Close-up view of the loop domain linking β2 to α2 (in front of the figure) at the outer edge of the C-terminal domain intracellular part of the 3D structure of the wild-type ZnT-8 protein. Note that Arg325 points away from the interface with the other monomer. The bacterial structure is indicated in green, the human structure in yellow, and the other monomer in blue. The four front spheres at the interface of the intracellular domains represent the positioning of the two binuclear zinc ion clusters in sites Z3 and Z4 in each of the protomers, and the back spheres the zinc ion in site Z2 in each of the protomers. At the back of the figure, the faint sphere represents a zinc ion in site Z1. The model predicts salt bridges linking the ε-NH3 + group of Arg325 and the β-COO- group of Asp326 as well as Asp295. The side-chains of Arg325, Asp326, and Asp295 are presented by a "ribbon structure".