Conformational changes in CDF transporters . The dimer is depicted schematically with transmembrane helices in rainbow colors from blue to red and the CTD shown as a triangle. Zn ions are pink spheres and residues composing the hydrophobic gate (Leu154 and Leu199) as bifurcated sticks. The checkered blue/red rectangle symbolizes a conserved salt-bridge between the protomers. Twofold symmetry axes are indicated as gray lines as well as the operator symbol for those dimers that retain overall symmetry. (A) Inward-facing (IF) conformation obtained for soYiiP, both from tubular crystals and from the YiiP:Fab complex. Zn ions are bound at all three sites, the hydrophobic gate is open, and the structure retains twofold symmetry. (B) Removal of Zn from site B causes disordering of the M2-M3 loop of soYiiP and bending of M2 and M5 helices, thus closing the hydrophobic gate. This hypothetical conformation may correspond to an occluded state and thus an intermediate in the transport pathway. A bend between the TMD and CTD breaks the overall twofold symmetry of the complex, though local twofold symmetry within each domain is preserved. (C) Architecture of the outward-facing (OF) state as seen in the cryo-EM structure of Znt8 (Xue et al., 2020). All the Zn sites are occupied, though they are formed by different structural elements in this homologue; overall twofold symmetry is retained. (D) Full removal of Zn from soYiiP leads to a loosening of interactions in the CTD. (E) The x-ray structure of ecYiiP shows splaying of TMDs and unwinding of several transmembrane helices on the extracellular side of the membrane. Nevertheless, the general architecture of the TMD is consistent with an outward-facing conformation and the overall twofold symmetry of the dimer is intact.