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Regulation of One-Carbon Metabolism in Arabidopsis: The N-Terminal Regulatory Domain of Cystathionine γ-Synthase Is Cleaved in Response to Folate Starvation

Laboratoire de Physiologie Cellulaire Végétale, Institut National de la Recherche Agronomique, Université Joseph Fourier Grenoble I, Grenoble, France.
Plant physiology (Impact Factor: 7.39). 11/2007; 145(2):491-503. DOI: 10.1104/pp.107.105379
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ABSTRACT In all organisms, control of folate homeostasis is of vital importance to sustain the demand for one-carbon (C1) units that are essential in major metabolic pathways. In this study we induced folate deficiency in Arabidopsis (Arabidopsis thaliana) cells by using two antifolate inhibitors. This treatment triggered a rapid and important decrease in the pool of folates with significant modification in the distribution of C1-substituted folate coenzymes, suggesting an adaptive response to favor a preferential shuttling of the flux of C1 units to the synthesis of nucleotides over the synthesis of methionine (Met). Metabolic profiling of folate-deficient cells indicated important perturbation of the activated methyl cycle because of the impairment of Met synthases that are deprived of their substrate 5-methyl-tetrahydrofolate. Intriguingly, S-adenosyl-Met and Met pools declined during the initial period of folate starvation but were further restored to typical levels. Reestablishment of Met and S-adenosyl-Met homeostasis was concomitant with a previously unknown posttranslational modification that consists in the removal of 92 amino acids at the N terminus of cystathionine gamma-synthase (CGS), the first specific enzyme for Met synthesis. Rescue experiments and analysis of different stresses indicated that CGS processing is specifically associated with perturbation of the folates pool. Also, CGS processing involves chloroplastic serine-type proteases that are expressed in various plant species subjected to folate starvation. We suggest that a metabolic effector, to date unidentified, can modulate CGS activity in vivo through an interaction with the N-terminal domain of the enzyme and that removal of this domain can suppress this regulation.

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    • "CH 3 - THF pool was selected for analysis in this experiment , as previous studies have indi - cated that , under folate deficiency , most folate - dependent transcriptional and metabolic alterations are correlated with the cellular levels of this folate species ( Loizeau et al . , 2007 , 2008"
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    • "Taken as a whole, these results are indicative of important modifications within the methyl cycle activity, Hcy and AdoHcy concentrations being the most affected by the MTX treatment. Similar observations were previously reported for Arabidopsis cells having a folate concentration lowered by 25-fold (Loizeau et al., 2007). Our present data indicate that these events also occur with a moderate decrease of the folate content, suggesting a tight coupling between methyl cycle and folate synthesis. "
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    • "The crystal structure of tobacco cystathionine γ-synthase shows that the Nterminal domain is outside of the protein globule and probably accessible to proteases (Steegborn et al., 1999). Similar to the case of the internally deleted protein (Hacham et al., 2006), proteolytic removal of the regulatory domain increases cystathionine γ-synthase activity and thereby the production of methionine and S-adenosylmethionine (Loizeau et al., 2007). Cystathionine β-lyase (EC 4.4.1.8), "
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