Abstract
Main conclusionA synthetic peptide from the C-terminal end of C4-phosphoenolpyruvate carboxylase is implicated in the proteolysis of the enzyme, and Glc-6P or phosphorylation of the enzyme modulate this effect.Phosphoenolpyruvate carboxylase (PEPC) is a cytosolic, homotetrameric enzyme that performs a variety of functions in plants. Among them, it is primarily responsible for CO2 fixation in the C4 photosynthesis pathway (C4-PEPC). Here we show that proteolysis of C4-PEPC by cathepsin proteases present in a semi-purified PEPC fraction was enhanced by the presence of a synthetic peptide containing the last 19 amino acids from the C-terminal end of the PEPC subunit (pC19). Threonine (Thr)944 and Thr948 in the peptide are important requirements for the pC19 effect. C4-PEPC proteolysis in the presence of pC19 was prevented by the PEPC allosteric effector glucose 6-phosphate (Glc-6P) and by phosphorylation of the enzyme. The role of these elements in the regulation of PEPC proteolysis is discussed in relation to the physiological context.
Highlights
Phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) catalyzes the addition of bicarbonate to phosphoenolpyruvate (PEP) to form oxaloacetate, which is reduced to malate by the enzyme malate dehydrogenase (MDH)
The family of plant-type PEPCs (PTPCs) in sorghum includes four C3-type PEPCs and the photosynthetic C4-PEPC (Paterson et al 2009). C4-PEPC catalyzes the first carboxylation step in C4 photosynthesis, and gene expression is activated during the greening of the C4 leaf when PEPC accumulates in the cytosol of mesophyll cells as required for the functioning of the C4 pathway (Chollet et al 1996). C4-PEPC has been further studied in relation to its catalytic and regulatory properties and the biochemical and signaling mechanisms that control its subcellular activity (Chollet et al 1996; Echevarría and Vidal 2003; Izui et al 2004). C3-PEPCs are key enzymes in the metabolism of carbon and nitrogen, 1 3 Vol.:(0123456789)
All PTPCs have a conserved N-terminal seryl residue that is phosphorylated by PEPC kinases (PEPCks; Echevarría and Vidal 2003), whereas this residue is absent from bacterial-type PEPC (BTPC; Sánchez and Cejudo 2003)
Summary
Phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) catalyzes the addition of bicarbonate to phosphoenolpyruvate (PEP) to form oxaloacetate, which is reduced to malate by the enzyme malate dehydrogenase (MDH). C4-PEPC catalyzes the first carboxylation step in C4 photosynthesis, and gene expression is activated during the greening of the C4 leaf when PEPC accumulates in the cytosol of mesophyll cells as required for the functioning of the C4 pathway (Chollet et al 1996). C4-PEPC has been further studied in relation to its catalytic and regulatory properties and the biochemical and signaling mechanisms that control its subcellular activity (Chollet et al 1996; Echevarría and Vidal 2003; Izui et al 2004). Monoubiquitination of Arabidopsis C3-PEPC is selectively degraded by autophagy (Baena et al 2021) It is not known whether C 4-PEPC is modified by monoubiquitination, and evidence of a possible degradation mechanism controlling the amount of C4-PEPC in the cytosol of mesophyll cells is lacking
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