Identification and characterisation of the molecular determinants of zinc transport is essential for a full understanding of the mechanisms of zinc homeostasis and zinc absorption. We report the sequence and localisation in polarised intestinal Caco-2 cells of a novel human zinc transporter, hZTL1, with homology to the zinc transporter ZnT1 (Palmiter & Findley, 1995). We demonstrate by heterologous expression that hZTL1 functions as a zinc transporter.

Rapid amplification of cDNA ends (RACE) was used to generate from human small intestinal cDNA the 5Ì portion of a 3Ì expressed sequence tag that was identified through homology to mouse ZnT1. The full-length cDNA includes an open reading frame coding for 523 amino acids sharing 34 % identity with mouse ZnT1 over a region spanning 122 amino acid residues. Topology analysis predicts 12 membrane-spanning domains with extracellular amino and carboxyl termini. HZTL1 expressed from a cDNA construct tagged at the C-terminus with the myc epitope was localised at the apical membrane of Caco-2 cells transfected after 14 days growth on polycarbonate filters. Efflux of injected ⁶⁵Zn²⁺ from Xenopus laevis oocytes expressing hZTL1 cRNA was greater than from water-injected controls (K_e after 100 min of -0.265 ± 0.028 compared with -0.171 ± 0.027 (mean ± S.E.M.), n = 8, P < 0.05 by Student’s unpaired t test). Efflux was maintained in the presence of 1 mM extracellular Zn²⁺, indicating an energy-dependent process. In addition, injection with hZTL1 cRNA increased the rate of 12 mM ⁶⁵Zn²⁺ uptake into oocytes in comparison with water-injected controls (73.70 ± 6.26 compared with 39.09 ± 4.82 pmol oocyte^-1 (4 h)^-1 (mean ± S.E.M.), n = 8, P < 0.001 by Student’s t test).

The data indicate that hZTL1 is a functional zinc transporter whose expression in Xenopus laevis oocytes increases both the efflux and uptake of tracer Zn²⁺. The data do not exclude the possibility that upregulation of endogenous zinc transport also contributes to the increased Zn²⁺ fluxes. Net flux of Zn²⁺ through hZTL1 may depend on prevailing transmembrane zinc gradients. The localisation and functional activity of hZTL1 indicate a role in moderating intestinal zinc absorption to maintain zinc homoeostasis.

This work was funded, in part, by BBSRC grant 13/D11912.