Light Regulation of the Thylakoid LHCII Protein Phosphorylation at the Substrate Level

The distribution of light energy between the two photosystems as well as the light-induced turnover of PSII proteins are regulated by the reversible phosphorylation of LHCII and the PSII-core proteins. The thylakoid protein kinase(s) is activated by a signal transduction system involving the interaction of reduced plastoquinone with the quinol oxidation site of the cytochrome bf complex [1]. Phosphorylation of the mobile pool of LHCII induces dissociation of this antenna from PSII and allows its interaction with the PSI in the stroma exposed membranes (state transition)[21. Dephosphorylation of LHCII by a membrane -bound phosphatase appears to be regulated by a cyclophilinlike protein located in the thylakoid lumen [1, 31. So far, it was assumed that the role of light in the above processes is only to drive the photosynthetic electron flow thus reducing plastoquinone and triggering the signal transduction system resulting in the protein kinase(s) activation. In this work we have used an in vitro reconstituted system consisting of partially purified, solubilized thylakoid protein-kinase(s) and isolated native as well as recombinant LHCII. We demonstrate that light induces conformational changes in the LHCII structure exposing the specific threonine phosphorylation site to the kinase activity, thus suggesting a regulation of protein phosphorylation also at the substrate level.