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.
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ABSTRACT: This last decade, many efforts were undertaken to understand how coenzymes, including vitamins, are synthesized in plants. Surprisingly, these metabolic pathways were often "quartered" between different compartments of the plant cell. Among these compartments, mitochondria often appear to have a key role, catalyzing one or several steps in these pathways. In the present review we will illustrate these new and important biosynthetic functions found in plant mitochondria by describing the most recent findings about the synthesis of two vitamins (folate and biotin) and one non-vitamin coenzyme (lipoate). The complexity of these metabolic routes raise intriguing questions, such as how the intermediate metabolites and the end-product coenzymes are exchanged between the various cellular territories, or what are the physiological reasons, if any, for such compartmentalization.Photosynthesis Research 06/2007; 92(2):149-62. DOI:10.1007/s11120-007-9167-z · 3.19 Impact Factor
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ABSTRACT: Chloroplasts are bounded by a pair of outer membranes, the envelope, that is the only permanent membrane structure of the different types of plastids. Chloroplasts have had a long and complex evolutionary past and integration of the envelope membranes in cellular functions is the result of this evolution. Plastid envelope membranes contain a wide diversity of lipids and terpenoid compounds serving numerous biochemical functions and the flexibility of their biosynthetic pathways allow plants to adapt to fluctuating environmental conditions (for instance phosphate deprivation). A large body of knowledge has been generated by proteomic studies targeted to envelope membranes, thus revealing an unexpected complexity of this membrane system. For instance, new transport systems for metabolites and ions have been identified in envelope membranes and new routes for the import of chloroplast-specific proteins have been identified. The picture emerging from our present understanding of plastid envelope membranes is that of a key player in plastid biogenesis and the co-ordinated gene expression of plastid-specific protein (owing to chlorophyll precursors), of a major hub for integration of metabolic and ionic networks in cell metabolism, of a flexible system that can divide, produce dynamic extensions and interact with other cell constituents. Envelope membranes are indeed one of the most complex and dynamic system within a plant cell. In this review, we present an overview of envelope constituents together with recent insights into the major functions fulfilled by envelope membranes and their dynamics within plant cells.Photosynthesis Research 06/2007; 92(2):225-44. DOI:10.1007/s11120-007-9195-8 · 3.19 Impact Factor
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ABSTRACT: S-Adenosylmethionine (SAM) is formed exclusively in the cytosol but plays a major role in plastids; SAM can either act as a methyl donor for the biogenesis of small molecules such as prenyllipids and macromolecules or as a regulator of the synthesis of aspartate-derived amino acids. Because the biosynthesis of SAM is restricted to the cytosol, plastids require a SAM importer. However, this transporter has not yet been identified. Here, we report the molecular and functional characterization of an Arabidopsis thaliana gene designated SAM TRANSPORTER1 (SAMT1), which encodes a plastid metabolite transporter required for the import of SAM from the cytosol. Recombinant SAMT1 produced in yeast cells, when reconstituted into liposomes, mediated the counter-exchange of SAM with SAM and with S-adenosylhomocysteine, the by-product and inhibitor of transmethylation reactions using SAM. Insertional mutation in SAMT1 and virus-induced gene silencing of SAMT1 in Nicotiana benthamiana caused severe growth retardation in mutant plants. Impaired function of SAMT1 led to decreased accumulation of prenyllipids and mainly affected the chlorophyll pathway. Biochemical analysis suggests that the latter effect represents one prominent example of the multiple events triggered by undermethylation, when there is decreased SAM flux into plastids.The Plant Cell 12/2006; 18(11):3088-105. DOI:10.1105/tpc.105.040741 · 9.58 Impact Factor