Zip14 is a complex broad-scope metal-ion transporter whose functional properties support roles in the cellular uptake of zinc and nontransferrin-bound iron

摘要

Recent studies have shown that overexpression of the transmembrane protein Zrt- and Irt-like protein 14 (Zip14) stimulates the cellular uptake of zinc and nontransferrin-bound iron (NTBI). Here, we directly tested the hypothesis that Zip14 transports free zinc, iron, and other metal ions by using the Xenopus laevis oocyte heterologous expression system, and use of this approach also allowed us to characterize the functional properties of Zip14. Expression of mouse Zip14 in RNA-injected oocytes stimulated the uptake of ⁵⁵Fe in the presence of l-ascorbate but not nitrilotriacetic acid, indicating that Zip14 is an iron transporter specific for ferrous ion (Fe²⁺) over ferric ion (Fe³⁺). Zip14-mediated ⁵⁵Fe²⁺ uptake was saturable (K0.5 ≈ 2 μM), temperature-dependent (apparent activation energy, Ea = 15 kcal/mol), pH-sensitive, Ca²⁺-dependent, and inhibited by Co²⁺, Mn²⁺, and Zn²⁺. HCO3⁻ stimulated ⁵⁵Fe²⁺ transport. These properties are in close agreement with those of NTBI uptake in the perfused rat liver and in isolated hepatocytes reported in the literature. Zip14 also mediated the uptake of ¹⁰⁹Cd²⁺, ⁵⁴Mn²⁺, and ⁶⁵Zn²⁺ but not ⁶⁴Cu (I or II). ⁶⁵Zn²⁺ uptake also was saturable (K0.5 ≈ 2 μM) but, notably, the metal-ion inhibition profile and Ca²⁺ dependence of Zn²⁺ transport differed from those of Fe²⁺ transport, and we propose a model to account for these observations. Our data reveal that Zip14 is a complex, broad-scope metal-ion transporter. Whereas zinc appears to be a preferred substrate under normal conditions, we found that Zip14 is capable of mediating cellular uptake of NTBI characteristic of iron-overload conditions.