Cation diffusion facilitator

The Cation Diffusion Facilitator (CDF) Family

The CDF family (TC# 2.A.4) 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, copper and mercuric ions. There are 9 mammalian paralogues, ZnT1 - 8 and 10. Most members of the CDF family possess six putative transmembrane spanners with N- and C-termini on the cytoplasmic side of the membrane, but MSC2 of S. cerevisiae (TC# 2.A.4.4.1) and Znt5 and hZTL1 (TC# 2.A.4.4.3) of H. sapiens exhibit 15 and 12 putative TMSs, respectively. 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 such as vacuoles of plants and yeast and the golgi of animals. They catalyze cation:proton antiport, have a single essential zinc-binding site within the transmembrane domains of each monomer within the dimer, and have a binuclear zinc-sensing and binding site in the cytoplamsic C-terminal region. A representative list of proteins belonging to the CDF family can be found in the Transporter Classification Database.

Phylogeny

Prokaryotic and eukaryotic proteins cluster separately but may function with the same polarity by similar mechanisms. These proteins are secondary carriers which utilize the proton motive force (pmf) and function by H⁺ antiport (for metal efflux). One member, CzcD of Bacillus subtilis (TC# 2.A.4.1.3) , 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. Another, ZitB of E. coli (TC #2.A.4.1.4), has been reconstituted in proteoliposomes and studied kinetically. It appears to function by simple Me²⁺:H⁺ antiport with a 1:1 stoichiometry.

Montanini et al. (2007) have conducted phylogenetic analysis of CDF family members. Their analysis revealed three major and two minor phylogenetic groups. They suggest that the three major groups segregated according to metal ion specificity:

1. Mn²⁺
2. Fe²⁺ and Zn²⁺ as well as other metal ions
3. Zn²⁺ plus other metals, but not Iron.

Structure

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. The two Zn²⁺/Cd²⁺ binding sites consist of two interacting conserved aspartyl residues (Asp-157 and Asp-49), both in 2 fold symmetry-related TMS 5 and TMS 2, respectively, at the dimer interface of the homodimer. The (Asp-49 and Asp-157) may form a bimetal binding center. Two bound Cd²⁺ were transported cooperatively with sigmoidal dependency on the Cd²⁺ concentration. A translocation pathway for metal ions at the dimer interface has been proposed. CDF family members may generally be homodimeric.

Lu and Fu (2007) have reported the x-ray structure of YiiP of E. coli (TC# 2.A.4.7.1) in complex with zinc at 3.8 angstrom resolution. YiiP is a homodimer held together in a parallel orientation through four Zn²⁺ ions at the interface of the cytoplasmic domains. The two transmembrane domains swing out to yield a Y-shaped structure. In each protomer, the cytoplasmic domain adopts a metallochaperone-like protein fold. The transmembrane domain features a bundle of six transmembrane helices and a tetrahedral Zn²⁺ binding site located in a cavity that is open to both the membrane outer leaflet and the periplasm.

Coudray et al. (2013) used cryoelectron microscopy to determine a 13 Å resolution structure of a YiiP homolog from Shewanella oneidensis within a lipid bilayer in the absence of Zn²⁺. Starting from the x-ray structure in the presence of Zn²⁺, they used molecular dynamic flexible fitting to build a model. Comparison of the structures suggested a conformational change that involves pivoting of a transmembrane, four-helix bundle (M1, M2, M4, and M5) relative to the M3-M6 helix pair. Although accessibility of transport sites in the x-ray model indicates that it represents an outward-facing state, their model was consistent with an inward-facing state, suggesting that the conformational change is relevant to the alternating access mechanism for transport. They speculated that the dimer may coordinate rearrangement of the transmembrane helices.

Involved in metal tolerance/resistance by efflux, most CDF proteins share a two-modular architecture consisting of a transmembrane domain (TMD) and a C-terminal domain (CTD) that protrudes into the cytoplasm. A Zn²⁺ and Cd²⁺ CDF transporter from the marine bacterium, Maricaulis maris, that does not possess the CTD is a member of a new, CTD-lacking subfamily of CDFs.

Transport Reaction

The generalized transport reaction for CDF family members is:

Me²⁺ (in) H⁺ (out) ± K⁺ (out) → Me²⁺ (out) H⁺ (in) ± K⁺ (in).