The size and charge characteristics of zinc mandate the use of carriers to traverse biological membranes. Two families of transporters have been described and partially characterized. The ZIP family (ZRT (zinc-regulated transporter)- and IRT (iron-regulated transporter)-related proteins, named after homologous transport proteins in yeast and plants) appears to move zinc into the cytoplasm of the cell, either from outside the cell or from subcellular compartments. The second group of transporters, the CDF (cation diffusion factor) family, is responsible for zinc egress from the cytoplasm. This latter family includes ZnT-1, which has been localized to plasma membranes and functions as a cellular efflux protein, and ZnT-2, which transports zinc into storage vesicles under conditions of high cellular zinc. Collectively, the ZIP and CDF proteins are likely to underlie the homeostatic control of zinc distribution around the body.
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