Abstract
The enzyme UDP-glucose dehydrogenase (UGD) competes with sucrose-phosphate synthase for the common photosynthesis product UDP-glucose. Sucrose-phosphate synthase is part of a pathway for the export of sucrose from source leaves to neighboring cells or the phloem. UGD is a central enzyme in a pathway for many nucleotide sugars used in local cell wall biosynthesis. Here, we identify a highly conserved phosphorylation site in UGD which is readily phosphorylated by MAP-kinase 3 in Arabidopsis. Phosphorylation occurs at a surface-exposed extra loop in all plant UGDs that is absent in UGDs from bacteria or animals. Phosphorylated sucrose-phosphate synthase is shifted to an inactive form which we did not measure for phosphorylated UGD. Plant UGDs have an extra loop which is phosphorylated by AtMPK3. Phosphorylation is not causing a reduction of UGD activity as found for the competitor enzymes and thus sets a preference for maintaining UDP-sugars at a constant level to prioritize cell wall biosynthesis.
Highlights
The conversion of CO2 into carbohydrates by green plants is the basis for our environment and the primary food chain
The serine S393 within the –P-X-S-P- motif was found to be phosphorylated in phosphoproteomic studies (Benschop et al 2007; van Bentem et al 2008) confirming that this Ser-residue is phosphorylated in vivo at least in certain conditions
The enzyme UDP-glucose dehydrogenase is a competitor with sucrose-phosphate synthase in the cell for the shared substrate UDP-Glc
Summary
The conversion of CO2 into carbohydrates by green plants is the basis for our environment and the primary food chain. Fossil energy is only available in limited amounts and fixation of CO2 in energy rich compounds by plants is of particular importance. This includes the use as feed, as combustible material, and as a raw product for future biofuels. The partitioning of assimilates between the competing pathways at the cellular and at the organ level is of major importance for the energy conversion of sunlight into useful chemicals This partitioning is influenced by the regulation of a complex developmental program that decides, whether to use carbohydrates for storage (for example, in seeds) or whether to invest them in plant growth, tightly connected with the biosynthesis of new cell wall polymers
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