Effect of Protein Phosphorylation on the Functional Activity of Photosystem 2 under Photoinhibitory Conditions

The effect of protein phosphorylation on the functional activity of PS2 under control and photoinhibitory conditions has been investigated. The time course of phosphorylated and non-phosphorylated PS2 enriched membranes (BBYs) has been monitored by chlorophyll fluorescence using a PAM fluorimeter. The results show that photochemical activity did not change during treatment in the dark but decreased sharply as a result of high light (2000 μmol.m–2.s–1) in both phosphorylated and non-phosphorylated preparations. No difference could be detected between the polypeptide profiles of control and phosphorylated BBYs as judged by SDS-PAGE and Western blotting. The results suggest that the decrease of photochemical activity took place prior to protein degradation. In order to check if protein phosphorylation had some protective effect at lower light intensities we subjected the BBYs to 1000 μmol.m–2.s–1. The experiment showed that the photochemical activity of PS2 was slightly more resistant to photoinhibiiton in the phosphorylated compared with the non-phosphorylated samples. Again, there was no difference between the polypeptide profiles of the phosphorylated and non-phosphorylated preparations even after one hour of exposure to illumination. Overall the results indicate that the phosphorylation of PS2 proteins does not significantly influence the rate of photochemical inactivation of PS2 but rather may serve to regulate the conformational state of the complex.