Archives

  • 2018-07
  • 2019-04
  • 2019-05
  • 2019-06
  • 2019-07
  • 2019-08
  • 2019-09
  • 2019-10
  • 2019-11
  • 2019-12
  • 2020-01
  • 2020-02
  • 2020-03
  • 2020-04
  • 2020-05
  • 2020-06
  • 2020-07
  • 2020-08
  • 2020-09
  • 2020-10
  • 2020-11
  • 2020-12
  • 2021-01
  • 2021-02
  • 2021-03
  • 2021-04
  • 2021-05
  • 2021-06
  • 2021-07
  • 2021-08
  • 2021-09
  • 2021-10
  • 2021-11
  • 2021-12
  • 2022-01
  • 2022-02
  • 2022-03
  • 2022-04
  • 2022-05
  • 2022-06
  • 2022-07
  • 2022-08
  • 2022-09
  • 2022-10
  • 2022-11
  • 2022-12
  • 2023-01
  • 2023-02
  • 2023-03
  • 2023-04
  • 2023-05
  • 2023-06
  • 2023-08
  • 2023-09
  • 2023-10
  • 2023-11
  • 2023-12
  • 2024-01
  • 2024-02
  • 2024-03
  • 2024-04

    • Some members from the AAAP family

    2023-01-27

    Some members from the AAAP family, namely the AUX1 and the LAX proteins, transport auxin instead of amino acids [48]. While there is some structural relationship between IAA and Trp, the substrate recognition characteristics seem different for these proteins: the auxins do not have any amino group on the α-carbon, and the molecules that compete with IAA transport contain one or two aromatic rings and one carboxylic group [48]. While sequence-related to amino Bryostatin 1 transporters, the AUX-LAX proteins have modified substrate-binding properties, which are at least based on a carboxylic group. Another member of an amino acid transporter family, WAT1/UMAMIT5, has been described as a vacuolar transporter involved in the import of IAA from the vacuole to the cytosol [49,50]. The results from these studies suggest that transport is dependent on the proton-motive force, but the transport mechanism, the Km for IAA, and the substrate specificity remain to be determined. Meticulous study of AtGAT1, a GABA transporter from the AAAP family related to the ProTs, has shown that amino acids are not a good substrate of AtGAT1. In addition, it appeared that the presence of a terminal carboxylic group is not required for transport, but that the terminal amino group is critical for transport, independent of the length of the transported molecule [51]. The binding pocket thus seems different from the transporters whose substrates are amino acids, since the latter require the presence of both amino and carboxylic groups on the α-carbon (see above). Some members of the APC family show another substrate specificity: the L-Type Amino acid Transporters/Putrescine Transporters (LAT/PUT) transport polyamines [52], which are amino compounds not structurally related to amino acids, but contain terminal amine groups. Mutants in some LAT/PUT genes display enhanced resistance to the herbicide paraquat, a structural analog of polyamines that induces oxidative stress and cell death. As a final example of the diversity of possible substrates, some amino acid transporters have been shown to transport synthetic compounds: a recent paper showed that the uptake of a derivative of the insecticide fipronyl by Ricinus plants is competitively inhibited by amino acids, suggesting that its transport across membranes is mediated by amino acid importers. Furthermore, the authors identified some amino acid transporter genes that are induced by the application of the compound, suggesting that they may be good candidates for mediating its uptake [21].
    Amino acid transporters as target for crop improvement and resistance to pathogens?
    Amino acid sensors in plants Amino acid metabolism is regulated by feedback inhibition and both transcriptional and post-translational enzyme activity (for a review see [2]). Yet, how amino acids are sensed and the corresponding signaling pathways are not understood in plants. Gent and Forde [77] have recently reviewed the status of our knowledge on amino acid sensing in plants. They identified five potential sensing processes involving different types of proteins: (1) the PII protein, which is thought to control Arg biosynthesis and fatty acid metabolism in the plant chloroplast; (2) the TOR signaling pathway, which receives input from unidentified amino acid sensors and carbon status to promote protein synthesis, and is a negative regulator of protein turnover; (3) the GCN2 protein kinase pathway that senses uncharged tRNAs and inhibits protein synthesis (but only a few steps of the signaling cascade are presently identified); (4) genes similar to glutamine synthetase type I that could also be involved in amino acid sensing, but here again little is known about their function; (5) glutamate receptors that have been extensively studied in Arabidopsis, and even if their ability to mediate Ca2+ transport to the cytosol upon binding extracellular amino acids is proven, the downstream events and their role as putative amino acid sensor are currently unclear (see [77] and references therein). It thus appears that, while the list of candidate genes is long, no amino acid or nitrogen sensor has been unequivocally identified in plants. We earlier raised the question of plant cells being able to sense alteration of amino acid flow to the apoplasm during pathogen infection, which leads to the question of how cells sense external amino acids. Glutamate receptors are certain candidates, but looking to sensors identified in yeast and mammals points towards other kinds of proteins that warrant further study.