2.A.4 The Cation Diffusion Facilitator (CDF) Family The CDF family is a ubiquitous family, members of which are found in bacteria, archaea and eukaryotes. They transport heavy metals including cobalt, cadmium, zinc and possibly nickel and mercuric ions. Most members of the CDF family possess six putative transmembrane spanners, but MSC2 of S. cerevisiae (TC #2.A.4.4.1) and Znt5 and hZTL1 of H. sapiens exhibit 15 and 12 putative TMSs, respectively (Cragg et al., 2002). These proteins exhibit an unusual degree of sequence divergence and size variation (300-750 residues). Eukaryotic proteins exhibit differences in cell localization. Some catalyze heavy metal uptake from the cytoplasm into various intracellular eukaryotic organelles (ZnT2-7) while others (ZnT1) catalyze efflux from the cytoplasm across the plasma membrane into the extracellular medium. Thus, some are found in plasma membranes while others are in organellar membranes (Chao and Fu, 2004b; MacDiarmid et al., 2003). Prokaryotic and eukaryotic proteins cluster separately but probably function with the same polarity by similar mechanisms. These proteins are secondary carriers which utilize the pmf and function by H⁺ antiport (for metal efflux). One member, CzcD of Bacillus subtilis, has been shown to exchange the divalent cation (Zn²⁺ or Cd²⁺) for two monovalent cations (K⁺ and H⁺) in an electroneutral process energized by the transmembrane pH gradient (Guffanti et al., 2002). Another, ZitB of E. coli (TC #2.A.4.1.4), has been reconstituted in proteoliposomes and studied kinetically (Chao and Fu, 2004a). It appears to function by simple Me²⁺:H⁺ antiport with a 1:1 stoichiometry. At least two metal binding sites have been identified in the E. coli paralogue, YiiP (TC #2.A.4.1.5), and one plays a role in H⁺ binding as well (Chao and Fu, 2004b). The generalized transport reaction for CDF family members is: Me²⁺ (in) + H⁺ (out) ą K⁺ (out) Me²⁺ (out) + H⁺ (in) ą K⁺ (in).
References:
Anton, A., A. Weltrowski, C.J. Haney, S. Franke, G. Grass, C. Rensing, and D.H. Nies. (2004). Characteristics of zinc transport by two bacterial cation diffusion facilitators from Ralstonia metallidurans CH34 and Escherichia coli. J. Bacteriol. 186: 7499-7507.
Chao, Y. and D. Fu. (2004a). Kinetic study of the antiport mechanism of an Escherichia coli zinc transporter, ZitB. J. Biol. Chem. 279: 12043-12050.
Chao, Y. and D. Fu. (2004b). Thermodynamic studies of the mechanism of metal binding to the Escherichia coli zinc transporter YiiP. J. Biol. Chem. 279: 17173-17180.
Clemens, S., T. Bloss, C. Vess, D. Neumann, D.H. Nies, and U. zur Nieden. (2002). A transporter in the endoplasmic reticulum of Schizosaccharomyces pombe cells mediates zinc storage and differentially affects transition metal tolerance. J. Biol. Chem. 277: 18215-18221.
Cragg, R.A., G.R. Christie, S.R. Phillips, R.M. Russi, S. Kury, J.C. Mathers, P.M. Taylor, and D. Ford. (2002). A novel zinc-regulated human zinc transporter, hZTL1, is localized to the enterocyte apical membrane. J. Biol. Chem. 277: 22789-22797.
Guffanti, A.A., Y. Wei, S.V. Rood, and T.A. Krulwich. (2002). An antiport mechanism for a member of the cation diffusion facilitator family: divalent cations efflux in exchange for K⁺ and H⁺. Mol. Microbiol. 45: 145-153.
Kambe, T., H. Narita, Y. Yumaguchi-Iwa, J. Hirose, T. Amano, N. Sugiura, R. Sasaki, K. Mori. T. Iwanaga, and M. Nagano. Cloning and characterization of a novel mammalian J. Biol. Chem. 277: 19049-1955.
Lee, S.M., G. Grass, C.J. Haney, B. Fan, B.P. Rosen, A. Anton, D.H. Nies, and C. Rensing. (2002). Functional analysis of the Escherichia coli zinc transporter ZitB. FEMS Microbiol. Lett. 215: 273-278.
Li, L. and J. Kaplan. (2001). The yeast gene MSC2, a member of the cation diffusion facilitator family, affects the cellular distribution of zinc. J. Biol. Chem. 276: 5036-5043.
MacDiarmid, C.W., M.A. Milanick, and D.J. Eide. (2003). Induction of the ZRC1 metal tolerance gene in zinc-limited yeast confers resistance to zinc shock. J. Biol. Chem. 278: 15065-15072.
Nies, D.H. and S. Silver. (1995). Ion efflux systems involved in bacterial metal resistances. J. Industr. Microbiol. 14: 186-199.
Paulsen, I.T. and M.H. Saier, Jr. (1997). A novel family of ubiquitous heavy metal ion transport proteins. J. Membr. Biol. 156: 99-103.
Xiong, A. and R.K. Jayaswal. (1998). Molecular characterization of a chromosomal determinant conferring resistance to zinc and cobalt ions in Staphylococcus aureus. J. Bacteriol. 180: 4024-4029.
Examples:
TC#	Name	Organismal Type	Example
2.A.4.1.1	Cd²⁺, Zn²⁺, Co²⁺ efflux permease (Anton et al., 2004)	Bacteria	*CzcD of Ralstonia metallidurans* (Alcaligenes eutrophus previously)**

2.A.4.1.2	Zn²⁺, Co²⁺ efflux permease	Bacteria	ZntA of Staphylococcus aureus

2.A.4.1.3	Cd²⁺ or Zn²⁺:H⁺ + K⁺ antiporter, CzcD	Bacteria	CzcD of Bacillus subtlis

2.A.4.1.4	Zn²⁺ (Km=105 μM), Cd²⁺ (Km=90 μM): proton (Km=20 nM) antiport metal ion efflux permease, ZitB (Chao and Fu, 2004a)	Bacteria	ZitB of E. coli

2.A.4.1.5	Zn²⁺/Cd²⁺/Hg²⁺:H⁺ antiporter (Chao and Fu, 2004b)	Bacteria	*YiiP of E. coli* (P32159)**

2.A.4.2.1	Mitochondrial Co²⁺/Zn²⁺ uptake permease	Yeast	Cotl of Saccharomyces cerevisiae

2.A.4.2.2	Vacuolar Zn²⁺, Cd²⁺ uptake permease (Zn²⁺/Cd²⁺:H⁺ antiporter)	Yeast	Zrclp (ZnrP) of Saccharomyces cerevisiae

2.A.4.2.3	Plasma membrane Zn²⁺ efflux permease	Animals	Znt1 of Rattus norvegicus

2.A.4.3.1	Vesicular Zn²⁺ uptake permease	Animals	Znt2 of Rattus norvegicus

2.A.4.4.1	Nuclear/ER Zn²⁺ uptake permease	Yeast	MSC2 of Saccharomyces cerevisiae

2.A.4.4.2	Apical enterocyte Zn²⁺ uptake permease (probable splice variant of Znt5 (2.A.4.4.3)).	Animals	hZTL1 of Homo sapiens

2.A.4.4.3	Golgi/secretory granule Zn²⁺ uptake permease	Animals	Znt5 of Homo sapiens