The Cation Selectivity of the ZIP Transporters

Zinc is a required micronutrient for cellular homeostasis and is essential for the structure and/or function of 100s of biological processes. Despite the central importance of zinc in physiology, the mechanism by which this transition metal is transported into cells is not well understood. The first human zinc importer was identified in 2000. Since that time, a new family of proteins, named ZIP, for Zrt-, Irt-like proteins, has been shown to be expressed in a tissue-specific manner in humans. There are a total of 14 members of this family, which can be further divided into four subfamilies based on sequence similarities: ZIPI, ZIPII, gufA and LIV-1. It has been shown that these proteins are expressed on the plasma membrane as well as on intracellular organelles and each function or are proposed to function to increase the cytosolic concentration of zinc. While the subcellular localization of most of the ZIP family of proteins has been elucidated, the mechanistic details of these proteins including the driving force of cation translocation, residues essential for transport and the molecular determinants which define the differing cation selectivity between the ZIP family of proteins have not been well resolved. The objective of this review is to describe the current understanding of cation transport, mediated by the mammalian family of ZIP proteins and to present some molecular determinants that may contribute to the differing substrate specificity for this family of proteins.