GmMATE100 Is Involved in the Import of Soyasaponins A and B into Vacuoles in Soybean Plants (Glycine max L.)

Triterpenoid saponins, synthesized via the mevalonic acid (MVA) pathway in the cytoplasm, provide protection against pathogens and pests in plants and health benefits for humans. However, the mechanisms by which triterpenoid saponins are transported between cellular compartments remain uncharacterized. Here, we characterize a tonoplast localized multidrug and toxic compound extrusion transporter, GmMATE100 (encoded by Glyma.18G143700), from soybean (Glycine max L.). GmMATE100 is co-expressed with soyasaponin biosynthetic genes, and its expression was induced by MeJA treatment, which also led to soyasaponin accumulation in soybean roots. GmMATE100 efficiently transports multiple type-B soyasaponins as well as type-A soyasaponins with low affinity from the cytosol to the vacuole in a yeast system. The GmMATE100 loss-of-function mutant showed a significant decrease in type-A and type-B soyasaponin contents in soybean roots. This study not only characterized the first soybean triterpenoid saponin transporter but also provided new knowledge for the rational engineering of soyasaponin content and composition in soybean plants to modulate their levels within crop environments.

Disentangling hydroxynitrile glucoside biosynthesis in a barley (Hordeum vulgare) metabolon provides access to elite malting barleys for ethyl carbamate-free whisky production

Barley produces several specialized metabolites, including five α-, β-, and γ-hydroxynitrile glucosides (HNGs). In malting barley, presence of the α-HNG epiheterodendrin gives rise to undesired formation of ethyl carbamate in the beverage production, especially after distilling. Metabolite-GWAS identified QTLs and underlying gene candidates possibly involved in the control of the relative and absolute content of HNGs, including an undescribed MATE transporter. By screening 325 genetically diverse barley accessions, we discovered three H. vulgare ssp. spontaneum (wild barley) lines with drastic changes in the relative ratios of the five HNGs. Knock-out (KO)-lines, isolated from the barley FIND-IT resource and each lacking one of the functional HNG biosynthetic genes (CYP79A12, CYP71C103, CYP71C113, CYP71U5, UGT85F22 and UGT85F23) showed unprecedented changes in HNG ratios enabling assignment of specific and mutually dependent catalytic functions to the biosynthetic enzymes involved. The highly similar relative ratios between the five HNGs found across wild and domesticated barley accessions indicate assembly of the HNG biosynthetic enzymes in a metabolon, the functional output of which was reconfigured in the absence of a single protein component. The absence or altered ratios of the five HNGs in the KO-lines did not change susceptibility to the fungal phytopathogen Pyrenophora teres causing net blotch. The study provides a deeper understanding of the organization of HNG biosynthesis in barley and identifies a novel, single gene HNG-0 line in an elite spring barley background for direct use in breeding of malting barley, eliminating HNGs as a source of ethyl carbamate formation in whisky production.

CcNFYB3-CcMATE35 and LncRNA CcLTCS-CcCS modules jointly regulate the efflux and synthesis of citrate to enhance aluminium tolerance in pigeon pea

Aluminium (Al) toxicity decreases crop production in acid soils in general, but many crops have evolved complex mechanisms to resist it. However, our current understanding of how plants cope with Al stress and perform Al resistance is still at the initial stage. In this study, the citrate transporter CcMATE35 was identified to be involved in Al stress response. The release of citrate was increased substantially in CcMATE35 over-expression (OE) lines under Al stress, indicating enhanced Al resistance. It was demonstrated that transcription factor CcNFYB3 regulated the expression of CcMATE35, promoting the release of citrate from roots to increase Al resistance in pigeon pea. We also found that a Long noncoding RNA Targeting Citrate Synthase (CcLTCS) is involved in Al resistance in pigeon pea. Compared with controls, overexpression of CcLTCS elevated the expression level of the Citrate Synthase gene (CcCS), leading to increases in root citrate level and citrate release, which forms another module to regulate Al resistance in pigeon pea. Simultaneous overexpression of CcNFYB3 and CcLTCS further increased Al resistance. Taken together, these findings suggest that the two modules, CcNFYB3-CcMATE35 and CcLTCS-CcCS, jointly regulate the efflux and synthesis of citrate and may play an important role in enhancing the resistance of pigeon pea under Al stress.

Genome-wide identification of MATE, functional analysis and molecular dynamics of DcMATE21 involved in anthocyanin accumulation in Daucus carota

Anthocyanins are a subclass of flavonoids that are synthesized in the endoplasmic reticulum and then transported to the vacuole in plants. Multidrug and toxic compound extrusion transporters (MATE) is a family of membrane transporters that transport ions and secondary metabolites, such as anthocyanins, in plants. Although various studies on MATE transporters have been carried out on different plant species, this is the first comprehensive report to mine the Daucus carota genome to identify the MATE gene family. Our study identified 45 DcMATEs through genome-wide analysis and detected five segmental and six tandem duplications from the genome. The chromosome distribution, phylogenetic analysis, and cis-regulatory elements revealed the structural diversity and numerous functions associated with the DcMATEs. In addition, we analyzed RNA-seq data obtained from the European Nucleotide Archive to screen for the expression of DcMATEs involved in anthocyanin biosynthesis. Among the identified DcMATEs, DcMATE21 correlated with anthocyanin content in the different D. carota varieties. In addition, the expression of DcMATE21 and anthocyanin biosynthesis genes was correlated under abscisic acid, methyl jasmonate, sodium nitroprusside, salicylic acid, and phenylalanine treatments, which were substantiated by anthocyanin accumulation in the in vitro cultures. Further molecular membrane dynamics of DcMATE21 with anthocyanin (cyanidin-3-glucoside) identified the binding pocket, showing extensive H-bond interactions with 10 crucial amino acids present in the transmembrane helix of 7, 8, and 10 of DcMATE21. The current investigation, using RNA-seq, in vitro cultures, and molecular dynamics studies revealed the involvement of DcMATE21 in anthocyanin accumulation in vitro cultures of D. carota.

MATE transporter GFD1 cooperates with sugar transporters, mediates carbohydrate partitioning and controls grain-filling duration, grain size and number in rice

More than half of the world's food is provided by cereals, as humans obtain >60% of daily calories from grains. Producing more carbohydrates is always the final target of crop cultivation. The carbohydrate partitioning pathway directly affects grain yield, but the molecular mechanisms and biological functions are poorly understood, including rice (Oryza sativa L.), one of the most important food sources. Here, we reported a prolonged grain filling duration mutant 1 (gfd1), exhibiting a long grain-filling duration, less grain number per panicle and bigger grain size without changing grain weight. Map-based cloning and molecular biological analyses revealed that GFD1 encoded a MATE transporter and expressed high in vascular tissues of the stem, spikelet hulls and rachilla, but low in the leaf, controlling carbohydrate partitioning in the stem and grain but not in the leaf. GFD1 protein was partially localized on the plasma membrane and in the Golgi apparatus, and was finally verified to interact with two sugar transporters, OsSWEET4 and OsSUT2. Genetic analyses showed that GFD1 might control grain-filling duration through OsSWEET4, adjust grain size with OsSUT2 and synergistically modulate grain number per panicle with both OsSUT2 and OsSWEET4. Together, our work proved that the three transporters, which are all initially classified in the major facilitator superfamily family, could control starch storage in both the primary sink (grain) and temporary sink (stem), and affect carbohydrate partitioning in the whole plant through physical interaction, giving a new vision of sugar transporter interactome and providing a tool for rice yield improvement.

Two homeologous MATE transporter genes, NtMATE21 and NtMATE22, are involved in the modulation of plant growth and flavonol transport in Nicotiana tabacum

The multidrug and toxic compound extrusion (MATE) protein family has been implicated in the transport of a diverse range of molecules, including specialized metabolites. In tobacco (Nicotiana tabacum), only a limited number of MATE transporters have been functionally characterized, and no MATE transporter has been studied in the context of flavonoid transport in this plant species so far. In the present study, we characterize two homeologous tobacco MATE genes, NtMATE21 and NtMATE22, and demonstrate their role in flavonol transport and in plant growth and development. The expression of these two genes was reported to be up-regulated in trichomes as compared with the trichome-free leaf. The transcript levels of NtMATE21 and NtMATE22 were found to be higher in flavonol overproducing tobacco transgenic lines as compared with wild type tobacco. The two transporters were demonstrated to be localized to the plasma membrane. Genetic manipulation of NtMATE21 and NtMATE22 led to altered growth phenotypes and modulated flavonol contents in N. tabacum. The β-glucuronidase and green fluorescent protein fusion transgenic lines of promoter regions suggested that NtMATE21 and NtMATE22 are exclusively expressed in the trichome heads in the leaf tissue and petals. Moreover, in a transient transactivation assay, NtMYB12, a flavonol-specific MYB transcription factor, was found to transactivate the expression of NtMATE21 and NtMATE22 genes. Together, our results strongly suggest the involvement of NtMATE21 and NtMATE22 in flavonol transport as well as in the regulation of plant growth and development.

Identifying Quantitative Trait Loci and Candidate Genes Conferring Resistance to Soybean Mosaic Virus SC7 by Quantitative Trait Loci-Sequencing in Soybean

Soybean mosaic virus (SMV) is detrimental to soybean (Glycine max) breeding, seed quality, and yield worldwide. Improving the basic resistance of host plants is the most effective and economical method to reduce damage from SMV. Therefore, it is necessary to identify and clone novel SMV resistance genes. Here, we report the characterization of two soybean cultivars, DN50 and XQD, with different levels of resistance to SMV. Compared with XQD, DN50 exhibits enhanced resistance to the SMV strain SC7. By combining bulked-segregant analysis (BSA)-seq and fine-mapping, we identified a novel resistance locus, R _SMV -11, spanning an approximately 207-kb region on chromosome 11 and containing 25 annotated genes in the reference Williams 82 genome. Of these genes, we identified eleven with non-synonymous single-nucleotide polymorphisms (SNPs) or insertion-deletion mutations (InDels) in their coding regions between two parents. One gene, GmMATE68 (Glyma.11G028900), harbored a frameshift mutation. GmMATE68 encodes a multidrug and toxic compound extrusion (MATE) transporter that is expressed in all soybean tissues and is induced by SC7. Given that MATE transporter families have been reported to be linked with plant disease resistance, we suggest that GmMATE68 is responsible for SC7 resistance in DN50. Our results reveal a novel SMV-resistance locus, improving understanding of the genetics of soybean disease resistance and providing a potential new tool for marker-assisted selection breeding in soybean.

Identification of an Isoflavonoid Transporter Required for the Nodule Establishment of the Rhizobium-Fabaceae Symbiotic Interaction

Nitrogen (N) as well as Phosphorus (P) are key nutrients determining crop productivity. Legumes have developed strategies to overcome nutrient limitation by, for example, forming a symbiotic relationship with N-fixing rhizobia and the release of P-mobilizing exudates and are thus able to grow without supply of N or P fertilizers. The legume-rhizobial symbiosis starts with root release of isoflavonoids that act as signaling molecules perceived by compatible bacteria. Subsequently, bacteria release nod factors, which induce signaling cascades allowing the formation of functional N-fixing nodules. We report here the identification and functional characterization of a plasma membrane-localized MATE-type transporter (LaMATE2) involved in the release of genistein from white lupin roots. The LaMATE2 expression in the root is upregulated under N deficiency as well as low phosphate availability, two nutritional deficiencies that induce the release of this isoflavonoid. LaMATE2 silencing reduced genistein efflux and even more the formation of symbiotic nodules, supporting the crucial role of LaMATE2 in isoflavonoid release and nodulation. Furthermore, silencing of LaMATE2 limited the P-solubilization activity of lupin root exudates. Transport assays in yeast vesicles demonstrated that LaMATE2 acts as a proton-driven isoflavonoid transporter.

AtDTX25, a member of the multidrug and toxic compound extrusion family, is a vacuolar ascorbate transporter that controls intracellular iron cycling in Arabidopsis

Iron (Fe) is an essential element, its transport is regulated by the cell redox balance. In seeds, Fe enters the embryo as Fe²⁺ and is stored in vacuoles as Fe³⁺ . Through its ferric reduction activity, ascorbate plays a major role in Fe redox state and therefore Fe transport within the seed. We searched for ascorbate membrane transporters responsible for controlling Fe reduction by screening the yeast ferric reductase-deficient fre1 strain and isolated AtDTX25, a member of the Multidrug And Toxic compound Extrusion (MATE) family. AtDTX25 was shown to mediate ascorbate efflux when expressed in yeast and Xenopus oocytes, in a pH-dependent manner. In planta, AtDTX25 is highly expressed during germination and encodes a vacuolar membrane protein. Isolated vacuoles from AtDTX25-1 knockout mutant contained less ascorbate and more Fe than wild-type (WT), and mutant seedlings were highly sensitive to Fe deficiency. Iron imaging further showed that the remobilisation of Fe from vacuoles was highly impaired in mutant seedlings. Taken together, our results established AtDTX25 as a vacuolar ascorbate transporter, required during germination to promote the reduction of the pool of stored Fe³⁺ and its remobilisation to feed the developing seedling.

Large-scale genome-wide association study, using historical data, identifies conserved genetic architecture of cyanogenic glucoside content in cassava (Manihot esculenta Crantz) root

Manihot esculenta (cassava) is a root crop originating from South America that is a major staple in the tropics, including in marginal environments. This study focused on South American and African germplasm and investigated the genetic architecture of hydrogen cyanide (HCN), a major component of root quality. HCN, representing total cyanogenic glucosides, is a plant defense component against herbivory but is also toxic for human consumption. We genotyped 3354 landraces and modern breeding lines originating from 26 Brazilian states and 1389 individuals were phenotypically characterized across multi-year trials for HCN. All plant material was subjected to high-density genotyping using genotyping by sequencing. We performed genome-wide association mapping to characterize the genetic architecture and gene mapping of HCN. Field experiments revealed strong broad- and narrow-sense trait heritability (0.82 and 0.41, respectively). Two major loci were identified, encoding for an ATPase and a MATE protein, and contributing up to 7 and 30% of the HCN concentration in roots, respectively. We developed diagnostic markers for breeding applications, validated trait architecture consistency in African germplasm and investigated further evidence for the domestication of sweet and bitter cassava. Fine genomic characterization revealed: (i) the major role played by vacuolar transporters in regulating HCN content; (ii) the co-domestication of sweet and bitter cassava major alleles are dependent upon geographical zone; and (iii) the major loci allele for high HCN in M. esculenta Crantz seems to originate from its ancestor, M. esculenta subsp. flabellifolia. Taken together, these findings expand our insights into cyanogenic glucosides in cassava roots and its glycosylated derivatives in plants.

Membrane transporters: the key drivers of transport of secondary metabolites in plants

This review summarizes the recent updates in the area of transporters of plant secondary metabolites, including their applied aspects in metabolic engineering of economically important secondary metabolites. Plants have evolved biosynthetic pathways to produce structurally diverse secondary metabolites, which serve distinct functions, including defense against pathogens and herbivory, thereby playing a pivotal role in plant ecological interactions. These compounds often display interesting bioactivities and, therefore, have been used as repositories of natural drugs and phytoceuticals for humans. At an elevated level, plant secondary metabolites could be cytotoxic to the plant cell itself; therefore, plants have developed sophisticated mechanisms to sequester these compounds to prevent cytotoxicity. Many of these valuable natural compounds and their precursors are biosynthesized and accumulated at diverse subcellular locations, and few are even transported to sink organs via long-distance transport, implying the involvement of compartmentalization via intra- and intercellular transport mechanisms. The transporter proteins belonging to different families of transporters, especially ATP binding cassette (ABC) and multidrug and toxic compound extrusion (MATE) have been implicated in membrane-mediated transport of certain plant secondary metabolites. Despite increasing reports on the characterization of transporter proteins and their genes, our knowledge about the transporters of several medicinally and economically important plant secondary metabolites is still enigmatic. A comprehensive understanding of the molecular mechanisms underlying the whole route of secondary metabolite transportome, in addition to the biosynthetic pathways, will aid in systematic and targeted metabolic engineering of high-value secondary metabolites. The present review embodies a comprehensive update on the progress made in the elucidation of transporters of secondary metabolites in view of basic and applied aspects of their transport mechanism.

On the ion coupling mechanism of the MATE transporter ClbM

Bacteria use a number of mechanisms to defend themselves from antimicrobial drugs. One important defense strategy is the ability to export drugs by multidrug transporters. One class of multidrug transporter, the so-called multidrug and toxic compound extrusion (MATE) transporters, extrude a variety of antibiotic compounds from the bacterial cytoplasm. These MATE transporters are driven by a Na⁺, H⁺, or combined Na⁺/H⁺ gradient, and act as antiporters to drive a conformational change in the transporter from the outward to the inward-facing conformation. In the inward-facing conformation, a chemical compound (drug) binds to the protein, resulting in a switch to the opposite conformation, thereby extruding the drug. Using molecular dynamics simulations, we now report the structural basis for Na⁺ and H⁺ binding in the dual ion coupled MATE transporter ClbM from Escherichia coli, which is connected to colibactin-induced genotoxicity, yielding novel insights into the ion/drug translocation mechanism of this bacterial transporter.

Inward-facing conformation of a multidrug resistance MATE family transporter

Multidrug and toxic compound extrusion (MATE) transporters mediate excretion of xenobiotics and toxic metabolites, thereby conferring multidrug resistance in bacterial pathogens and cancer cells. Structural information on the alternate conformational states and knowledge of the detailed mechanism of MATE transport are of great importance for drug development. However, the structures of MATE transporters are only known in V-shaped outward-facing conformations. Here, we present the crystal structure of a MATE transporter from Pyrococcus furiosus (PfMATE) in the long-sought-after inward-facing state, which was obtained after crystallization in the presence of native lipids. Transition from the outward-facing state to the inward-facing state involves rigid body movements of transmembrane helices (TMs) 2-6 and 8-12 to form an inverted V, facilitated by a loose binding of TM1 and TM7 to their respective bundles and their conformational flexibility. The inward-facing structure of PfMATE in combination with the outward-facing one supports an alternating access mechanism for the MATE family transporters.

The Xenobiotic Extrusion Mechanism of the MATE Transporter NorM_PS from Pseudomonas stutzeri

Multidrug resistance (MDR) in bacterial pathogens has become a severe threat to public health. Membrane transporters of the multidrug and toxic compound extrusion (MATE) family contribute critically to MDR, making them promising drug targets. Despite recent advances, structures in different conformations and the mechanistic details of their antiport cycle are still elusive. Here we studied NorM_PS, a representative MATE transporter from Pseudomonas stutzeri, using biochemical assays in combination with hydrogen/deuterium exchange-mass spectrometry. Our results confirm that the antiport is proton dependent and electroneutral with a stoichiometry of two protons per one doubly positively charged substrate. We investigated the conformational dynamics upon substrate binding, and our hydrogen/deuterium exchange-mass spectrometry analysis revealed an occlusion in the proposed binding site as well as a closure of the cytoplasmic cavity and formation of a periplasmic cavity. Together with the results of selected variants (D38N, D373N and Q376A), we propose a six-step rocker-switch model for NorM_PS, which also increases our understanding of related MATE transporters and may help to fight the burden of MDR.

Genome-wide analysis of MATE transporters and molecular characterization of aluminum resistance in Populus

Ionic aluminum (Al) in acidic soils, comprising approximately 50% of arable land globally, is highly toxic to most plant species. Populus grow naturally in acidic soils and tolerate high concentrations of Al. Multidrug and toxic compound extrusion (MATE) family genes in plants are involved in responses to Al tolerance. To date, however, the functional roles of the MATE genes in Populus remain unclear. In the present study, 71 putative MATE transporters were predicted in the genome of Populus trichocarpa. The chromosome distribution, phylogenetic relationships, and expression level analysis revealed that four candidate MATE genes belonging to subgroup IIIc might contribute to high Al tolerance in poplar. Further, the expression levels of two subgroup IIIc members, PtrMATE1 and PtrMATE2, were induced by Al stress. Transient expression in onion epidermal cells showed that PtrMATE1 was localized to the plasma membrane. Overexpression of PtrMATE1 increased Al-induced secretion of citrate from the root apex of transgenic plants. Al-induced inhibition of root growths were alleviated in both PtrMATE1 overexpression lines in Populus and in Arabidopsis compared with wild-type plants. In addition, PtrMATE1 expression was induced at 12 h after exposure to Al stress whereas PtrMATE2 expression was induced at 24 h, indicating that these proteins coordinately function in response to Al stress in poplar. Taken together, these results provide important insights into the molecular mechanisms involved in Al tolerance in poplar.

Diverse functions of multidrug and toxin extrusion (MATE) transporters in citric acid efflux and metal homeostasis in Medicago truncatula

The multidrug and toxin extrusion (MATE) transporter family comprises 70 members in the Medicago truncatula genome, and they play seemingly important, yet mostly uncharacterized, physiological functions. Here, we employed bioinformatics and molecular genetics to identify and characterize MATE transporters involved in citric acid export, Al³⁺ tolerance and Fe translocation. MtMATE69 is a citric acid transporter induced by Fe-deficiency. Overexpression of MtMATE69 in hairy roots altered Fe homeostasis and hormone levels under Fe-deficient or Fe-oversupplied conditions. MtMATE66 is a plasma membrane citric acid transporter primarily expressed in root epidermal cells. The mtmate66 mutant had less root growth than the wild type under Al³⁺ stress, and seedlings were chlorotic under Fe-deficient conditions. Overexpression of MtMATE66 rendered hairy roots more tolerant to Al³⁺ toxicity. MtMATE55 is involved in seedling development and iron homeostasis, as well as hormone signaling. The mtmate55 mutant had delayed development and chlorotic leaves in mature plants. Both knock-out and overexpression mutants of MtMATE55 showed altered Fe accumulation and abnormal hormone levels compared with the wild type. We demonstrate that the zinc-finger transcription factor MtSTOP is essentially required for MtMATE66 expression and plant resistance to H⁺ and Al³⁺ toxicity. The proper expression of two previously characterized MATE flavonoid transporters MtMATE1 and MtMATE2 also depends on several transcription factors. This study reveals not only functional diversity of MATE transporters and regulatory mechanisms in legumes against H⁺ and Al³⁺ stresses, but also casts light on their role in metal nutrition and hormone signaling under various stresses.

LCP crystallization and X-ray diffraction analysis of VcmN, a MATE transporter from Vibrio cholerae

Multidrug and toxic compound extrusion (MATE) transporters, one of the multidrug exporter families, efflux xenobiotics towards the extracellular side of the membrane. Since MATE transporters expressed in bacterial pathogens contribute to multidrug resistance, they are important therapeutic targets. Here, a MATE-transporter homologue from Vibrio cholerae, VcmN, was overexpressed in Escherichia coli, purified and crystallized in lipidic cubic phase (LCP). X-ray diffraction data were collected to 2.5 Å resolution from a single crystal obtained in a sandwich plate. The crystal belonged to space group P212121, with unit-cell parameters a = 52.3, b = 93.7, c = 100.2 Å. As a result of further LCP crystallization trials, crystals of larger size were obtained using sitting-drop plates. X-ray diffraction data were collected to 2.2 Å resolution from a single crystal obtained in a sitting-drop plate. The crystal belonged to space group P212121, with unit-cell parameters a = 61.9, b = 91.8, c = 100.9 Å. The present work provides valuable insights into the atomic resolution structure determination of membrane transporters.

ELS1, a novel MATE transporter related to leaf senescence and iron homeostasis in Arabidopsis thaliana

The multidrug and toxic compound extrusion (MATE) transporters mediate the coupled exchange of organic substrates and monovalent cations have been recently implicated in various plant biological activities. In this work, we isolated a dominant mutant from an Arabidopsis activation-tagging mutant pool. This mutant exhibits pleiotropic phenotype including early flowering, dwarf and bushy architecture, minified lateral organs and early leaf senescence, and is therefore designated early leaf senescence 1-Dominaint (els1-D). Genotyping assays showed that els1-D is a gain-of-function mutant of a novel MATE transporter gene, ELS1, which encodes a close homolog of the previously reported ADP1, BCD1 and DTX50. Further investigations revealed that the overexpression of ELS1 reduces iron content in els1-D, and the accelerated senescence of the detached els1-D leaves can be recovered by exogenous iron supply. In addition, we also found that ELS1 is an iron responsive gene. Based on these findings, we proposed that ELS1 is related to leaf senescence and iron homeostasis in Arabidopsis.

Lipid Flippases for Bacterial Peptidoglycan Biosynthesis

The biosynthesis of cellular polysaccharides and glycoconjugates often involves lipid-linked intermediates that need to be translocated across membranes. Essential pathways such as N-glycosylation in eukaryotes and biogenesis of the peptidoglycan (PG) cell wall in bacteria share a common strategy where nucleotide-sugars are used to build a membrane-bound oligosaccharide precursor that is linked to a phosphorylated isoprenoid lipid. Once made, these lipid-linked intermediates must be translocated across a membrane so that they can serve as substrates in a different cellular compartment. How translocation occurs is poorly understood, although it clearly requires a transporter or flippase. Identification of these transporters is notoriously difficult, and, in particular, the identity of the flippase of lipid II, an intermediate required for PG biogenesis, has been the subject of much debate. Here, I will review the body of work that has recently fueled this controversy, centered on proposed flippase candidates FtsW, MurJ, and AmJ.

QTLs Regulating the Contents of Antioxidants, Phenolics, and Flavonoids in Soybean Seeds Share a Common Genomic Region

Soybean seeds are a rich source of phenolic compounds, especially isoflavonoids, which are important nutraceuticals. Our study using 14 wild- and 16 cultivated-soybean accessions shows that seeds from cultivated soybeans generally contain lower total antioxidants compared to their wild counterparts, likely an unintended consequence of domestication or human selection. Using a recombinant inbred population resulting from a wild and a cultivated soybean parent and a bin map approach, we have identified an overlapping genomic region containing major quantitative trait loci (QTLs) that regulate the seed contents of total antioxidants, phenolics, and flavonoids. The QTL for seed antioxidant content contains 14 annotated genes based on the Williams 82 reference genome (Gmax1.01). None of these genes encodes functions that are related to the phenylpropanoid pathway of soybean. However, we found three putative Multidrug And Toxic Compound Extrusion (MATE) transporter genes within this QTL and one adjacent to it (GmMATE1-4). Moreover, we have identified non-synonymous changes between GmMATE1 and GmMATE2, and that GmMATE3 encodes an antisense transcript that expresses in pods. Whether the polymorphisms in GmMATE proteins are major determinants of the antioxidant contents, or whether the antisense transcripts of GmMATE3 play important regulatory roles, awaits further functional investigations.

Arabidopsis pab1, a mutant with reduced anthocyanins in immature seeds from banyuls, harbors a mutation in the MATE transporter FFT

Forward genetics approaches have helped elucidate the anthocyanin biosynthetic pathway in plants. Here, we used the Arabidopsis banyuls (ban) mutant, which accumulates anthocyanins, instead of colorless proanthocyanidin precursors, in immature seeds. In contrast to standard screens for mutants lacking anthocyanins in leaves/stems, we mutagenized ban plants and screened for mutants showing differences in pigmentation of immature seeds. The pale banyuls1 (pab1) mutation caused reduced anthocyanin pigmentation in immature seeds compared with ban. Immature pab1 ban seeds contained less anthocyanins and flavonols than ban, but showed normal expression of anthocyanin biosynthetic genes. In contrast to pab1, introduction of a flavonol-less mutation into ban did not produce paler immature seeds. Map-based cloning showed that two independent pab1 alleles disrupted the MATE-type transporter gene FFT/DTX35. Complementation of pab1 with FFT confirmed that mutation in FFT causes the pab1 phenotype. During development, FFT promoter activity was detected in the seed-coat layers that accumulate flavonoids. Anthocyanins accumulate in the vacuole and FFT fused to GFP mainly localized in the vacuolar membrane. Heterologous expression of grapevine MATE-type anthocyanin transporter gene partially complemented the pab1 phenotype. These results suggest that FFT acts at the vacuolar membrane in anthocyanin accumulation in the Arabidopsis seed coat, and that our screening strategy can reveal anthocyanin-related genes that have not been found by standard screening.

Crystallographic study of a MATE transporter presents a difficult case in structure determination with low-resolution, anisotropic data and crystal twinning

NorM from Neisseria gonorrhoeae (NorM-NG) belongs to the multidrug and toxic compound extrusion (MATE) family of membrane-transport proteins, which can extrude cytotoxic chemicals across cell membranes and confer multidrug resistance. Here, the structure determination of NorM-NG is described, which had been hampered by low resolution (∼ 4 Å), data anisotropy and pseudo-merohedral twinning. The crystal structure was solved using molecular replacement and was corroborated by conducting a difference Fourier analysis. The NorM-NG structure displays an extracellular-facing conformation, similar to that of NorM-NG bound to a crystallization chaperone. The approaches taken to determine the NorM-NG structure and the lessons learned from this study are discussed, which may be useful for analyzing X-ray diffraction data with similar shortcomings.

A subgroup of MATE transporter genes regulates hypocotyl cell elongation in Arabidopsis

The growth of higher plants is under complex regulation to ensure the elaboration of developmental programmes under a changing environment. To dissect these regulatory circuits, we carried out genetic screens for Arabidopsis abnormal shoot (abs) mutants with altered shoot development. Here, we report the isolation of two dominant mutants, abs3-1D and abs4-1D, through activation tagging. Both mutants showed a 'bushy' loss of apical dominance phenotype. ABS3 and ABS4 code for two closely related putative Multidrug and Toxic Compound Extrusion (MATE) family of efflux transporters, respectively. ABS3 and ABS4, as well as two related MATE genes, ABS3-Like1 (ABS3L1) and ABS3L2, showed diverse tissue expression profiles but their gene products all localized to the late endosome/prevacuole (LE/PVC) compartment. The over-expression of these four genes individually led to the inhibition of hypocotyl cell elongation in the light. On the other hand, the quadruple knockout mutant (mateq) showed the opposite phenotype of an enhanced hypocotyl cell elongation in the light. Hypocotyl cell elongation and de-etiolation processes in the dark were also affected by the mutations of these genes. Exogenously applied sucrose attenuated the inhibition of hypocotyl elongation caused by abs3-1D and abs4-1D in the dark, and enhanced the hypocotyl elongation of mateq under prolonged dark treatment. We determined that ABS3 genetically interacts with the photoreceptor gene PHYTOCHROME B (PHYB). Our results demonstrate that ABS3 and related MATE family transporters are potential negative regulators of hypocotyl cell elongation and support a functional link between the endomembrane system, particularly the LE/PVC, and the regulation of plant cell elongation.

Towards understanding promiscuity in multidrug efflux pumps

Drug export from cells is a major factor in the acquisition of cellular resistance to antimicrobial and cancer chemotherapy, and poses a significant threat to future clinical management of disease. Many of the proteins that catalyse drug efflux do so with remarkably low substrate specificity, a phenomenon known as multidrug transport. For these reasons we need a greater understanding of drug recognition and transport in multidrug pumps to inform research that attempts to circumvent their action. Structural and computational studies have been heralded as being great strides towards a full elucidation of multidrug recognition and transport. In this review we summarise these advances and ask how close we are to a molecular understanding of this remarkable phenomenon.