Zinc transporter ZIP6
plasma membrane
Structure-function analysis of LIV-1, the breast cancer-associated protein that belongs to a new subfamily of zinc transporters.
The LIV-1 gene has been previously associated with oestrogen-positive breast cancer and its metastatic spread to the regional lymph nodes. We have investigated the protein product of this gene as a marker for disease progression of breast cancer. The protein sequence contains a potential metalloprotease motif (HEX P H E XGD), which fits the consensus sequence for the catalytic zinc-binding site motif of the zincin metalloproteases. This motif has identified a new subfamily of ZIP (Zrt-, Irt-like proteins) zinc transporters, which we have termed LZT (LIV-1 subfamily of ZIP zinc transporters). Expression of recombinant LIV-1 in Chinese-hamster ovary cells confirmed the prediction that LZT proteins can act as zinc-influx transporters. Zinc is essential for growth and zinc transporters have an important role in maintaining intracellular zinc homoeostasis, aberrations of which could lead to diseases such as cancer. This is the first report of the expression of a recombinant human LZT protein in mammalian cells. Recombinant LIV-1 locates to the plasma membrane, concentrated in lamellipodiae, similar to membrane-type metalloproteases. Examination of LIV-1 tissue expression located it mainly to hormonally controlled tissues with widespread expression in the brain. Interestingly, the LIV-1 sequence contains a strong PEST site and other potential degradation motifs, which, combined with our evidence that recombinant LIV-1 associates with ubiquitin, may explain the low-level expression of LIV-1. Combining the crucial role that zinc plays in cell growth and the proven role of metalloproteases in metastasis presents an exciting indication of how LIV-1 plays a role in breast cancer progression.