Received July 12, 2007
Accepted August 20, 2007
Regulation of C1 Metabolism in Arabidopsis: The N-terminal Regulatory Domain of Cystathionine 
Synthase Is Cleaved in Response to Folate Starvation
Karen Loizeau , Bernadette Gambonnet , Guo-Fang Zhang , Gilles Curien , Samuel Jabrin , Dominique Van Der Straeten , Willy E. Lambert , Fabrice Rébeillé , and Stéphane Ravanel *
Laboratoire de Physiologie Cellulaire Végétale, UMR5168 CNRS-CEA-INRA-Université Joseph Fourier Grenoble I, Institut de Recherches en Technologies et Sciences pour le Vivant, CEA-Grenoble, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France; Laboratory of Toxicology, Ghent University, Harelbekestraat 72, B-9000 Ghent, Belgium; Unit Plant Hormone Signaling and Bio-imaging, Department of Molecular Genetics, Ghent University, K.L. Ledeganckstraat 35, B-9000 Ghent, Belgium
* Corresponding author; email: sravanel{at}cea.fr.
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 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-adenosylmethionine (AdoMet) and Met pools declined during the initial period of folate starvation but were further restored to typical levels. Re-establishment of Met and AdoMet homeostasis was concomitant with a previously unknown post-translational regulation that consists in the removal of 92 amino acids at the N-terminus of cystathionine 
-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.
