Structure-function analysis of HKE4, a member of the new LIV-1 subfamily of zinc transporters

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Abstract
The KE4 proteins are an emerging group of proteins with little known functional data. In the present study, we report the first characterization of the recombinant human KE4 protein in mammalian cells. The KE4 sequences are included in the subfamily of ZIP (Zrt-, Irt-like Proteins) zinc transporters, which we have termed LZT (LIV-1 subfamily of ZIP zinc Transporters). All these LZT sequences contain similarities to ZIP transporters, including the consensus sequence in transmembrane domain IV, which is essential for zinc transport. However, the new LZT subfamily can be separated from other ZIP transporters by the presence of a highly conserved potential metalloprotease motif (HEXPHEXGD) in transmembrane domain V. Here we report the location of HKE4 on intracellular membranes, including the endoplasmic reticulum, and its ability to increase the intracellular free zinc as measured with the zinc-specific fluorescent dye, Newport Green, in a time-, temperature- and concentration-dependent manner. This is in contrast with the zinc influx ability of another LZT protein, LIV-1, which was due to its plasma membrane location. Therefore we have added to the functionality of LZT proteins by reporting their ability to increase intracellular-free zinc, whether they are located on the plasma membrane or on intracellular membranes. This result, in combination with the crucial role that zinc plays in cell growth, emphasizes the importance of this new LZT subfamily, including the KE4 sequences, in the control of intracellular zinc homoeostasis, aberrations of which can lead to diseases such as cancer, immunological disorders and neurological dysfunction.