Structural defects caused by Acrodermatitis Enteropathica mutations in the extracellular domain account for mistrafficking and malfunction of ZIP4

ZIP4 is a representative member of the Zrt-/Irt-like protein (ZIP) transporter family and responsible for zinc uptake from diet. Loss-of-function mutations of human ZIP4 (hZIP4) drastically reduce zinc absorption, causing a life-threatening autosomal recessive disorder, Acrodermatitis Enteropathica (AE). Although the zinc transport machinery is located in the transmembrane domain conserved in the entire ZIP family, half of the missense mutations occur in the extracellular domain (ECD) of hZIP4, which is only present in a fraction of mammalian ZIPs. How the AE-causing mutations in the ECD lead to ZIP4 malfunction has not be fully clarified. In this work, we characterized all the seven confirmed AE-causing missense mutations in hZIP4-ECD and found that the variants exhibited completely abolished zinc transport activity measured in a cell-based transport assay. Although the variants were able to be expressed in HEK293T cells, they failed to traffic to cell surface and were largely retained in the ER with immature glycosylation. When the corresponding mutations were introduced in the ECD of ZIP4 from Pteropus Alecto, a close homolog of hZIP4, the variants exhibited impaired protein folding and reduced thermal stability, which likely account for intracellular mistrafficking of the AE-associated variants and as such a total loss of zinc uptake in cells. This work provides a molecular pathogenic mechanism for AE, which lays out a basis for potential therapy using small molecular chaperones.