Functional Characterization of a Novel Mammalian Zinc Transporter, ZnT6

Abstract

We describe ZnT6, a new member of the CDF (cation diffusion facilitator) family of heavy metal transporters. The human ZNT6 gene was mapped at 2p21-22, while the mouse Znt6 was localized to chromosome 17. Overexpression of ZnT6 in both wild-type yeast and mutants that are deficient in cytoplasmic zinc causes growth inhibition, but this inhibition is abolished in mutant cells with high cytoplasmic zinc. ZnT6 may function in transporting the cytoplasmic zinc into the Golgi apparatus as well as the vesicular compartment, as evidenced by its overlapping intracellular localization with TGN38 and transferrin receptor in the normal rat kidney cells. We also demonstrate that the intracellular distributions of ZnT6 as well as ZnT4 are regulated by zinc in the normal rat kidney cells. The results from this report, combined with those from other studies, suggest that the intracellular zinc homeostasis is mediated by many ZnT proteins, which act in tissue-, cell-, and organelle-specific manners.

Expression of the zinc transporter ZnT4 is decreased in the progression from early prostate disease to invasive prostate cancer

“…Indeed, clear delineation of plasma membrane staining may be confounded because ZnT4 may be constantly recycling. Thus, while the exact function of ZnT4 in the prostate remains unknown, our results are consistent with a role in the transport of cytoplasmic zinc into the Golgi apparatus, the vesicular compartment and potentially in efflux of zinc (Huang et al, 2002).…”

“…Previous studies have implied that ZnT4 (Murgia et al, 1999) is located to endosomal vesicles, although a subpopulation of ZnT4-containing vesicles also contained the transferrin receptor that recycles between the endosomal compartment and the plasma membrane (Murgia et al, 1999). The punctate cytoplasmic IHC staining seen in prostate tissue and the immunoflourescence studies performed on ZnTh infected overexpressing PC-3 prostate cancer cells are consistent with the intracytoplasmic vesicular location of ZnT4 described in ZnT4 in prostate cancer SM Henshall et al the rat intestine (Murgia et al, 1999) and the more detailed studies on subcellular localization to the trans-Golgi network and the endosomal compartment that it shares with ZnT6 (Huang et al, 2002). However, our biotinylation studies suggest the presence of ZnT4 at the plasma membrane, indicating membrane cycling between endosomal vesicles and the cell membrane.…”

“…However, our biotinylation studies suggest the presence of ZnT4 at the plasma membrane, indicating membrane cycling between endosomal vesicles and the cell membrane. While other authors share this view (Huang et al, 2002;Michalczyk et al, 2002), clear demonstration of cell surface expression has, to date, been confined to overexpression studies with no detectable cell membrane labelling in PMC42 breast cancer (Michalczyk et al, 2002), or LNCaP prostate cancer cells (date not shown). Indeed, clear delineation of plasma membrane staining may be confounded because ZnT4 may be constantly recycling.…”

“…ZnT proteins are members of the cation diffusion facilitator (CDF) family and function by either transporting zinc out of cells or by sequestering it into intracellular compartments, while ZIP proteins appear to be involved in the cellular uptake of zinc. The ZNT family of mammalian zinc transporters, of which there are now six members, share similar structures consisting of six transmembrane domains and a histidine-rich cytoplasmic loop, but differ from each other in their tissue specificity and intracellular location (McMahon and Cousins, 1998;Murgia et al, 1999;Huang et al, 2002;Kambe et al, 2002). ZNT1 is located at the plasma membrane and functions as a zinc efflux transporter (Palmiter and Findley, 1995), while ZNT2 and ZNT3 are thought to be involved in zinc sequestration in endosomal vesicles (Palmiter et al, 1996a, b).…”

The Zinc Transporter ZnT3 Interacts with AP-3 and It Is Preferentially Targeted to a Distinct Synaptic Vesicle Subpopulation

“…On the other hand, we estimate that most of the zinquin fluorescence detected by us is due to ZnT3 because ZnT3 Ϫ/Ϫ mice completely lack ionic zinc in their hippocampus (Cole et al, 1999). However, the contribution of other zinc transporters expressed in brain, although likely to be minor (Huang, 1997;Huang et al, 2002;Kambe et al, 2002), cannot be ruled out yet. Nonetheless, our confocal microscopy immuno-localization studies in hippocampal neurons and the biochemical analysis of brain SV from wild-type and AP-3-deficient mocha brains support the hypothesis that vesicles possess variable stoichiometry of their constituents.…”