Fig. 2: Cryo-EM structure of the Zn²⁺-unbound hZnT7 homodimer composed of the outward-facing (OF) protomers (OF-OF form).

a Side view of the cryo-EM map of Zn²⁺-unbound hZnT7 homodimer. Chain A (violet), chain B (green), TMs 4-5 (yellow) and His-loop 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 homodimer in the same orientation as in (a). Both left and right protomers adopt an OF conformation. 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. The black dashed-line squares indicate the regions that are highlighted in panel (f). 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 surface map of an hZnT7 protomer in the OF form. Surface colors indicate Coulombic potentials (red, negative; white, neutral; blue, positive). e Zoomed-in view of the visible segments of the His-loop near the C-terminus of TM4 and the N-terminus of TM5. TM4 and TM5 are represented by yellow ribbon. The density map is shown by blue mesh at a contour level of 0.17σ. f Zoomed-in view of the CTD-CTD interface in the OF-OF homodimer of hZnT7. Yellow dotted lines indicate interactions formed between polar residues.