A canonical mechanism for metal-proton symport. In a conventional transport mechanism, metal and protons bind to central binding sites in the transporter in an outward-facing conformation. In APC/FIRL transporters, binding sites are found near the interface between the scaffold (blue) and bundle (orange) domains. After binding metal (green) and proton(s) (red) to the outward-facing transporter (top), reorganization of the bundle domain (right) closes the extracellular pathway and opens a pathway between the binding sites and the cytoplasm (lower right). After release of metal and proton(s) to the cytoplasm (bottom), the bundle domain isomerizes (left), returning to the outward-facing conformation (top left).