Phosphorylation of PS II polypeptides inhibits D1 protein-degradation and increases PS II stability |
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Authors: | Volker Ebbert Doris Godde |
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Institution: | (1) Lehrstuhl für Biochemie der Pflanzen, Ruhr-Universität, D-44780 Bochum, Germany |
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Abstract: | To study the significance of Photosystem (PS) II phosphorylation for the turnover of the D1 protein, phosphorylation was compared with the synthesis and content of the D1 protein in intact chloroplasts. As shown by radioactive labelling with 32Pi] phosphorylation of PS II polypeptides was saturated at light intensities of 125 mol m-2 s-1. Under steady state conditions, in intact chloroplasts D1 protein, once it was phosphorylated, was neither dephosphorylated nor degraded in the light. D1 protein-synthesis was measured as incorporation of 14C] leucine. As shown by non-denaturing gel-electrophoresis followed by SDS-PAGE newly synthesised D1 protein was assembled to intact PS II-centres and no free D1 protein could be detected. D1 protein-synthesis was saturated at light intensities of 500 mol m-2 s-1. The content of D1 protein stayed stable even after illumination with 5000 mol m-2 s-1 showing that D1 protein-degradation was saturated at the same light intensities. The difference in the light saturation points of phosphorylation and of D1 protein-turnover indicates a complex regulation of D1 protein-turnover by phosphorylation. Separation of the phosphorylated and dephosphorylated D1 protein by LiDS-gelelectrophoresis combined with radioactive pulse-labelling with 14C] leucine and 32Pi] revealed that D1 protein, synthesised under steady state conditions in the light, did not become phosphorylated but instead was rapidly degraded whereas the phosphorylated form of the D1 protein was not a good substrate for degradation. According to these observations phosphorylation of the D1 protein creates a pool of PS II centres which is not involved in D1 to these observations phosphorylation of the D1 protein creates a pool of PS II centres which is not involved in D1 protein-turnover. Fractionation of thylakoid membranes confirms that the phosphorylated, non-turning over pool of PS II-centres was located in the central regions of the grana, whereas PS II-centres involved in D1 protein-turnover were found exclusively in the stroma-lamellae and in the grana-margins.Abbreviations chl
chlorophyll
- Fv
yield of variable fluorescence, difference between Fm, the maximal fluorescence yield at saturating light, when all reaction-centres are closed, and Fo, the fluorescence yield in the dark, when all reaction-centres are open
- LHC
light harvesting complex
- PFD
photon flux density
- PS
photosystem |
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Keywords: | D1 protein-turnover lateral distribution photoinhibition phosphorylation Photosystem II stability |
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