2.A.16 The Tellurite-resistance/Dicarboxylate Transporter (TDT) Family
The TDT family includes members from the bacterial (E. coli and Haemophilus influenzae), archaeal (Methanococcus jannaschii) and eukaryotic (Schizosaccharomyces pombe) kingdoms and therefore occurs ubiquitously. Only a few members of the family have been functionally characterized. One is the TehA protein of E. coli which functions as a tellurite-resistance uptake permease; the second is the Mae1 protein of S. pombe which functions in the uptake of malate and other dicarboxylates by a proton symport mechanism, the third is the sulfite efflux pump of Saccharomyces cerevisiae (Park and Bakalinsky, 2000). These proteins are 320-460 aas, but some are larger. The homologues contain an internal repeat. They exhibit 10 putative transmembrane α-helical spanners (TMSs). The phylogenetic tree for the TDT family exhibits three major branches, one for the bacterial proteins, one for the archaeal proteins and one for the yeast protein. (Saier et al., 1999).
Stomatal pores, formed by two surrounding guard cells in the epidermis of plant leaves, allow influx of atmospheric carbon dioxide in exchange for transpirational water loss. Stomata also restrict the entry of ozone - an important air pollutant that has an increasingly negative impact on crop yields, and thus global carbon fixation and climate change. The aperture of stomatal pores is regulated by the transport of osmotically active ions and metabolites across guard cell membranes.
Guard cell anion channels function as important regulators of stomatal closure and are essential in mediating stomatal responses to physiological and stress stimuli. Vahisalu et al. (2008) and Negi et al. (2008) have identified an ozone-sensitive Arabidopsis thaliana mutant, slac1. They found that SLAC1 (SLOW ANION CHANNEL-ASSOCIATED 1) is preferentially expressed in guard cells and encodes a distant homologue of the Tellurite-resistance/Dicarboxylate transporter family with closest resemblance to TehA (2.A.16.1.1; 20% identity, 38% similarity, e-11). The plasma membrane protein SLAC1 is essential for stomatal closure in response to CO2, abscisic acid, ozone, light/dark transitions, humidity change, calcium ions, hydrogen peroxide and nitric oxide. Mutations in SLAC1 impair slow (S-type) anion channel currents that are activated by cytosolic Ca2+ and abscisic acid, but do not affect rapid (R-type) anion channel currents or Ca2+ channel function.
The transport reaction catalyzed by the TehA protein of E. coli is:
Tellurite (out) + nH+ (out) → Tellurite (in) + nH+ (in).
The transport reaction catalyzed by the Mae1 protein of S. pombe is:
C4-Dicarboxylate (out) + nH+ (out) → C4-Dicarboxylate (in) + nH+ (in).
The transport reaction catalyzed by the Ssu1 protein is:
Sulfite (in)→ Sulfite (out)
Tellurite resistance protein, TehA. Encoded in an operon with the gene for a tellurite S-adenosylmethionine-dependent methyl transferase. Together they confer tellurite resistance (Moraes and Reithmeier 2012).
TehA of E. coli
Tellurite resistance protein TehA homologue. An anion channel involved in tellurite resistance. A quasi-symmetrical homotrimer in which each subunit has 10 TMSs and forms a channel. The crystal structure is known at 1.2 A resolution (Chen et al. 2010). The helices are arranged from helical hairpin pairs to form a central 5-helix transmembrane pore that is gated by a conserved phenylalanine residue. Gating is controlled by kinase activation. Selectivity for various anions may be a function of the energetic cost of ion dehydration (Chen et al. 2010).
Tellurite resistance protein tehA homologue of Haemophilus influenzae
Mae1 malate:proton symport protein. May also transport other dicarboxylates such as oxaloacetate, malonate, succinate and fumarate (Camarasa et al. 2001). May also transport thio-malate (Osawa and Matsumoto 2006).
Mae1 of Schizosaccharomyces pombe
C4-dicarboxylate transporter/malic acid transport protein, Mae1 of 395 aas and 10 TMSs. It has been overexpressed for the production of L-malate (Liu et al. 2017).
Mae1 of Emericella nidulans (Aspergillus nidulans)
The sulfite efflux (sulfite sensitivity) protein, SSU1. Expression is controlled by the FZF1-4 transcriptional activator; only free sulfite (not complexed sulfite) is exported (Park and Bakalinsky, 2000). Can also export nitrite and nitrate (Cabrera et al. 2014). SSU1 has a putative 10 TMS topology in a (S-L)5 arrangement where S= a small putative TMS and L= a large TMS.
SSU1 of Saccharomyces cerevisiae (P41930)
Sulfite, nitrate exporter of 384 aas, Ssu1 (Cabrera et al. 2014).
Ssu1 of Pichia angusta (Yeast) (Hansenula polymorpha)
Sulfite/nitrate exporter of 392 aas, Ssu2 (Cabrera et al. 2014).
Ssu2 of Pichia angusta (Yeast) (Hansenula polymorpha)
The unknown homologue, UnkH (same topology as 2.A.16.3.1)
UnkH of Aspergillus niger (A2QYD7)
TDT homologue of Sulfolobus acidocaldarius
TDT homologue of Streptomyces coelicolor
The plant guard cell S (Slow)-type anion channel, SLAC1 (based on activation kinetics of anion channel currents in response to voltage changes); functions in stomatal signalling, controls turgor pressure, and regulates the exchange of water and CO2 (Chen et al. 2010). Also called carbon dioxide insensitive (CDI3) and ozone sensitive (OZS1) (Kollist et al., 2011). Heterotrimeric G proteins regulate guard cell ion channels (Zhang, 2011). Evolutionary studies have been reported (Dreyer et al. 2012). The transmembrane region of guard cell SLAC1 channels detect CO2 signals via an abscisic acid (ABA)-independent pathway (Yamamoto et al. 2016). SLAC1 is activated by the protein kinase OST1 (OPEN STOMATA 1), the Ca2+-dependent protein kinases (CPKs), the GHR1 (GUARD CELL HYDROGEN PEROXIDE-RESISTANT 1) transmembrane receptor-like protein (TC# 1.A.87.2.8), or the PYL5 abscisic acid (ABA) receptor (Q9FLB1) (Wang et al. 2017).
SLAC1 of Arabidopsis thaliana
Slow anion channel homologue-3, SLAH3; nitrate is both a substrate and a gate opener (Geiger et al. 2011). Slow, weak voltage-dependent S-type anion efflux channel involved in maintenance of anion homeostasis (Negi et al. 2008). Binds to the highly selective inward-rectifying potassium channel KAT1 and inhibits its activity. Functions as an essential negative regulator of inward potassium channels in guard cells. Essential for the efficient stomatal closure and opening in guard cells (Zhang et al. 2016). The plasma membrane Glycine soja (soy bean) GsSLAH3 protein contains ten TMSs. GsSLAH3 expression is induced by NaHCO3 treatment, suggesting an involvement to alkaline stress, and ectopic expression of GsSLAH3 in yeast increased sensitivity to alkali treatment (Duan et al. 2017). Overexpression of GsSLAH3 in Arabidopsis thaliana enhanced alkaline tolerance during germination, seedling and adult stages. Transgenic lines improved plant tolerance to KHCO3 rather than high pH treatment. Overexpressing lines accumulated more NO3- than wild type (Duan et al. 2017).
SLAH3 of Arabidopsis thaliana
TDT homologue, TehA of 302aas and 10 TMSs
TehA of Streptococcus pyogenes (Q9A061)
TehA homologue of 314aas and 10 TMSs
TehA of Clostridium butyricum (C4IKV8)
Uncharacterized protein of 332 aas and 10 TMSs.
UP of Lactobacillus dextrinicus