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). Fungal carboxylate transporters have been reviewed (Wu et al. 2023).
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. Stomatal movement is orchestrated by diverse signaling cascades and metabolic activities in guard cells (Chang et al. 2023). Light triggers the opening of the pores through the phototropin-mediated pathway, which leads to the activation of plasma membrane H+-ATPase and thereby facilitates potassium accumulation through K+(in) channels. Chang et al. 2023 showed that the stomatal response to light is negatively regulated by 12-oxo-phytodienoic acid (OPDA), an oxylipin metabolite produced through enzymatic oxygenation of polyunsaturated fatty acids (PUFAs). A set of phospholipase-encoding genes, phospholipase (PLIP)1/2/3 are transactivated rapidly in guard cells upon illumination in a phototropin-dependent manner. These phospholipases release PUFAs from the chloroplast membrane, which is oxidized by guard-cell lipoxygenases and further metabolized to OPDA. OPDA-deficient mutants have wider stomatal pores, whereas mutants containing elevated levels of OPDA showed the opposite effect on the stomatal aperture. Transmembrane solute fluxes that drive stomatal aperture were enhanced in lox6-1 guard cells, indicating that OPDA signaling ultimately impacts on activities of proton pumps and K+(in) channels. The accelerated stomatal kinetics in lox6-1 leads to increased plant growth without cost in water or macronutrient use. Thus, a new role is revealed for chloroplast membrane oxylipin metabolism in stomatal regulation. The accelerated stomatal opening kinetics in OPDA-deficient mutants benefits plant growth and water use efficiency (Chang et al. 2023).
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). High level cell-free expression and specific labeling of TehA from E. coli has been achieved (Klammt et al. 2004).
TehA of E. coli
Tellurite resistance protein TehA homologue of 328 aas and 10 TMSs. 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). High level cell-free expression and specific labeling of TehA from E. coli has been achieved (Klammt et al. 2004).
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). The structure of SLAC1 in an inactivated, closed state has been determined (Li et al. 2022). The cytosolic N-terminus and C-terminus are partially resolved and form a plug-like structure which packs against the TM domain. Breaking the interactions between the cytosolic plug and the TMD triggers channel activation. An inhibition-release model is proposed for SLAC1 activation by phosphorylation, that the cytosolic plug dissociates from the TMD upon phosphorylation, and induces conformational changes to open the pore. These findings facilitate an understanding of the regulation of SLAC1 activity and stomatal aperture in plants (Li et al. 2022).
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
SLAC1/SLAH nitrate transporter (slow anion channel-associated homologue) of 565 aas and 12 TMSs in a 6 + 6 TMS arrangement. This protein was identified and partially characterized in wheat (Kumar et al. 2022). Increased transmembrane transport of per- and polyfluoroalkyl substances (PFASs), stimulated by humic acid (HA) is mainly driven by the slow-type anion channel pathways interacting with Ca2+-dependent protein kinases (Ca2+-CDPK-SLAC1). The promoted transmembrane transport of PFASs might cause adverse effects on the plant cell wall (Liu et al. 2023).
SLAC1/SLAH of Triticum aestivum
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