Control of Plant Enzyme Activity by Reversible Protein Phosphorylation

Protein phosphorylation is considered one of the most studied posttranslational-modification mechanisms affecting protein structure and function. This chapter reviews the regulation of plant enzymes by protein phosphorylation. It focuses on plant enzymes known to be regulated by reversible phosphorylation and explores how this mechanism of post-translational modification may serve to coordinate flux through major metabolic pathways. Most of the enzymes known to be phosphorylated are leaf proteins and the phosphorylation status of many of the phosphoproteins is altered in response to light. Phosphorylation of numerous chloroplast stromal proteins can be observed when intact spinach chloroplasts are provided [³²P]P_i in the light or when stromal extracts are given [γ-³²P] in vitro. One chloroplast stromal enzyme that undergoes reversible phosphorylation is pyruvate P_i dikinase (PPDK). This enzyme is found in highest activities in leaves of C₄ species and in certain crassulacean acid metabolism (CAM) plants, where it plays an important role in photosynthesis by catalyzing the regeneration of phosphoenolpyruvate (PEP), which is the primary CO₂ acceptor. The light or dark modulation of PPDK activity in leaves can be mimicked in vitro by an ADP-dependent inactivation and a P_i-dependent activation.