Chapter Nine - The Families of Zinc (SLC30 and SLC39) and Copper (SLC31) Transporters

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Abstract

The solute carriers families 30 (SLC30; ZnT), 39 (SLC39; ZIP), and 31 (SLC31; CTR) are involved in the essential maintenance of cellular zinc (Zn2 +) and copper (Cu2 +) homeostasis, respectively.

ZnTs mediate Zn2 + extrusion from cells (SLC30A1) or transport Zn2 + into organelles and secretory vesicles/granules (SLC30A2–SLC30A8). SLC39 family members are predominantly localized to the cell membrane where they perform Zn2 + uptake and increase the availability of cytosolic Zn2 +. SLC39A1 is ubiquitously expressed, whereas other ZIP transporters (e.g., SLC39A2 and SLC39A3) show a more tissue-restricted expression consistent with organ-specific functions of these proteins.

The members A1 (CTR1) and A2 (CTR2) of the SLC31 family of solute carriers belong to a network of proteins that acts to regulate the intracellular Cu2 + concentration within a certain range. SLC31A1 is predominantly localized to the plasma membrane, whereas SLC31A2 is mainly found in intracellular membranes of the late endosome and lysosome. The specific function of SLC31A2 is not known. SLC31A1 is ubiquitously expressed and has been characterized as a high-affinity importer of reduced copper (Cu+). Cu2 + transport function of CTR proteins is associated with oligomerization; SLC31A1 trimerizes and thereby forms a channel-like structure enabling Cu2 + translocation across the cell membrane.

The molecular characteristics and structural details (e.g., membrane topology, conserved Zn2 +, and Cu2 + binding sites) and mechanisms of translational and posttranslational regulation of expression and/or activity have been described for SLC30 and SLC39 family members, and for SLC31A1.

For SLC31A1, data on tissue-specific functions (e.g., in the intestine, heart, and liver) are also available. A link between SLC31A1, immune function, and disorders such as Alzheimer's disease or cancer makes the protein a candidate therapeutic target.

In secretory tissues (e.g., the mammary gland and pancreas), Zn2 + transporters of SLC families 30 and 39 are involved in specific functions such as insulin synthesis and secretion, metallation of digestive proenzymes, and transfer of nutrients into milk. Defective or dysregulated Zn2 + metabolism in these organs is associated with disorders such as diabetes and cancer, and impaired Zn2 + secretion into milk.

Introduction

Zinc (Zn2 +) and copper (Cu2 +) are cofactors for a multitude of enzymes (Festa and Thiele, 2011, Leary et al., 2004, Lutsenko et al., 2007, McCall et al., 2000, Vallee and Auld, 1990). Zn2 + is also a structural component of metallothioneins (MTs) and insulin and serves a specific structural role by stabilizing the conformation of various protein domains found in gene regulatory proteins (Brown et al., 1985, Kadhim et al., 2006, Vallee and Falchuk, 1993). Therefore, Zn2 + and Cu2 + play an essential role in important biological processes such as growth, development, immune and neuronal functions, reproduction, and metabolism (Beach et al., 1980, Danks, 1988, Festa and Thiele, 2011, Lutsenko et al., 2007, Pena et al., 1999, Prasad et al., 1961, Todd et al., 1934, Vallee and Falchuk, 1993, Vulpe and Packman, 1995).

Although Zn2 + and Cu2 + are essential, excess of these trace elements can be toxic to cells (Koh et al., 1996, Lutsenko et al., 2007). For example, excess Cu2 + leads to the generation of reactive oxygen species (ROS) and is highly toxic to cells because of its redox-active property (Lutsenko et al., 2007).

These examples clearly illustrate that intracellular Zn2 + and Cu2 + homeostasis needs to be tightly controlled by influx, intracellular storage/sequestration, and efflux of the ions. Zn2 + and Cu2 + transport proteins encoded by three solute-linked carrier (SLC) gene families named SLC30 (zinc transporter; ZnT), SLC39 (Zrt- and Irt-like Protein; ZIP), and SLC31 (CTR) are involved in this process.

SLC30 and SLC39 Zn2 + transport proteins have opposite roles in the regulation of Zn2 + homeostasis. ZnT transporters reduce cytosolic Zn2 + availability by mediating either Zn2 + efflux from cells or Zn2 + uptake into intracellular vesicles. On the other hand, ZIP transporters promote uptake of extracellular Zn2 + and release of vesicular Zn2 + into the cytosol. Intracellular Zn2 + is bound by small cysteine-rich proteins named MTs (Coyle, Philcox, Carey, & Rofe, 2002).

Cellular Cu2 + homeostasis is regulated by a complex network of proteins that mediates Cu2 + uptake across the plasma membrane, for example, by SLC31 proteins, and its subcellular and systemic distribution, for example, by cytosolic Cu2 + chaperons such as ATOX1 and the Cu2 +-transporting P-type ATPases, ATP7A and ATP7B (Culotta et al., 2006, Kim et al., 2008, Lutsenko et al., 2007, Robinson and Winge, 2010). SLC31A1 (CTR1) is the main transporter for cellular uptake of monovalent copper (Cu+), whereas SLC31A2 (CTR2) appears to be a vacuolar/vesicular transporter—most probably responsible for the recycling of Cu2 + from intracellular stores (Bertinato et al., 2008, Rees et al., 2004, Zhou and Gitschier, 1997).

This chapter is a brief review of the structural and cell biological aspects of SLC30-, SLC39-, and SLC31-mediated Zn2 +/Cu2 + transport, as well as the function of these proteins in physiological and pathophysiological processes.

Section snippets

Discovery of Zinc Transporters

SLC30 and SLC39 together constitute a superfamily of Zn2 + transporters. The first mammalian SLC30 genes, SLC30A1 (ZnT1) and SLC30A2 (ZnT2), were identified by transfection of a rat kidney cDNA expression library into a Zn2 +-sensitive baby hamster kidney cell line followed by the selection of clones showing resistance to high extracellular Zn2 + concentrations (Palmiter et al., 1996, Palmiter and Findley, 1995). SLC30A3 (ZnT3) was cloned by hybridizing a mouse genomic library to a rat SLC30A2

The SLC30 (ZnT) Family

At present, 10 members of the SLC30 (ZnT) family have been identified that are responsible for Zn2 + transport into organelles or out of cells (Palmiter & Findley, 1995). The transport mechanism is not clear. Ohana et al. (2009) have shown that the proton gradient generated by V-type ATPases drives Zn2 + transport into the Golgi apparatus (GA). This finding is in agreement with vH+-ATPase-dependent Zn2 +/H+ exchange observed in bacteria, yeast, and plants (Guffanti et al., 2002, Kawachi et al.,

The SLC39 (ZIP) Family

Proteins belonging to the SLC39 family are involved in Zn2 + uptake and localize mainly to the plasma membrane (Dufner-Beattie et al., 2003, Gaither and Eide, 2000). Consistent with the ability of ZIPs to promote Zn2 + uptake, overexpression of human and mice ZIP1, ZIP2, and ZIP3 in various cell systems (e.g., HEK293, K562, and prostate cells) increased 65Zn2 + accumulation (~ 2- to 4-fold) (Dufner-Beattie et al., 2003, Gaither and Eide, 2000, Wang et al., 2004). The specific mechanism for this

The Role of SLC31A1 (CTR1) in Cu2 + Uptake and Body Cu2 + Homeostasis

SLC31A1 (CTR1) has been characterized as a high-affinity transporter specific for reduced Cu+. Copper accumulates in cells overexpressing human CTR1 (Lee et al., 2000, Zhou and Gitschier, 1997).

Structure and Mode of Action of SLC31 Family Members

The need for copper in important cell functions such as growth, development, and metabolism (Kim et al., 2008) is highly conserved across all phylogenetic levels. As such, the yeast Saccharomyces cerevisiae has served as a crucial model for identifying Cu2 + transporters. The human CTR1 cDNA was isolated based on its capacity to rescue Cu2 + transport in yeast cells with defective Cu2 + influx systems (Zhou & Gitschier, 1997). Human genes Slc31a1 (with five exons of 4744 bps) and Slc31a2 (with four

Regulation of Copper Acquisition by SLC31A1

Strict regulation of uptake, distribution, and excretion of Cu2 + is necessary for maintaining optimal activities of copper-dependent cellular pathways.

The SLC31A2 (CTR2) Copper Transporter

CTR2 is the second member of the mammalian SLC31 family of Cu2 + transporters and shows structural homology to the second transmembrane domain of CTR1 that is essential for Cu2 + transport (Zhou & Gitschier, 1997). Its overall organization mirrors that of CTR1 with three putative TMs and an intramembranous MX3M motif, but without an extended N-terminal domain. According to biochemical studies, CTR2 functions as an oligomeric protein, but structural details are currently lacking. Highest levels of

Cancer

CTR1 is an important factor in determining toxicity of platinum-based anticancer drugs such as cisplatin (Howell et al., 2010, Ishida et al., 2002, Kuo et al., 2007). Cisplatin and related drugs appear to bind with high affinity to Cu2 +-binding residues and domains of CTR1 and other molecules involved in Cu2 + metabolism (Crider et al., 2010, Guo et al., 2004). Compared with wild-type cells, mice embryonic fibroblasts with CTR1-knockout display only 30–35% of cisplatin transport activity (

Conclusion

The SLC30/39 families of Zn2 + transporters and the SLC31 family of Cu2 + transporters are important components of a network of proteins that balance the homeostasis of these essential trace elements within a narrow range between toxicity and deficiency.

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