Identification and cloning of a β-cell-specific zinc transporter, ZnT-8, localized into insulin secretory granules

SLC3OA8, a novel member of the zinc transporter (ZnT) family, was identified by searching the human genomic and expressed sequence tag (EST) databases with the amino acid sequence of all known human ZnT. The protein (369 amino acids) predicted from this gene, ZnT-8, contains six transmembrane domains and a histidine-rich loop between transmembrane domains IV and V, like the other ZnT proteins. We demonstrated by RT-PCR on cDNA libraries and human tissue extracts that the ZnT-8 gene is solely transcribed in the pancreas, mainly in the islets of Langerhans. The gene, named SLC3OA8, was cloned and sequenced. Confocal immunofluorescence analysis revealed that a ZnT-8-EGFP (enhanced green fluorescent protein) fusion product colocalized with insulin in the secretory pathway granules of the insulin-secreting INS-1 cells. Exposure of the ZnT-8-EGFP stably expressing HeLa cells to 75 mumol/l zinc caused an accumulation of zinc in intracellular vesicles compared with cells expressing EGFP alone. These results identified ZnT-8 as a ZnT specific to the pancreas and expressed in beta-cells. Because ZnT-8 facilitates the accumulation of zinc from the cytoplasm into intracellular vesicles, ZnT-8 may be a major component for providing zinc to insulin maturation and/or storage processes in insulin-secreting pancreatic beta-cells.