Influence of protein phosphorylation on the electron-transport properties of Photosystem II.

Many of the core proteins in Photosystem II (PS II) undergo reversible phosphorylation. It is known that protein phosphorylation controls the repair cycle of Photosystem II. However, it is not known how protein phosphorylation affects the partial electron transport reactions in PS II. Here we have applied variable fluorescence measurements and EPR spectroscopy to probe the status of the quinone acceptors, the Mn cluster and other electron transfer components in PS II with controlled levels of protein phosphorylation. Protein phosphorylation was induced in vivo by varying illumination regimes. The phosphorylation level of the D1 protein varied from 10 to 58% in PS II membranes isolated from pre-illuminated spinach leaves. The oxygen evolution and Q(A) (-) to Q(B)(Q(B) (-)) electron transfer measured by flash-induced fluorescence decay remained similar in all samples studied. Similar measurements in the presence of DCMU, which reports on the status of the donor side in PS II, also indicated that the integrity of the oxygen-evolving complex was preserved in PS II with different levels of D1 protein phosphorylation. With EPR spectroscopy we examined individual redox cofactors in PS II. Both the maximal amplitude of the charge separation reaction (measured as photo-accumulated pheophytin(-)) and the EPR signal from the Q(A) (-) Fe(2+) complex were unaffected by the phosphorylation of the D1 protein, indicating that the acceptor side of PS II was not modified. Also the shape of the S(2) state multiline signal was similar, suggesting that the structure of the Mn-cluster in Photosystem II did not change. However, the amplitude of the S(2) multiline signal was reduced by 35% in PS II, where 58% of the D1 protein was phosphorylated, as compared to the S(2) multiline in PS II, where only 10% of the D1 protein was phosphorylated. In addition, the fraction of low potential Cyt b (559) was twice as high in phosphorylated PS II. Implications from these findings, were precise quantification of D1 protein phosphorylation is, for the first time, combined with high-resolution biophysical measurements, are discussed.