Zinc is the second most abundant transition metal in biological systems. This cation is required for the catalytic activity of hundreds of enzymes which mediate protein synthesis, DNA replication and cell division. Despite the central importance of zinc in cellular homeostasis, the mechanism of zinc uptake, compartmentalization and efflux is unknown. Recently, a family of proteins, called ZIP, has been shown to control zinc uptake. Mutations in one of the genes coding for these proteins (ZIP4) can lead to potentially life-threatening diseases like Acrodermatitis Enteropathica and high levels of ZIP4 have been detected in patients suffering from pancreatic cancer. Therefore our goal is to investigate the mechanism of ZIP4 transport and regulation. It was previously shown that the intracellular loop between transmembrane III and IV (M3M4) of ZIP4 is ubiquitinated in the presence of high intracellular zinc which lead to protein degradation. Our initial hypothesis was that the large intracellular domain of ZIP4 (M3M4) is a sensor which detects the intracellular concentration of zinc and regulates the surface expression of ZIP4. In order to test this hypothesis we expressed and purified the M3M4 domain to examine the ability of M3M4 to bind zinc. Our results have demonstrated that M3M4 binds zinc with a 2:1 zinc:protein stoichiometry with nanomolar affinity. We have also shown that upon binding of zinc, M3M4 undergoes a large conformational change.