The Regulation of Plant Phosphoenolpyruvate Carboxylase by Reversible Phosphorylation

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Phosphoenolpyruvate carboxylase (PEPc) is a multifaceted enzyme that serves different physiological functions in plants. In C3 plants, an important role is in the anaplerotic supply of carbon skeletons for biosynthetic functions such as amino acid synthesis, whereas C4 and crassulacean acid metabolism (CAM) species also have a specific, highly active isoform that catalyses primary CO2 fixation in the photosynthesis pathway. More effort has been concentrated to date on the regulation of the latter, photosynthetic form of PEPc. It has long been known that this form of the enzyme is subject to allosteric control by opposing photosynthesis-related metabolites in the cytosol of the mesophyll cells. The discovery of a phosphorylation process acting on photosynthetic PEPc revitalized interest in this enzyme and the ensuing wealth of data has highlighted signaling mechanisms acting in the regulation of plant metabolism. In C4 plants, the cascade depends upon a cross-talk between the two neighboring photosynthetic cell types, involves classical second messengers like pH, phosphoinositide-specific phospholipase C, inositol-1,4,5-trisphosphate and calcium, leading to up-regulation of the activity of a Ca2+-independent, C4 PEPc-specific protein-serine/threonine kinase (PEPcK), which finally phosphorylates PEPc. The final activity of C4 PEPc and the resulting carbon flux to bundle sheath cells are dependent on the mutual interaction between metabolite and covalent control mechanisms acting on this enzyme. Recent results have suggested that a similar regulatory circuit is operative at night in mesophyll cells of CAM leaves. It has become clear that the anaplerotic PEPc which is found in all plant types, is also regulated by a PEPcK and that phosphorylation of PEPc in C3 plant leaves functions in the coordination of carbon and nitrogen assimilation. We discuss the extent to which parallels can be drawn between the regulation of the different isoforms of PEPc.