A Novel Zinc-regulated Human Zinc Transporter, hZTL1, Is Localized to the Enterocyte Apical Membrane

Abstract

Zinc is essential to a wide range of cellular processes; therefore, it is important to elucidate the molecular mechanisms of zinc homeostasis. To date, no zinc transporters expressed at the enterocyte apical membrane, and so essential to mammalian zinc homeostasis, have been discovered. We identified hZTL1 as a human expressed sequence tag with homology to the basolateral enterocyte zinc transporter ZnT1 and deduced the fulllength cDNA sequence by PCR. The protein of 523 amino acids belongs to the cation diffusion facilitator family of membrane transporters. Unusually, the predicted topology comprises 12 rather than 6 transmembrane domains. ZTL1 mRNA was detected by reverse transcription-PCR in a range of mouse tissues. A Myc-tagged hZTL1 clone was expressed in transiently transfected polarized human intestinal Caco-2 cells at the apical membrane. Expression of hZTL1 mRNA in Caco-2 cells increased with zinc supplementation of the nutrient medium; however, in the placental cell line JAR hZTL1 appeared not to be regulated by zinc. Heterologous expression of hZTL1 in Xenopus laevis oocytes increased zinc uptake across the plasma membrane. The localization, regulatory properties, and function of hZTL1 indicate a role in regulating the absorption of dietary zinc across the apical enterocyte membrane.

Altered expression of two zinc transporters, SLC30A5 and SLC30A6, underlies a mammary gland disorder of reduced zinc secretion into milk

“…A 55 kDa protein was also the single dominant band in the normal BT. The band sizes detected in both mothers and control were consistent with previously reported sizes of 55 and 57 and 60 kDa found in a range of human cells and tissues (Cragg 2008;Cragg et al 2002;Kambe et al 2002). Identified bands of 55 kDa for the SLC30A6 protein corresponded with predicted size of the splice variant found in lymphoblasts where exon IV was missing 42 nucleotides (Fig.…”

Demonstration and Characterization of the Heterodimerization of ZnT5 and ZnT6 in the Early Secretory Pathway

“…Considering our present results showing that the cytosolic C-terminal tail is important for the functional interaction with hZnT6 and that the N-terminal half is not required for localization to the secretory compartments, ZnT5B (hZTL1) would fail to form heterodimers with hZnT6 and so the extra putative TMD in the C terminus would probably function as a localization signal to the plasma membrane. We have attempted to confirm this theory but have not been able to obtain ZnT5B (hZTL1) cDNA by reverse transcription-PCR from total RNA prepared from human cell lines, including the CaCo-2 cell line used to analyze ZnT5B (hZTL1) (38). The ZnT5B (hZTL1) gene was not found in any mRNA/cDNA data bases, which suggests that its expression is quite low and may increase only in response to some unknown stimulus in specific human cell types.…”

“…Human ZnT5 has one splice variant, ZnT5B (hZTL1), that has quite different characteristics from hZnT5 (38,39). ZnT5B (hZTL1) is localized to the plasma membrane and functions to enable zinc uptake from the extracellular environment -the opposite direction of zinc transport by hZnT5/hZnT6 heterodimers.…”

Molecular and ionic mimicry and the transport of toxic metals

“…It has been hypothesized that Cd 2+ mimics Zn 2+ and is transported into enterocytes via a Zn 2+ transporter, such as the human, zinc-regulated zinc transporter 1 (hZTL1). This transporter, which is present in the luminal plasma membrane of enterocytes, is responsible for the inward transport of Zn 2+ in these cells (Cragg et al, 2001). Given this, it is possible that Cd 2+ can mimic Zn 2+ at the site of hZTL1 to gain access to the intracellular compartment of enterocytes.…”

“…The results of these experiments would suggest that a Zn 2+ transporter most likely mediates the testicular uptake of Cd 2+ . A possible candidate for this transport is hZTL1 or a similar Zn 2+ transporter (Cragg et al, 2001). The expression of this hZTL1 has not been characterized in testis.…”