Abstract
Nitrogen is an essential mineral nutrient and it is often transported within living organisms in its reduced form, as amino acids. Transport of amino acids across cellular membranes requires proteins, and here we report the phylogenetic analysis across taxa of two amino acid transporter families, the amino acid permeases (AAPs) and the lysine–histidine-like transporters (LHTs). We found that the two transporter families form two distinct groups in plants supporting the concept that both are essential. AAP transporters seem to be restricted to land plants. They were found in Selaginella moellendorffii and Physcomitrella patens but not in Chlorophyte, Charophyte, or Rhodophyte algae. AAPs were strongly represented in vascular plants, consistent with their major function in phloem (vascular tissue) loading of amino acids for sink nitrogen supply. LHTs on the other hand appeared prior to land plants. LHTs were not found in chlorophyte algae Chlamydomonas reinhardtii and Volvox carterii. However, the characean alga Klebsormidium flaccidum encodes KfLHT13 and phylogenetic analysis indicates that it is basal to land plant LHTs. This is consistent with the hypothesis that characean algae are ancestral to land plants. LHTs were also found in both S. moellendorffii and P. patens as well as in monocots and eudicots. To date, AAPs and LHTs have mainly been characterized in Arabidopsis (eudicots) and these studies provide clues to the functions of the newly identified homologs.
Highlights
Nitrogen is a critical mineral nutrient in all living organisms since it is required for synthesis of a large number of compounds including hormones, nucleotides, and amino acids
amino acid permeases (AAPs) AND LHT TRANSPORTERS FORM TWO DISTINCT GROUPS Database searches for AAP and LHT proteins in red algae, green algae, basal non-vascular and vascular land plants, and seed plants resulted in 44 AAP and 39 LHT protein sequences (Figure 2; Tables 1 and 2)
Phylogenetic analysis supports that AAPs, which generally present moderate and low affinity systems for neutral and acidic amino acids, are important to land plants with a main function in phloem loading and that they are not required in red algae or green algae (Charophytes or Chlorophytes)
Summary
Nitrogen is a critical mineral nutrient in all living organisms since it is required for synthesis of a large number of compounds including hormones, nucleotides, and amino acids. Amino acid uptake into cells and cellular compartments depends on membraneintegral transporter proteins, and amino acid transporters have been identified in many organisms including bacteria, fungi, animals, and plants (Chang et al, 2004; Boudko, 2010). Amino acid transporters are found in two families within the amino acid–polyamine–choline (APC) transporter superfamily, the amino acid/auxin permease (AAAP), and the APC family. The AAAP family includes transporters from plants, animals, and fungi (Chang et al, 2004), and in plants contains the amino acid permeases (AAPs), lysine–histidine-like transporters (LHTs), proline transporters (ProTs), γ-aminobutyric acid transporters (GATs), ANT1-like aromatic, and neutral amino acid transporters and auxin transporters (AUXs; Wipf et al, 2002; Rentsch et al, 2007). Cationic amino acid transporters (CATs) belong to the APC family and are present in both animals and plants1)
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