Regulation of photosynthesis by chloroplast protein phosphorylation

Abstract

Several polypeptides of the chloroplast photosynthetic membrane are reversibly phosphorylated in vivo and in vitro . The most conspicuous phosphoproteins belong to the light-harvesting chlorophyll a/b complex (LHC), which accounts for about half of the photons absorbed by the pigments of the photosynthetic membrane and can transfer excitation energy to either photosystem I or photosystem II. Phosphorylation of LHC increases (and dephosphorylation decreases) the proportion of excitation energy transferred to photosystem I at the expense of photosystem II. The LHC kinase is activated in vivo and in vitro by overexcitation of photosystem II and inactivated by overexcitation of photosystem I. The redox state of the plastoquinone pool governs the activity of the kinase and enables the photosynthetic membrane to detect and then correct any imbalance in the rate of excitation of the two photosystems. Reversible phosphorylation of LHC also enables the chloroplast to regulate the relative rates of cyclic and non-cyclic electron transport and thereby coordinates the rates of synthesis of ATP and NADPH with the demands of the Calvin cycle and other metabolic pathways operating within the organelle.