Folate metabolism in plants - An Arabidopsis homolog of the mammalian mitochondrial folate transporter mediates folate import into chloroplasts
ABSTRACT The distribution of folates in plant cells suggests a complex traffic of the vitamin between the organelles and the cytosol. The Arabidopsis thaliana protein AtFOLT1 encoded by the At5g66380 gene is the closest homolog of the mitochondrial folate transporters (MFTs) characterized in mammalian cells. AtFOLT1 belongs to the mitochondrial carrier family, but GFP-tagging experiments and Western blot analyses indicated that it is targeted to the envelope of chloroplasts. By using the glycine auxotroph Chinese hamster ovary glyB cell line, which lacks a functional MFT and is deficient in folates transport into mitochondria, we showed by complementation that AtFOLT1 functions as a folate transporter in a hamster background. Indeed, stable transfectants bearing the AtFOLT1 cDNA have enhanced levels of folates in mitochondria and can support growth in glycine-free medium. Also, the expression of AtFOLT1 in Escherichia coli allows bacterial cells to uptake exogenous folate. Disruption of the AtFOLT1 gene in Arabidopsis does not lead to phenotypic alterations in folate-sufficient or folate-deficient plants. Also, the atfolt1 null mutant contains wild-type levels of folates in chloroplasts and preserves the enzymatic capacity to catalyze folate-dependent reactions in this subcellular compartment. These findings suggest strongly that, despite many common features shared by chloroplasts and mitochondria from mammals regarding folate metabolism, the folate import mechanisms in these organelles are not equivalent: folate uptake by mammalian mitochondria is mediated by a unique transporter, whereas there are alternative routes for folate import into chloroplasts.
- SourceAvailable from: Edmund R S Kunji
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- "The matrix gate mutations include residues of the signature motif PX[DE]XX[RK] that are highly conserved in all carriers and can therefore not have a discriminatory role in substrate recognition (Figure 5B). Carriers transporting NAD + (Todisco et al. 2006, Palmieri et al. 2009), pyrimidine nucleotides (Marobbio et al. 2006, Floyd et al. 2007), FAD/ folate (Tzagoloff et al. 1996, Titus and Moran, 2000, Bedhomme et al. 2005), and coenzyme A, FAD and NAD + in peroxisomes (Agrimi et al. 2012a, 2012b, Bernhardt et al. 2012) have a conserved W instead of [DE] in the second signature motif; FAD/folate carriers and ATP-Mg 2+ /phosphate carriers (Fiermonte et al. 2004, Traba et al. 2008, 2009) have a glutamine and [QNAT], respectively, instead of the negatively charged residue of the third signature motif; dicarboxylate carriers (Palmieri et al. 1996b, 2008, Fiermonte et al. 1998) have an asparagine or methionine instead of the positively charged residue of the second motif; the fungal oxaloacetate/sulphate/ a-isopropylmalate carrier (Palmieri et al. 1999, Marobbio et al. 2008) lacks the negatively charged residue and the positively charged residue in the second and third motifs, the former residue being replaced by [FY] and the latter by [LM]; and the phosphate carriers (Runswick et al. 1987, Dolce et al. 1994, Wohlrab and Briggs 1994) have a hydrophobic substitution instead of the positively charged residue in the third motif. These mostly polar modifications would be capable to either cation-p or hydrogen bond interactions, which have an interaction energy that is approximately half of an ionic bond. "
ABSTRACT: Abstract Mitochondrial carriers transport inorganic ions, nucleotides, amino acids, keto acids and cofactors across the mitochondrial inner membrane. Structurally they consist of three domains, each containing two transmembrane α-helices linked by a short α-helix and loop. The substrate binds to three major contact points in the central cavity. The class of substrate (e.g., adenine nucleotides) is determined by contact point II on transmembrane α-helix H4 and the type of substrate within the class (e.g., ADP, coenzyme A) by contact point I in H2, whereas contact point III on H6 is most usually a positively charged residue, irrespective of the type or class. Two salt bridge networks, consisting of conserved and symmetric residues, are located on the matrix and cytoplasmic side of the cavity. These residues are part of the gates that are involved in opening and closing of the carrier during the transport cycle, exposing the central substrate binding site to either side of the membrane in an alternating way. Here we revisit the plethora of mutagenesis data that have been collected over the last two decades to see if the residues in the proposed binding site and salt bridge networks are indeed important for function. The analysis shows that the major contact points of the substrate binding site are indeed crucial for function and in defining the specificity. The matrix salt bridge network is more critical for function than the cytoplasmic salt bridge network in agreement with its central position, but neither is likely to be involved in substrate recognition directly.Molecular Membrane Biology 03/2013; 30(2):149-159. DOI:10.3109/09687688.2012.737936 · 1.69 Impact Factor
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- "online). MCF members are not confined to mitochondria but are also found in other cell compartments, namely, chloroplasts, peroxisomes , the endoplasmic reticulum, or the plasma membrane (Bedhomme et al., 2005; Bouvier et al., 2006; Thuswaldner et al., 2007; Kirchberger et al., 2008; Leroch et al., 2008; Linka et al., 2008; Palmieri et al., 2008, 2009; Rieder and Neuhaus, 2011; Bernhardt et al., 2012). They mediate the transport of various substrates, including nucleotides, mainly in an antiport manner (Palmieri et al., 2011). "
ABSTRACT: 3'-Phosphoadenosine 5'-phosphosulfate (PAPS) is the high-energy sulfate donor for sulfation reactions. Plants produce some PAPS in the cytosol, but it is predominantly produced in plastids. Accordingly, PAPS has to be provided by plastids to serve as a substrate for sulfotransferase reactions in the cytosol and the Golgi apparatus. We present several lines of evidence that the recently described Arabidopsis thaliana thylakoid ADP/ATP carrier TAAC transports PAPS across the plastid envelope and thus fulfills an additional function of high physiological relevance. Transport studies using the recombinant protein revealed that it favors PAPS, 3'-phosphoadenosine 5'-phosphate, and ATP as substrates; thus, we named it PAPST1. The protein could be detected both in the plastid envelope membrane and in thylakoids, and it is present in plastids of autotrophic and heterotrophic tissues. TAAC/PAPST1 belongs to the mitochondrial carrier family in contrast with the known animal PAPS transporters, which are members of the nucleotide-sugar transporter family. The expression of the PAPST1 gene is regulated by the same MYB transcription factors also regulating the biosynthesis of sulfated secondary metabolites, glucosinolates. Molecular and physiological analyses of papst1 mutant plants indicate that PAPST1 is involved in several aspects of sulfur metabolism, including the biosynthesis of thiols, glucosinolates, and phytosulfokines.The Plant Cell 10/2012; 24(10). DOI:10.1105/tpc.112.101964 · 9.34 Impact Factor
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- "At first glance Arabidopsis does not encode a clearly identifiable second FOLT isoform. The absence of a visible phenotype in Arabidopsis mutant plants lacking functional FOLT1 however is indicative for the existence of an alternative plastidial folate transport system (Bedhomme et al., 2005). Interestingly, a transporter of the major facilitator superfamily was shown to mediate FAD transport into plastids (Klaus et al., 2005). "
ABSTRACT: Mitochondria play a key role in respiration and energy production and are involved in multiple eukaryotic but also in several plant specific metabolic pathways. Solute carriers in the inner mitochondrial membrane connect the internal metabolism with that of the surrounding cell. Because of their common basic structure, these transport proteins affiliate to the mitochondrial carrier family (MCF). Generally, MCF proteins consist of six membrane spanning helices, exhibit typical conserved domains and appear as homodimers in the native membrane. Although structurally related, MCF proteins catalyze the specific transport of various substrates, such as nucleotides, amino acids, dicarboxylates, cofactors, phosphate or H(+). Recent investigations identified MCF proteins also in several other cellular compartments and therefore their localization and physiological function is not only restricted to mitochondria. MCF proteins are a characteristic feature of eukaryotes and bacterial genomes lack corresponding sequences. Therefore, the evolutionary origin of MCF proteins is most likely associated with the establishment of mitochondria. It is not clear whether the host cell, the symbiont, or the chimerical organism invented the ancient MCF sequence. Here, we try to explain the establishment of different MCF proteins and focus on the characteristics of members from plants, in particular from Arabidopsis thaliana.Frontiers in Plant Science 01/2012; 3:2. DOI:10.3389/fpls.2012.00002 · 3.95 Impact Factor