

a Side view of the cryo-EM map of Zn²⁺-unbound hZnT7 heterodimer. Chain A (violet), chain B (green), TMs 4-5 (yellow) and histidine-rich segment (magenta) are highlighted. Numbers indicate the transmembrane (TM) helix number from the N-terminus. TMD, transmembrane domain; CTD, cytosolic domain. b Cartoon representation of the hZnT7 heterodimer in the same orientation as in (a). Left and right protomers adopt IF and OF conformations, respectively. TM1 to TM3 and TM6 are shown in green or violet, respectively, while TM4 and TM5 are both in yellow. Visible regions of the histidine-rich loop (His-loop) are shown in magenta. Numbers indicate the TM helix number from the N-terminus. TMD, transmembrane domain; CTD, cytosolic domain. c Golgi luminal view of the TMD of hZnT7, where the CTD and TM loops are removed for clarity. The red double-headed arrows indicate the proximity between TM2 and TM3’ (or TM2’ and TM3) at the dimer interface. Numbers indicate the TM helix number from the N-terminus. d Coronal section of the electrostatic potential map of the IF protomer in the Zn²⁺-unbound hZnT7 heterodimer. Surface colors indicate Coulombic potentials (red, negative; white, neutral; blue, positive). e Coronal section of the electrostatic potential map of the OF protomer in the Zn²⁺-unbound hZnT7 heterodimer. f Superimposition of the Zn²⁺-unbound OF-OF homodimer (gray) and IF-OF heterodimer (colored as indicated) of hZnT7 viewed from the Golgi lumen (left), cytosol (middle), and side (right). The cytosolic domain and TM loops are omitted for clarity. Numbers indicate the TM helix number from the N-terminus. Dark blue arrows indicate the movements of TM helices during the conversion from the OF to IF conformations in the left protomer.