1.A.43 The Camphor Resistance or Fluoride Channel (Fluc) Family
CrcB of E. coli appears to confer camphor resistance when overexpressed and induces resA expression (Sand et al. 2003). Deletion is not lethal but increases sensitivity to camphor and fluoride. CrcB has 127 aas and 4 TMSs. Its many close homologues are small proteins, usually with 4 TMSs. These proteins mediate fluoride efflux and resistance (Ji et al. 2014). The active transporter is a dimer of 4 TMS subunits arranged antiparallel (Stockbridge et al. 2014).
Fluc family fluoride channels are assembled as primitive antiparallel homodimers (McIlwain et al. 2020). Crystallographic studies revealed a cation bound at the center of the protein, where it is coordinated at the dimer interface by mainchain carbonyl oxygens from the mid-membrane breaks in two corresponding TMSs. This cation is a stably bound sodium ion, and, although it is not a transported substrate, its presence is required for the channel to adopt an open, fluoride conducting conformation. The interfacial site is selective for sodium over other cations, except for Li+, which competes with Na+ for binding, but does not support channel activity. Fluoride, F-, ubiquitous in soil, water and marine environments, is a chronic threat to microorganisms. Many bacteria, archaea, unicellular eukaryotes and plants use F- exporters to lower cytoplasmic F- levels to counteract the anion's toxicity (McIlwain et al. 2020).
Stockbridge et al. 2013 and Li et al. 2013 showed that these ''Fluc'' proteins, purified and reconstituted in liposomes and planar phospholipid bilayers, form constitutively open anion channels with extreme selectivity for F- over Cl-. The active channel is a dimer of identical or homologous subunits arranged in antiparallel transmembrane orientation, or a single protein with two internal 4TMS repeats (as for the yeast proteins (TC# 1.A.43.2.4 and 5). This dual-topological assembly had not previously been seen in ion channels but is known in multidrug transporters of the SMR sub-family of the DMT superfamily (TC# 2.A.7.1) (Pornillos et al. 2005; Pornillos and Chang 2006).
Crystal structures showed that Fluc channels contain two separate ion-conduction pathways, each with two F- binding sites (Stockbridge et al. 2015). Last et al. 2016 examined the consequences of mutating two conserved F--coordinating phenylalanine residues. Substitution of each phenylalanine specifically extinguished its associated F- binding site in crystal structures and concomitantly inhibited F- permeation. Functional analysis of concatemeric channels, which permit mutagenic manipulation of individual pores, showed that each pore can be separately inactivated without blocking F- conduction through its symmetry-related twin. The results strongly supported a dual-pathway architecture of these channels (Last et al. 2016).
The camphor resistance protein, CrcB or FluC (Hu et al. 1996; Sand et al. 2003). Exports fluoride selectively over chloride by an anion open channel mechanism (Stockbridge et al. 2013). The active transporter is a dimer of 4 TMS subunits arranged in an antiparallel transmembrane orientation (Stockbridge et al. 2014). In bacteria lacking Fluc, F- accumulates when the external medium is acidified as a predicted function of the transmembrane pH gradient. This weak acid accumulation effect, which results from the high pKa (3.4) and membrane permeability of HF, is abolished by Fluc channels (Ji et al. 2014). A proper tubulin network is required for functional Cx43 GJ channels, and mefloquineis a gap junction inhibitor (Picoli et al. 2019).
CrcB of E. coli (P37002)
CrcB-like protein of 164 aas and 4 TMSs
CreB of Mobiluncus curtisii (Falcivibrio vaginalis)
Putative fluoride transporter of 122 aas, CrcB
CrcB of Campylobacter jejuni
CreB of 168 aas and 4 TMSs
CreB of Brachybacterium faecium
CreB of 123 aas and 4 TMSs.
CreB of Aequorivita sublithincola
CrcB, putative fluoride channel protein of 124 aas and 4 TMSs
CrcB of Lactobacillus kefiranofaciens
CrcB of 133 aas and 4 TMSs.
CrcB of Halorubrum coriense
Fluc homologue of 453 aas and 9 putative TMSs.
Fluc of Acanthamoeba castellanii
Fluoride ion channel of 128 aas and 4 TMSs, Fluc or CrcB. The crystal structure is known (PDB5A40; 5A43).
Fluc of Bordetella pertussis
Protein CrcB homologue 2
crcB2 of Bacillus subtilis
Putative fluoride-selective channel of 143 aas and 4 TMSs, CrcB.
CreB of Propionibacterium acnes
CreB homologue of 124 aas
CrcB homologue of Methanocaldococcus fervens
CrcB homologue of 172 aas and 4 TMSs
CrcB of Parvularcula bermudensis
Putative fluoride exporter, CrcB.
CrcB of Pelodictyon luteolum (Chlorobium luteolum)
Putative fluoride exporter, CrcB if 114 aas and 4 TMSs.
CrcB of Thermococcus barophilus
Uncharacterized protein of 151 aas and 4 TMSs.
UP of Rothia mucilaginosa (Stomatococcus mucilaginosus)
Putative fluoride exporter, CrcB of 113 aas and 4 TMSs.
UP of Haloquadratum walsbyi
CrcB-like protein of 307 aas and 6 TMSs in an apparent 3 + 3 arrangement.
CrcB homologue of Tetrahymena thermophila
Uncharacterized protein of 372 aas and 9 - 10 TMSs
UP of Kazachstania africana (Kluyveromyces africanus)
CrcB domain containing protein of 310 aas and 9 TMSs in a 4 + 5 arrangement, with both halves showing sequence similarity with the 4 TMS CrcB of E. coli.
CrcB homologue of Schizosaccharomyces cryophilus
Plasma membrane fluoride ( > chloride) export channel of 375 aas and 8 TMSs, FEX1 (Li et al. 2013). The two homologous 4 TMS domains are functionally assymetric (Smith et al. 2015). There are two very similar fex genes in S. cerevisiae, the other having TC# 1.A.43.2.5. Fex1 is consitutively synthesized (Smith et al. 2015).
FEX1 of Saccharomyces cerevisiae
Fluoride exporter, FEX2 of 375 aas and 8 TMSs (Li et al. 2013).
FEX2 of Saccharomyces cerevisiae
Fluoride exporter, FEX, of 526 aas and 10 putative TMSs (Li et al. 2013).
FEX of Neurospora crassa
Camphor resistance CrcB protein of 461 aas and 9 putative TMSs.
CrcB of Arabidopsis thaliana
Uncharacterized protein of 405 aas and 8 putative TMSs.
UP of Ciona intestinalis (Transparent sea squirt) (Ascidia intestinalis)
Uncharacterized protein of 460 aas and 9 TMSs/
UP of Phytophthora parasitica
CreB of 346 aas and 4 TMSs.
CreB of Bifidobacterium longum
Putative fluoride channel, CrcB, of 180 aas and 4 TMSs.
CrcB of Scardovia wiggsiae
Putative fluoride ion channnel, CrcB, of 178 aas and 4 TMSs
CrcB of Bifidobacterium longum
Putative fluoride ion channel, CrcB, with 310 aas and 4 TMSs.
CrcB of Bifidobacterium animalis subsp. lactis