Fig 1.
Structural arrangement of fluoride export proteins.
(A) Topology model of bacterial Fluc protein. (B) Tertiary structure of Fluc-Bp (PDB ID: 5FXB) showing two pores with fluoride (dark grey) ions. Sodium ion (purple) is in the middle of the protein dimer. Two monomers are colored brown and light grey. (C) Arrangement of transmembrane helices (top view). Helices TM3b from different monomers (yellow and green) separate two pores and have residues belonging to two different pores but located on the same helix. (D) Topology model of eukaryotic FEX protein with proposed interactions between TM3 and TM8 based on Fluc crystal structure.
Fig 2.
Alignment of protein sequences from six eukaryotic and two bacterial fluoride channels.
Transmembrane regions were predicted using TMHMM and TMpred servers. Conserved residues for all eukaryotic FEX proteins are highlighted with asterisks. Amino acid numbering corresponds to the protein sequence of Fex1p from S. cerevisiae. The residues highlighted in green are present within both pores of Fluc.
Fig 3.
Model of Fex1p from S. cerevisiae.
Comparison of the residues in two pores for Fex1p-Sc (red residues are in N-terminal domain and blue are in C-terminal domain) and Fluc-Bp (white). Residues in Pore II and not Pore I of FEX are similar to bacterial Fluc.
Fig 4.
Functional analysis of conserved fragments in FEX.
A, mutations to pore-lining Phe located in TM3 and TM8. B, mutations to PxGTxxxN motif located in TM7. C, mutations to YxxxS fragment located in TM9. The fex1Δfex2Δ strain was transformed with wild-type pRS416-FEX1 (WT), empty vector pRS416 (No FEX) or Fex1p mutants in pRS416-FEX1. Serial dilutions of yeast cultures expressing the indicated mutants were incubated on YPD media with different concentrations of NaF.
Table 1.
Effect of mutations to residues in the hypothetical pores of Fex1p.
Fig 5.
Functional analysis of conserved S/T fragments in domain 1 (red) and domain 2 (blue).
Conserved Ser/Thr residues are located near putative Na+ binding site.
Table 2.
Effect of mutations at conserved residues in two domains of Fex1p.
Table 3.
Growth tolerance of FEX-like genes from multicellular organisms relative to empty p426GPD vector.
Fig 6.
Growth rescue of fex1Δfex2Δ yeast expressing previously uncharacterized FEX-like genes.
(A) The strains were grown on YPD and YPD containing 5 mM NaF to determine rescue phenotype. Plant FEX-At is a putative FEX gene from A. thaliana cloned into the p426GPD vector. Animal FEX-Aq is a putative FEX gene from A. queenslandica cloned into the p426GPD vector. Yeast rescue plasmid (pRS416-FEX1) was used as a positive control and empty vector (p426GPD) was used as a negative control. (B) Quantification of the growth tolerance to NaF of yeast strains described in A.
Fig 7.
The helices from both domains that form functional and vestigial pores.