ZNT-2, A MAMMALIAN PROTEIN THAT CONFERS RESISTANCE TO ZINC BY FACILITATING VESICULAR SEQUESTRATION

A cDNA encoding a second zinc transporter (ZnT-2) was isolated from a rat kidney cDNA expression library by complementation of a zinc-sensit ive BHK cell line. The protein predicted from the open reading frame o f ZnT-2 cDNA has 359 amino acids and initiates with a CTG codon. It re sembles ZnT-1 (a plasma membrane protein that stimulates zinc efflux) in overall topology in that it has six membrane-spanning domains, a hi stidine-rich intracellular loop and a long C-terminal tail; however, t he overall amino acid identity is only 26%. Unlike ZnT-1, which is in the plasma membrane and lowers cellular zinc by stimulating zinc efflu x, ZnT-2 is localized on vesicles and allows the zinc-sensitive BHK ce lls to accumulate zinc to levels that are much higher than non-transfo rmed cells can tolerate. Zinc was visualized within these vesicles wit h zinquin, a zinc-specific fluorescent probe. The intracellular compar tment that accumulates zinc is acidic as revealed by staining with acr idine orange or LysoTracker. Prolonged exposure of cells expressing Zn T-2 to zinc causes an accretion of intracellular vesicles. We suggest that ZnT-2 protects these cells from zinc toxicity by facilitating zin c transport into an endosomal/lysosomal compartment.