The phosphorylation of proteins within the eukaryotic photosynthetic membrane is thought to regulate a number of photosynthetic processes in land plants and algae. Both light quality and intensity influence protein kinase activity via the levels of reductants produced by the thylakoid electron transport chain. We have isolated a family of proteins called TAKs, Arabidopsis thylakoid membrane threonine kinases that phosphorylate the light harvesting complex proteins. TAK activity is enhanced by reductant and is associated with the photosynthetic reaction center II and the cytochrome b 6 f complex. TAKs are encoded by a gene family that has striking similarity to transforming growth factor  receptors of metazoans. Thus thylakoid protein phosphorylation may be regulated by a cascade of reductantcontrolled membrane-bound protein kinases.
To survive fluctuations in quality and intensity of light, plants and algae are able to preferentially direct the absorption of light energy to either one of the two photosystems, PSI or PSII. This rapid process is referred to as a state transition and has been correlated with the phosphorylation and migration of the light-harvesting complex protein (LHCP) between PSII and PSI. We show here that thylakoid protein kinases (TAKs) are required for state transitions in Arabidopsis. Antisense TAK1 expression leads to a loss of LHCP phosphorylation and a reduction in state transitions. Preferential activation of PSII causes LHCP to accumulate with PSI, and TAK1 mutants disrupt this process. Finally, TAKs also influence the phosphorylation of multiple thylakoid proteins.Photosynthetic organisms have developed the ability to adjust to changes in the intensity and quality of light they utilize for energy production. Alterations in light exposure can lead to the excessive stimulation of either one of the two photosystems and may result in unequal light utilization and damage (1, 2). Most organisms, especially vascular plants, are unable to avoid these light changes, and they have evolved several mechanisms to alter the distribution of light energy between photosystem I (PSI) 1 and photosystem II (PSII) (Fig. 1). These include transcriptional regulation of photosystem components (3), regulation of protein import and turnover (4, 5), nonphotochemical quenching (6), and the adjustment of the light harvesting capacity of the two photosystems. The latter process, a more rapid regulatory mechanism, is referred to as a state transition and is commonly measured by the differing levels of PSII fluorescence (2,7,8,9). The excessive stimulation of PSII has been correlated with the phosphorylation of light harvesting complex II proteins (LHCP) and the migration of LHCP from PSII to PSI (7). It has been proposed that this migration balances the light harvesting and thus reaction center activity (9). When energy utilization between the two photosystems is balanced, LHCP becomes dephosphorylated and moves from PSI back to PSII (1, 2). However, until recently the protein kinase(s) responsible for the critical phosphorylation step of LHCP had not been isolated (10), and therefore a direct test of this model was not possible.The current kinase activation model states that light absorption by PSII and PSI leads to energy transduction along the thylakoid membrane (Fig. 1A). Electron flow from PSII to PSI results in the reduction of the plastoquinone to plastoquinol. The Rieske Fe-S center, cytochrome (cyt) f, and plastocyanin are also reduced in the membrane (2). Binding of the plastoquinol (Q r ; Fig. 1) to the cytochrome b 6 f complex (cyt b 6 f) quinol oxidation site (Q ox ) activates a kinase (Refs. 2 and 11; Fig. 1B). The model predicts that the kinase then phosphorylates LHCP (creating LHCP P ), resulting in LHCP P migration from PSII to PSI (Fig. 1C). The association of LHCP with PSI requires the PSI-H subunit (12). There are several ...
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