Regulation of thylakoid protein phosphorylation: Characterization of novel enzymes, mechanisms and physiological relevance for acclimation to light changes

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Phosphorylation of the light-harvesting complex II (LHCII) and PSII core proteins requires the kinases STN7 and STN8, respectively. LHCII dephosphorylation depends on the phosphatase TAP38. The PSII core protein phosphatase and an additional PSII core protein kinase still await their identification. Other open fundamental questions concern the actual physiological relevance of LHCII and PSII core protein phosphorylation, and the mechanisms by which STN7 is regulated and by which STN7 mediates the gene expression changes associated with the long-term photosynthetic acclimation (LTR). Moreover, although STN8 is re-quired for modulation of thylakoid ultrastructure, we found that this process does not require PSII core protein phosphorylation. To identify the PSII core protein phosphatase and the additional PSII core protein kinase we will combine genomics and subcellular localisation experiments. To clarify the actual physiological role of thylakoid protein phosphorylation we will characterise in detail mutants and overexpressor lines for the known thylakoid protein kinases and phosphatases. Moreover, we will identify and characterize those substrates of STN8 that confer the ultrastructural changes. In preliminary experiments we found that STN7 abundance and LHCII phosphorylation correlate; therefore, we will analyse STN7 expression at the transcript and protein level. To identify the substrate of STN7 that mediates LTR signalling, as well as the LTR target genes, comparative chloroplast phosphoproteome and whole-genome transcriptome analyses will be em-ployed.