title: Modeling of regulatory loops controlling galactolipid biosynthesis in the inner envelope membrane of chloroplasts.
abstract: In Angiosperms, the biosynthesis of galactolipids involves enzymes localized in the inner envelope membrane (IEM) of chloroplasts, including a phosphatidic acid phosphatase (PAP), dephosphorylating phosphatidic acid (PA) into diacylglycerol (DAG), and MGD1, transferring a galactose onto DAG thus generating monogalactosyldiacylglycerol (MGDG). It has been shown that PA and DAG could be synthesized in the plastid via the so-called 'prokaryotic' pathway or imported from the endoplasmic reticulum via the 'eukaryotic' pathway. In vitro studies support the existence of (1) a negative regulation of the plastid PAP by DAG and (2) an activation of MGD1 by PA. We developed a mathematical model of the IEM galactolipid biosynthesis pathway to understand the properties of the system ruled by the presence of these two regulatory motifs. We demonstrated that the design of the system implies that PA should accumulate to levels that are not observed experimentally, regardless of its prokaryotic or eukaryotic origin. PA should therefore be used for other syntheses, such as that of phosphatidylglycerol. Whereas a massive influx of eukaryotic PA appears unlikely, an influx of eukaryotic DAG in the IEM is supported by simulations. The model also implies that DAG cannot transiently accumulate and that PA mainly acts as a signal switching the whole system on. Eventually, this analysis highlights the fact that the PAP enzyme could easily become dispensable and that the design of the system, with the two regulatory motifs, could precede the loss of the PAP gene or activity in this pathway, a phenomenon that occurred independently in most clades of Angiosperms.
authors.affiliation: Laboratoire de Physiologie Cellulaire et Végétale, UMR 5168 CNRS-CEA-INRA-Univ., Grenoble Alpes, CEA Grenoble, 17 rue des Martyrs, 38054, Grenoble Cedex 09, France.
authors.forename: Eric
authors.initials: E
authors.lastname: Maréchal
authors.affiliation: Laboratoire de Physiologie Cellulaire et Végétale, UMR 5168 CNRS-CEA-INRA-Univ., Grenoble Alpes, CEA Grenoble, 17 rue des Martyrs, 38054, Grenoble Cedex 09, France. Electronic address: olivier.bastien@cea.fr.
authors.forename: Olivier
authors.initials: O
authors.lastname: Bastien
mesh_terms: D002736:Chloroplasts / Q000378:metabolism*; D038983:Galactolipids / Q000096:biosynthesis*; D007425:Intracellular Membranes / Q000378:metabolism*; D019684:Magnoliopsida / Q000378:metabolism*; D008954:Models, Biological
chemical_list: D038983:Galactolipids
keywords: Dynamical system; Galactolipid biosynthetic pathway; Metabolic control analysis; Metabolic processes