Insights on Na+ binding and conformational dynamics in multidrug and toxic compound extrusion transporter NorM

MATE (multidrug and toxic compound extrusion) transporter proteins mediate metabolite transport in plants and multidrug resistance in bacteria and mammals. MATE transporter NorM from Vibrio cholerae is an antiporter that is driven by Na+ gradient to extrude the substrates. To understand the molecular mechanism of Na+‐substrate exchange, molecular dynamics simulation was performed to study conformational changes of both wild‐type and mutant NorM with and without cation bindings. Our results show that NorM is able to bind two Na⁺ ions simultaneously, one to each of the carboxylic groups of E255 and D371 in the binding pocket. Furthermore, this di‐Na⁺ binding state is likely more efficient for conformational changes of NorM_VC toward the inward‐facing conformation than single‐Na⁺ binding state. The observation of two Na⁺ binding sites of NorM_VC is consistent with the previous study that two sites for ion binding (denoted as Na1/Na2 sites) are found in the transporter LeuT and BetP, another two secondary transporters. Taken together, our findings shed light on the structure rearrangements of NorM on Na⁺ binding and enrich our knowledge of the transport mechanism of secondary transporters. Proteins 2014; 82:240–249. © 2013 Wiley Periodicals, Inc.     

Enhanced exudation of malate in the rhizosphere due to AtALMT1 overexpression in blackgram (Vigna mungo L.) confers increased aluminium tolerance

• Worldwide, 50% of soil is acidic, which induces aluminium (Al) toxicity in plants, as the phyto‐availability of Al³⁺ increases in acidic soil. Plants responds to Al³⁺ toxicity by exuding organic acids into the rhizosphere. The organic acid responsible for Al³⁺ stress response varies from species to species, which in the case of blackgram (Vigna mungo L.) is citrate.
• In blackgram, an Arabidopsis malate transporter, AtALMT1, was overexpressed with the motive of inducing enhanced exudation of malate. Transgenics were generated using cotyledon node explants through Agrobacterium tumefaciens‐mediated transformation. The putative transgenics were initially screened by AtALMT1‐specific genomic DNA PCR, followed by quantitative PCR. Two independent transgenic events were identified and functionally characterized in the T₃ generation.
• The transgenic lines, Line 1 and 2, showed better root growth, relative water content and chlorophyll content under Al³⁺ stress. Both lines also accounted for less oxidative damage, due to reduced accumulation of ROS molecules. Photosynthetic efficiency, as measured in terms of F_v/F_m, NPQ and Y(II), increased when compared to the wild type (WT). Relative expression of genes (VmSTOP1, VmALS3, VmMATE) responsible for Al³⁺ stress response in blackgram showed that overexpression of a malate transporter did not have any effect on their expression. Malate exudation increased whereas citrate exudation did not show any divergence from the WT. A pot stress assay found that the transgenics showed better adaptation to acidic soil.
• This report demonstrates that the overexpression of a malate transporter in a non‐malate exuding species improves adaptation to Al³⁺ toxicity in acidic soil without effecting its stress response mechanism.

     

Conserved functions of MATE transporters and their potential to enhance plant tolerance to aluminium toxicity

Almost half of the world’s arable land has acidic pH. Aluminum salts present in acid soils dissociate to release Al³⁺ ions in the soil solution that inhibit root growth causing severe loss in crop yields. Aluminium toxicity accounts for the second highest loss in plant productivity after drought. Aluminium in high doses causes damage to the plant cell wall, cytoskeleton and DNA. One of the ways by which plants alleviate aluminium toxicity is by the exudation of citrate from the roots that chelates the free Al³⁺ and prevents its entry into the plant. In several crop plants Multidrug and Toxic Compound Extrusion (MATE) transporters regulate citrate exudation from the roots. The MATE proteins are ubiquitously present in bacteria, archaea, fungi, animals and plants. The origin and evolution of these membrane transporters in plants is not well known. Here, using protein sequence information we identify MATE transporters in major groups of land plants and their algal ancestors. Our study indicates that the MATE family members expanded in number and functionally diverse among the land plants. We also identify motifs present across the streptophyte clade and a conserved aspartate residue that might regulate citrate exudation. This study can provide leads to engineer MATE transporters to confer enhanced tolerance in acid soils.

     

Identification of novel natural inhibitor for NorM – a multidrug and toxic compound extrusion transporter – an insilico molecular modeling and simulation studies

The emergence of bacterial multidrug resistance is an increasing problem in treatment of infectious diseases. An important cause for the multidrug resistance of bacteria is the expression of multidrug efflux transporters. The multidrug and toxic compound extrusion (MATE) transporters are most recently recognized as unique efflux system for extrusion of antimicrobials and therapeutic drugs due to energy stored in either Na⁺ or H⁺ electrochemical gradient. In the present study, high throughput virtual screening of natural compound collections against NorM – a MATE transporter from Neisseria gonorrhea (NorM-NG) has been carried out followed by flexible docking. The molecular simulation in membrane environment has been performed for understanding the stability and binding energetic of top lead compounds. Results identified a compound from the Indian medicinal plant “Terminalia chebula” which has good binding free energy compared to substrates (rhodamine 6 g, ethidium) and more favorable interactions with the central cavity forming active site residues. The compound has restricted movement in TM7, TM8, and TM1, thus blocking the disruption of Na+ – coordination along with equilibrium state bias towards occlude state of NorM transporter. Thus, this compound blocks the effluxing pathway of antimicrobial drugs and provides as a natural bioactive lead inhibitor against NorM transporter in drug-resistant gonorrhea.     

An extracellular hydrophilic carboxy‐terminal domain regulates the activity of TaALMT1, the aluminum‐activated malate transport protein of wheat

Al³⁺‐resistant cultivars of wheat (Triticum aestivum L.) release malate through the Al³⁺‐activated anion transport protein Triticum aestivum aluminum‐activated malate transporter 1 (TaALMT1). Expression of TaALMT1 in Xenopus oocytes and tobacco suspension cells enhances the basal transport activity (inward and outward currents present in the absence of external Al³⁺), and generates the same Al³⁺‐activated currents (reflecting the Al³⁺‐dependent transport function) as observed in wheat cells. We investigated the amino acid residues involved in this Al³⁺‐dependent transport activity by generating a series of mutations to the TaALMT1 protein. We targeted the acidic residues on the hydrophilic C‐terminal domain of TaALMT1 and changed them to uncharged residues by site‐directed mutagenesis. These mutant proteins were expressed in Xenopus oocytes and their transport activity was measured before and after Al³⁺ addition. Three mutations (E274Q, D275N and E284Q) abolished the Al³⁺‐activated transport activity without affecting the basal transport activity. Truncation of the hydrophilic C‐terminal domain abolished both basal and Al³⁺‐activated transport activities. Al³⁺‐dependent transport activity was recovered by fusing the N‐terminal region of TaALMT1 with the C‐terminal region of AtALMT1, a homolog from Arabidopsis. These findings demonstrate that the extracellular C‐terminal domain is required for both basal and Al³⁺‐dependent TaALMT1 activity. Furthermore, we identified three acidic amino acids within this domain that are specifically required for the activation of transport function by external Al³⁺.     

Glucose transport byAspergillus niger: the low-affinity carrier is only formed during growth on high glucose concentrations

The filamentous fungusAspergillus niger accumulates large levels of citric acid in the medium when grown under conditions favouring a high rate of sugar catabolism. With the aim of understanding the mechanisms involved in this process we investigated glucose transport in this fungus. To this end a medium was designed that enables growth of the fungus into a fine, hairy filamentous mycelium, suitable for transport studies. It was found thatA. niger contains a single, high-affinity glucose transporter when grown on a low (1% w/v) glucose concentration, but forms an additional low-affinity transporter when grown on a high (15% w/v) glucose concentration. Both glucose transporters exhibit decreased activities at low pH and are inhibited by citric acid. However, the activity of the low-affinity transporter is much less affected by these conditions. Two 2-deoxyglucose-resistant (dgr) mutants ofA. niger, which produce citric acid at a much lower rate than the parent strain, are impaired in the formation of the low-affinity transporter, but form the high-affinity transporter with higher activities. We conclude that the low-affinity glucose transporter takes part in the mechanism by whichA. niger responds to high extracellular glucose concentrations leading to citric acid accumulation.     

HvAKT2 and HvHAK1 confer drought tolerance in barley through enhanced leaf mesophyll H+ homoeostasis

Plant K⁺ uptake typically consists low—affinity mechanisms mediated by Shaker K⁺ channels (AKT/KAT/KC) and high‐affinity mechanisms regulated by HAK/KUP/KT transporters, which are extensively studied. However, the evolutionary and genetic roles of both K⁺ uptake mechanisms for drought tolerance are not fully explored in crops adapted to dryland agriculture. Here, we employed evolutionary bioinformatics, biotechnological and electrophysiological approaches to determine the role of two important K⁺ transporters HvAKT2 and HvHAK1 in drought tolerance in barley. HvAKT2 and HvHAK1 were cloned and functionally characterized using barley stripe mosaic virus‐induced gene silencing (BSMV‐VIGS) in drought‐tolerant wild barley XZ5 and agrobacterium‐mediated gene transfer in the barley cultivar Golden Promise. The hallmarks of the K⁺ selective filters of AKT2 and HAK1 are both found in homologues from strepotophyte algae, and they are evolutionarily conserved in strepotophyte algae and land plants. HvAKT2 and HvHAK1 are both localized to the plasma membrane and have high selectivity to K⁺ and Rb⁺ over other tested cations. Overexpression of HvAKT2 and HvHAK1 enhanced K⁺ uptake and H⁺ homoeostasis leading to drought tolerance in these transgenic lines. Moreover, HvAKT2‐ and HvHAK1‐overexpressing lines showed distinct response of K⁺, H⁺ and Ca²⁺ fluxes across plasma membrane and production of nitric oxide and hydrogen peroxide in leaves as compared to the wild type and silenced lines. High‐ and low‐affinity K⁺ uptake mechanisms and their coordination with H⁺ homoeostasis play essential roles in drought adaptation of wild barley. These findings can potentially facilitate future breeding programs for resilient cereal crops in a changing global climate.     

The phage shock protein PspA facilitates divalent metal transport and is required for virulence of Salmonella enterica sv. Typhimurium

The phage shock protein (Psp) system is induced by extracytoplasmic stress and thought to be important for the maintenance of proton motive force. We investigated the contribution of PspA to Salmonella virulence. A pspA deletion mutation significantly attenuates the virulence of Salmonella enterica serovar Typhimurium following intraperitoneal inoculation of C3H/HeN (Ity^r) mice. PspA was found to be specifically required for virulence in mice expressing the natural resistance‐associated macrophage protein 1 (Nramp1) (Slc11a1) divalent metal transporter, which restricts microbial growth by limiting the availability of essential divalent metals within the phagosome. Salmonella competes with Nramp1 by expressing multiple metal uptake systems including the Nramp‐homologue MntH, the ABC transporter SitABCD and the ZIP family transporter ZupT. PspA was found to facilitate Mn²⁺ transport by MntH and SitABCD, as well as Zn²⁺ and Mn²⁺ transport by ZupT. In vitro uptake of ⁵⁴Mn²⁺ by MntH and ZupT was reduced in the absence of PspA. Transport‐deficient mutants exhibit reduced viability in the absence of PspA when grown under metal‐limited conditions. Moreover, the ZupT transporter is required for Salmonella enterica serovar Typhimurium virulence in Nramp1‐expressing mice. We propose that PspA promotes Salmonella virulence by maintaining proton motive force, which is required for the function of multiple transporters mediating bacterial divalent metal acquisition during infection.     

Translocation and accumulation of nicotine via distinct spatio-temporal regulation of nicotine transporters in Nicotiana tabacum

In plants, secondary metabolites play important roles in adaptation to the environment. Nicotine, a pyridine alkaloid in Nicotiana tabacum, functions as chemical barrier against herbivores. Nicotine produced in the root undergoes long-distance transport and accumulates mainly in the leaves. Since production of such defensive compounds is costly, plants must regulate the allocation of the products to their tissues; however, the molecular mechanism of nicotine translocation remains unclear. Our recent studies identified a novel multidrug and toxic compound extrusion (MATE)-type nicotine transporter, JAT2 (jasmonate-inducible alkaloid transporter 2). This transporter is specifically expressed in leaves, localizes to the tonoplast, and transports nicotine as its substrate. The specific induction of JAT2 expression in leaves by methyl jasmonate (MeJA) treatment suggests that this transporter plays an important role in nicotine distribution to leaves, especially under herbivore attack, by transporting nicotine into the vacuole. Considering JAT2, together with the previously identified MATE transporters JAT1, MATE1, and MATE2, and the PUP (purine permease) transporter NUP1 (nicotine uptake permease1), we show a model of nicotine translocation and accumulation via distinct spatio-temporal regulation of nicotine transporter expression. Furthermore, we discuss the possible role of nicotine transporters in determining outcrossing rates and seed production.     

Quantitative detection of changes in the leaf‐mesophyll tonoplast proteome in dependency of a cadmium exposure of barley (Hordeum vulgare L.) plants

Although the vacuole is the most important final store for toxic heavy metals like cadmium (Cd²⁺), our knowledge on how they are transported into the vacuole is still insufficient. It has been suggested that Cd²⁺ can be transported as phytochelatin‐Cd²⁺ by an unknown ABC transporter or in exchange with protons by cation/proton exchanger (CAX) transporters. To unravel the contribution of vacuolar transporters to Cd²⁺ detoxification, a quantitative proteomics approach was performed. Highly purified vacuoles were isolated from barley plants grown under minus, low (20 μM), and high (200 μM) Cd²⁺ conditions and protein levels of the obtained tonoplast samples were analyzed using isobaric tag for relative and absolute quantitation (iTRAQ?). Although 56 vacuolar transporter proteins were identified, only a few were differentially expressed. Under low‐Cd²⁺ conditions, an inorganic pyrophosphatase and a γ‐tonoplast intrinsic protein (γ‐TIP) were up‐regulated, indicating changes in energization and water fluxes. In addition, the protein ratio of a CAX1a and a natural resistance‐associated macrophage protein (NRAMP), responsible for vacuolar Fe²⁺ export was increased. CAX1a might play a role in vacuolar Cd²⁺ transport. An increase in NRAMP activity leads to a higher cytosolic Fe²⁺ concentration, which may prevent the exchange of Fe²⁺ by toxic Cd²⁺. Additionally, an ABC transporter homolog to AtMRP3 showed up‐regulation. Under high Cd²⁺ conditions, the plant response was more specific. Only a protein homologous to AtMRP3 that showed already a response under low Cd²⁺ conditions, was up‐regulated. Interestingly, AtMRP3 is able to partially rescue a Cd²⁺‐sensitive yeast mutant. The identified transporters are good candidates for further investigation of their roles in Cd²⁺ detoxification.     

The ScAACT1 gene at the Q alt5 locus as a candidate for increased aluminum tolerance in rye (Secale cereale L.)

Soluble aluminum (Al³⁺) is a major constraint to plant growth in highly acidic soils, which comprise up to 50% of the world??s arable land. The primary mechanism of Al resistance described in plants is the chelation of Al³⁺ cations by release of organic acids into the rhizosphere. Candidate aluminum tolerance genes encoding organic acid transporter of the ALMT (aluminum-activated malate transporter) and MATE (multi-drug and toxic compound extrusion) families have been characterized in several plant species. In this study, we have isolated in five different cultivars the rye ScAACT1 gene, homolog to barley aluminum activated citrate transporter HvAACT1. This gene mapped to the 7RS chromosome arm, 25?cM away from the ScALMT1 aluminum tolerance gene. The gene consisted of 13 exons and 12 introns and encodes a predicted membrane protein that contains the MatE domain and at least seven putative transmembrane regions. Expression of the ScAACT1 gene is Al-induced, but there were differences in the levels of expression among the cultivars analyzed. A new quantitative trait locus for Al tolerance in rye that co-localizes with the ScAACT1 gene was detected in the 7RS chromosome arm. These results suggest that the ScAACT1 gene is a candidate gene for increased Al tolerance in rye. The phylogenetic relationships between different MATE proteins are discussed.     

Two functionally distinct members of the MATE (multi‐drug and toxic compound extrusion) family of transporters potentially underlie two major aluminum tolerance QTLs in maize

Crop yields are significantly reduced by aluminum (Al) toxicity on acidic soils, which comprise up to 50% of the world’s arable land. Al‐activated release of ligands (such as organic acids) from the roots is a major Al tolerance mechanism in plants. In maize, Al‐activated root citrate exudation plays an important role in tolerance. However, maize Al tolerance is a complex trait involving multiple genes and physiological mechanisms. Recently, transporters from the MATE family have been shown to mediate Al‐activated citrate exudation in a number of plant species. Here we describe the cloning and characterization of two MATE family members in maize, ZmMATE1 and ZmMATE2, which co‐localize to major Al tolerance QTL. Both genes encode plasma membrane proteins that mediate significant anion efflux when expressed in Xenopus oocytes. ZmMATE1 expression is mostly concentrated in root tissues, is up‐regulated by Al and is significantly higher in Al‐tolerant maize genotypes. In contrast, ZmMATE2 expression is not specifically localized to any particular tissue and does not respond to Al. [¹⁴C]‐citrate efflux experiments in oocytes demonstrate that ZmMATE1 is a citrate transporter. In addition, ZmMATE1 expression confers a significant increase in Al tolerance in transgenic Arabidopsis. Our data suggests that ZmMATE1 is a functional homolog of the Al tolerance genes recently characterized in sorghum, barley and Arabidopsis, and is likely to underlie the largest maize Al tolerance QTL found on chromosome 6. However, ZmMATE2 most likely does not encode a citrate transporter, and could be involved in a novel Al tolerance mechanism.     

不同生境外生菌根真菌对铝胁迫的响应

以南北方不同生境下的10株外生菌根真菌为研究对象,采用液体培养的方法,研究了铝对不同菌根真菌的生物量、有机酸分泌及养分含量的影响,以期筛选出抗铝性强的优良菌株,并探讨其抗铝机理。结果表明:外生菌根真菌Sl 08抗铝性最强;Pt 715、Ld 03、Bo 11、Sl 01、Bo 15也具有不同程度的耐铝性;Sl 14、Gc 99、Cg 04抗铝性较差;Sg 11抗铝性最差。来自南方酸性森林土壤的菌株总体抗铝性强于来自北方石灰性土壤的菌株,这表明外生菌根真菌的铝耐受能力与其原始生境有着密切的联系。外生菌根真菌能分泌多种有机酸,且不同菌株分泌的有机酸种类不同。其中,受铝胁迫分泌量增加最多的是草酸。研究中,铝胁迫能增加大多数铝抗性菌株的草酸分泌量,其中铝抗性最强的Sl 08表现最为明显。但铝胁迫并没有促进具备一定铝抗性的Bo 11和Sl 01草酸的分泌量,同时在铝敏感的菌株中均观察到了草酸分泌量的增加。这表明分泌草酸可能并不是外生菌根真菌抵抗铝毒的唯一途径。对各菌株铝胁迫下对氮,磷及钾的吸收研究表明,除铝敏感菌株Sl 14外,铝胁迫均能促进各供试菌株对氮,磷或钾的吸收。综上,在一定铝浓度下,一些外生菌根真菌可通过增加草酸分泌来抵御铝毒。此外,铝胁迫下外生菌根真菌还可通过调控氮、磷、钾等营养元素的吸收来抵抗铝毒,即通过增加对营养元素的吸收来增强其在铝胁迫下的生存能力,这可能是其抵御铝胁迫的应激反应之一。     

Aluminum‐dependent dynamics of ion transport in Arabidopsis: specificity of low pH and aluminum responses

Low‐pH and Al³⁺ stresses are the major causes of poor plant growth in acidic soils. However, there is still a poor understanding of plant responses to low‐pH and Al³⁺ toxicity. Low‐pH or combined low‐pH and Al³⁺ stress was imposed in order to measure rhizosphere pH, ion fluxes, plasma membrane potential and intracellular H⁺ concentration in distal elongation and mature zones (MZs) along the longitudinal axis of Arabidopsis thaliana roots. Low‐pH stress facilitated H⁺ influx into root tissues and caused cytoplasmic acidification; by contrast, combined low‐pH/Al³⁺ treatment either decreased H⁺ influx in the distal elongation zone (DEZ) or induced H⁺ efflux in the MZ, leading to cytoplasmic alkalinization in both zones. Low‐pH stress induced an increase in rhizosphere pH in the DEZ, whereas combined low‐pH/Al³⁺ stress resulted in lower rhizosphere pH in both root zones compared with the low‐pH treatment alone. Low‐pH stress facilitated K⁺ efflux; the presence of Al³⁺ diminished K⁺ efflux or favored K⁺ influx into root tissues. In both zones, low‐pH treatment induced plasma membrane (PM) depolarization, which was significantly diminished (P≤ 0.05) when combined stresses (low‐pH/100 µM Al³⁺) were imposed. After 60 min of exposure, low pH caused PM depolarization, whereas low pH/100 µM Al³⁺ caused PM hyperpolarization. Thus, low pH and Al³⁺ toxicity differentially affect root tissues and, consequently, the rhizosphere, which might underpin the differential mechanisms of plant adaptation to these abiotic stresses.     

Intense green‐, red‐emitting Tb3+, Tb3+/Bi3+‐doped and Sm3+, Sm3+/La3+‐doped Ca2Al2SiO7 phosphors

This paper focuses on an optical study of a Tb³⁺/Bi³⁺‐doped and Sm³⁺/La³⁺‐ doped Ca₂Al₂SiO₇ phosphor synthesized using combustion methods. Here, Ca₂Al₂SiO₇:Sm³⁺ showed a red emission band under visible light excitation but, when it co‐doped with La³⁺ ions, the emission intensity was further enhanced. Ca₂Al₂SiO₇:Tb³⁺ shows the characteristic green emission band under near‐ultraviolet light excitation wavelengths, co‐doping with Bi³⁺ ions produced enhanced photoluminescence intensity with better colour tunable properties. The phosphor exhibited better phase purity and crystallinity, confirmed by X‐ray diffraction. Binding energies of Ca(2p), Al(2p), Si(2p), O(1s) were studied using X‐ray photoelectron spectroscopy. The reported phosphor may be a promising visible light excited red phosphor for light‐emitting diodes and energy conversion devices.     

Molecular cloning and functional expression in bacteria of the potassium transporters CnHAK1 and CnHAK2 of the seagrass Cymodocea nodosa

The cDNAs CnHAK1 and CnHAK2, encoding K⁺ transporters, were amplified from the leaves of the seagrass Cymodocea nodosa. None of these transporters suppressed the K⁺ deficiency of a Saccharomyces cerevisiae mutant, but both suppressed the equivalent defect of an Escherichia coli mutant. Overexpression of the transporter CnHAK1, but not CnHAK2, mediated very rapid K⁺ or Rb⁺ influxes in the E. coli mutant. The concentration dependence of these influxes demonstrated that CnHAK1 is a low-affinity K⁺ transporter, which does not discriminate between K⁺ and Rb⁺. CnHAK1 expressed in E. coli worked in reverse when the external K⁺ concentrations were low, and we established the condition of a simple functional test of K⁺ loss for transporters of the Kup-HAK family. In comparison with its homologue barley transporter HvHAK2, CnHAK1 was insensitive to Na⁺.     

Characterization of a highly Al3+‐selective fluorescence probe based on naphthalimide‐Schiff base and its application to practical water samples

A new fluorescent Al³⁺‐probe, N‐allyl‐4‐[3,3′‐((2‐aminoethyl)azanediyl)‐bis(N´‐(2‐hydroxybenzylidene)propanehy‐drazide)]‐1,8‐naphthalimide ( L ), was designed and synthesized based on 1,8‐naphthalimide. The probe L contains 1,8‐naphthalimide moiety as the fluorophore and a Schiff base as the recognition group. The structure of L was determined by single crystal X‐ray. L emission at 526 nm increased on addition of Al³⁺ under excitation wavelength at 350 nm. L exhibited high selectivity and sensitivity fluorescence emission towards to Al³⁺ in ethanol/Tris–HCl buffer solution (1:1, v/v, pH = 7.2) as compared with other tested metal ions. A good linearity with a correlation coefficient (R²) of 0.99 was observed in the concentration range 2–10 μM. The binding constant and the detection limit of L for Al³⁺ were calculated to 2.6 × 10⁴ M^?1 and 0.34 μM, respectively. The results of experiments that including Job plot, ultraviolet–visible (UV–Vis) light titration, fluorescence titration, ESI‐MS and ¹H NMR titration, indicated a 1:1 stoichiometric complex between L and Al³⁺. L was highly effective in monitoring Al³⁺ in real‐life Yellow River and tap water samples.     

A novel variant of mouse MATE-1 H+/organic cation antiporter with a long hydrophobic tail

Mammalian multidrug and toxic compound extrusion 1 (MATE1) are polyspecific H⁺-coupled exporters of organic cations (OCs) and responsible for excretion of metabolic waste products and xenobiotics. Here, we report a novel variant of mouse MATE1, mMATE1b, that has a long carboxyl terminal hydrophobic tail homologous to other MATE1 transporter proteins. Mouse MATE1b mediates tetraethylammonium (TEA) uptake with properties similar to that of mMATE1 and is localized in renal brush border membranes. Thus, mMATE1b is a functional variant of mMATE1 and seems to be the true counterpart to other MATE1 transporters.