A growing body of evidence continues to demonstrate the vital roles that zinc and its transporters play on human health. The mammalian solute carrier 30 (SLC30) family, with ten current members, controls zinc efflux transport in cells. TMEM163, a recently reported zinc transporter, has similar characteristics in both predicted transmembrane domain structure and function to the cation diffusion facilitator (CDF) protein superfamily. This review discusses past and present data indicating that TMEM163 is a zinc binding protein that transports zinc in cells. We provide a brief background on TMEM163’s discovery, transport feature, protein interactome, and similarities, as well as differences, with known SLC30 (ZnT) protein family. We also examine recent reports that implicate TMEM163 directly or indirectly in various human diseases such as Parkinson’s disease, Mucolipidosis type IV and diabetes. Overall, the role of TMEM163 protein in zinc metabolism is beginning to be realized, and based on current evidence, we propose that it is likely a new CDF member belonging to mammalian SLC30 (ZnT) zinc efflux transporter proteins.
A growing body of evidence continues to demonstrate the vital roles that zinc and its transporters play on human health. The solute carrier (SLC) 30 and 39 families, with ten and fourteen members, respectively, control zinc transport in cells. TMEM163, a recently characterized zinc transporter, has similar characteristics in both structure and function to the SLC30 family. This review examines recent data that reveal TMEM163 to be a zinc efflux transporter and a new member of the cation diffusion facilitator (CDF) family of mammalian zinc transporter (ZNT) proteins. It also discusses reports that implicate TMEM163 in various human diseases.
Our recent work showed that TMEM163 is a zinc efflux transporter that belongs to the cation diffusion facilitator (CDF) family of proteins. We propose that the TMEM163 be now called ZNT11 as a new member of the Group 1 CDF family ZNT efflux proteins that include 10 mammalian zinc transporters, ZNT1‐ZNT10. Accordingly, we hypothesize that TMEM163/ZNT11 interacts with at least one of the ZNT proteins, namely ZNT1 and ZNT2, based on its structural and functional characteristics. To show that TMEM163/ZNT11 interacts with either ZNT1 or ZNT2, we cloned their cDNAs into mammalian expression constructs containing either Myc‐DDK or HA peptide tag. Using HEK‐293 cells, we individually transfected TMEM163/ZNT11, ZNT1, and ZNT2 constructs as negative controls, while we co‐transfected TMEM163/ZNT11 and ZNT1, or TMEM163/ZNT11 and ZNT2. As a positive control, we co‐transfected HA‐tagged TMEM163/ZNT11 with a Myc‐DDK‐tagged TMEM163/ZNT11 construct in HEK‐293 cells, based on a previous report that its rodent counterpart exists as a homodimer. We then used a co‐immunoprecipitation assay using either anti‐HA‐ or anti‐DDK‐bound agarose beads to pull down one of the target proteins. Western blot analysis showed that TMEM163/ZNT11 physically bound ZNT1 or ZNT2 protein. To determine the functional relevance of the interaction, we performed zinc flux assays using two zinc‐specific fluorescence dyes, Fluozin‐3 (high affinity, membrane impermeable) and Newport Green (low affinity, membrane permeable) following single‐ and co‐transfection of TMEM163/ZNT11, ZNT1, and ZNT2 constructs in HeLa cells. Our results confirmed that homodimers of TMEM163/ZNT11, ZNT1, and ZNT2 proteins transport zinc out of the cells, but that the efflux activity of homodimer TMEM163/ZNT11 proteins varied slightly in magnitude when compared with heterodimers of TMEM163/ZNT11 and ZNT1, or TMEM163/ZNT11 and ZNT2 proteins. These results suggest that the interaction between TMEM163/ZNT11 and distinct ZNT proteins is physiologically relevant and may serve to modify the transport activity of ZNT protein interactor. Overall, our investigations showed for the first time that TMEM163/ZNT11 forms functional heterodimers with ZNT1 and ZNT2 proteins. Thus, TMEM163/ZNT11 by itself, or in combination with one of these specific ZNT proteins, may play a crucial role in maintaining intracellular zinc homeostasis in specific cell types. Support or Funding Information This work is funded by NIH R15 NS101594.
Recently, we reported that TMEM163 is a zinc efflux transporter that likely belongs to the mammalian solute carrier 30 (Slc30/ZnT) subfamily of the cation diffusion facilitator (CDF) protein superfamily. We hypothesized that human TMEM163 forms functional heterodimers with ZNT proteins based on their subcellular localization overlapping with TMEM163 and previous reports that certain ZNT monomers interact with each other. In this study, we heterologously expressed individual constructs with a unique peptide tag containing TMEM163, ZNT1, ZNT2, ZNT3, and ZNT4 (negative control) or co-expressed TMEM163 with each ZNT in HEK-293 cells for co-immunoprecipitation (co-IP) experiments. We also co-expressed TMEM163 with two different peptide tags as a positive co-IP control. Western blot analyses revealed that TMEM163 dimerizes with itself but that it also heterodimerizes with ZNT1, ZNT2, ZNT3, and ZNT4 proteins. Native co-IP using mouse tissues confirmed the interactions while confocal microscopy revealed that TMEM163 and ZNT proteins partially co-localize in cells, suggesting that they exist as homodimers and heterodimers in their respective subcellular sites. Functional zinc flux assays using Fluozin-3 and Newport Green dyes show that cells expressing TMEM163 homodimers extruded zinc slightly less efficiently than cells expressing TMEM163/ZNT heterodimers. Cell surface biotinylation revealed a subtle change in the plasma membrane localization of TMEM163 upon co-expression with certain ZNT proteins, which possibly explains why zinc efflux is marginally different for TMEM163 homodimers than TMEM163/ZNT heterodimers. Overall, our results show that the interaction between TMEM163 and distinct ZNT proteins is functionally relevant and that their heterodimerization may serve to influence their zinc efflux activity within specific tissues or cell types.
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