Reassessment of the Transport Mechanism of the Human Zinc Transporter SLC39A2

Abstract

The human zinc transporter SLC39A2, also known as ZIP2, was shown to mediate zinc transport that could be inhibited at pH <7.0 and stimulated by HCO, suggesting a Zn/HCO cotransport mechanism [Gaither, L. A., and Eide, D. J. (2000) J. Biol. Chem. 275, 5560-5564]. In contrast, recent experiments in our laboratory indicated that the functional activity of ZIP2 increases at acidic pH [Franz, M. C., et al. (2014) J. Biomol. Screening 19, 909-916]. The study presented here was therefore designed to reexamine the findings about the pH dependence and to extend the functional characterization of ZIP2. Our current results show that ZIP2-mediated transport is modulated by extracellular pH but independent of the H driving force. Also, in our experiments, ZIP2-mediated transport is not modulated by extracellular HCO. Moreover, a high extracellular [K], which induces depolarization, inhibited ZIP2-mediated transport, indicating that the transport mechanism is voltage-dependent. We also show that ZIP2 mediates the uptake of Cd ( K ∼ 1.57 μM) in a pH-dependent manner ( K ∼ 66 nM). Cd transport is inhibited by extracellular [Zn] (IC ∼ 0.32 μM), [Cu] (IC ∼ 1.81 μM), and to a lesser extent [Co], but not by [Mn] or [Ba]. Fe is not transported by ZIP2. Accordingly, the substrate selectivity of ZIP2 decreases in the following order: Zn > Cd ≥ Cu > Co. Altogether, we propose that ZIP2 is a facilitated divalent metal ion transporter that can be modulated by extracellular pH and membrane potential. Given that ZIP2 expression has been reported in acidic environments [Desouki, M. M., et al. (2007) Mol. Cancer 6, 37; Inoue, Y., et al. (2014) J. Biol. Chem. 289, 21451-21462; Tao, Y. T., et al. (2013) Mol. Biol. Rep. 40, 4979-4984], we suggest that the herein described H-mediated regulatory mechanism might be important for determining the velocity and direction of the transport process.

Unraveling the structural elements of pH sensitivity and substrate binding in the human zinc transporter SLC39A2 (ZIP2)

“…A series of titratable residues was also identified on TMH 2 (His-63, Glu-67, Glu-70, and Glu-71; Fig. 1 A ), which we suspected might play a role in substrate recruitment to the binding site, or pH sensitivity of transport previously described by our group (14). Additionally, two acidic residues (Glu-106 and Glu-120) were identified, which extend toward the membrane bilayer (Fig.…”

“…In contrast, prior work conducted in our laboratory suggested that ZIP2 activity is stimulated rather than inhibited by the presence of H + (13). In a more recent study, we showed that ZIP2-mediated transport in both microinjected Xenopus laevis oocytes and transiently transfected HEK293 cells is independent of both HCO 3 − - and H + -driving forces, but modulated by extracellular pH and voltage (14). However, structural details of how H + affects transport activity are still missing and remain to be described.…”

Water molecules mediate zinc mobility in the bacterial zinc diffusion channel ZIPB

“…Accordingly, zinc release in the transport cycle of YiiP is driven by conformational changes that alternatively expose the transport site to either side of the membrane surfaces (7). While mammalian ZIP transporters are thought to be operated by a similar alternating access mechanism (5,(24)(25)(26), our data from the bacterial ZIPB support a model in which water dynamics complement metal coordination chemistry to confer mobility to trapped zinc ions, highlighting the functional importance of solvated water in driving zinc transport through a sequence of high-affinity binding sites. Zinc is a defining feature of eukaryotic proteomes that are encoded by ~10% of genomes (27).…”

The Znt7-null mutation has sex dependent effects on the gut microbiota and goblet cell population in the mouse colon

“…Only Zip2 and Zip4 were significantly affected by the Znt7 knockout. Little is known about the function and expression profile of Zip2 [24]. Therefore, the significance of the differential expression of this gene in female Znt7 -/mice is unknown.…”