Abstract
BackgroundThe Cation Diffusion Facilitator (CDF) family is a ubiquitous family of heavy metal transporters. Much interest in this family has focused on implications for human health and bioremediation. In this work a broad phylogenetic study has been undertaken which, considered in the context of the functional characteristics of some fully characterised CDF transporters, has aimed at identifying molecular determinants of substrate selectivity and at suggesting metal specificity for newly identified CDF transporters.ResultsRepresentative CDF members from all three kingdoms of life (Archaea, Eubacteria, Eukaryotes) were retrieved from genomic databases. Protein sequence alignment has allowed detection of a modified signature that can be used to identify new hypothetical CDF members. Phylogenetic reconstruction has classified the majority of CDF family members into three groups, each containing characterised members that share the same specificity towards the principally-transported metal, i.e. Zn, Fe/Zn or Mn. The metal selectivity of newly identified CDF transporters can be inferred by their position in one of these groups. The function of some conserved amino acids was assessed by site-directed mutagenesis in the poplar Zn2+ transporter PtdMTP1 and compared with similar experiments performed in prokaryotic members. An essential structural role can be assigned to a widely conserved glycine residue, while aspartate and histidine residues, highly conserved in putative transmembrane domains, might be involved in metal transport. The potential role of group-conserved amino acid residues in metal specificity is discussed.ConclusionIn the present study phylogenetic and functional analyses have allowed the identification of three major substrate-specific CDF groups. The metal selectivity of newly identified CDF transporters can be inferred by their position in one of these groups. The modified signature sequence proposed in this work can be used to identify new hypothetical CDF members.
Highlights
The Cation Diffusion Facilitator (CDF) family is a ubiquitous family of heavy metal transporters
CDF members displayed the highest conservation in trans-membrane regions, especially in Transmembrane domain (TMD) II, where the following signature was previously defined for this family [15]
Since 1997, new CDF transporters have been characterised or identified by similarity with known CDFs, and the number of recognised CDF family members has dramatically expanded, in large part due to data coming from automated annotated genomic sequences; the first signature was derived from only 13 CDF members and appears too restrictive to allow the recognition of the newly identified CDF members
Summary
The Cation Diffusion Facilitator (CDF) family is a ubiquitous family of heavy metal transporters. Much interest in this family has focused on implications for human health and bioremediation. Cation diffusion facilitator (CDF, TC 2.A.4) transporters, first identified by Nies and Silver [5], are ubiquitous, spanning all three kingdoms of life: Archaea, Eubacteria and Eukaryotes. The majority of CDF proteins possess six putative transmembrane domains (TMDs), with cytoplasmic N and C termini, as experimentally demonstrated for bacterial members [10,14]. Plant CDF members are usually called Metal Tolerance Proteins (MTPs), while vertebrate members are named Zinc Transporter (ZnT) or Solute carrier family 30 (SLC30)
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