Phosphorylation of PSII Proteins in Low Light Grown Maize in Response to the Pb Ions

Abstract

Reversible protein phosphorylation plays a crucial role in the regulation of numerous cellular functions and signal transduction pathways. In thylakoids light- and redox-induced activation or deactivation of protein phosphorylation is involved in structural changes of these membranes and in regulation of protein turnover. Photosynthesis in C4 plants involves mesophyll (M) and bundle sheath (BS) chloroplasts, which differ structurally and functionally. We studied maize plants in which lead was introduced into detached leaves with transpiration stream. Thylakoids were isolated mechanically and then proteins were analyzed. We observed that PSII activity was not affected by Pb ions in M chloroplasts, whereas in BS it was reduced. The presence of Pb ions affected only slightly photochemical efficiency of PSII (Fv/Fm ratio). Protein phosphorylation in mesophyll and bundle sheath thylakoids was analyzed using mass spectrometry and western blotting before and after lead treatment. Both methods clearly demonstrated increase in phosphorylation of the PSII proteins upon treatment with the heavy metal. We found that D1 and PsbH proteins of PSII complex were strongly phosphorylated in the presence of Pb ions. These results suggest that Pb 2+ stimulates phosphorylation of PSII core proteins, which affects the stability of PSII complex by controlling the conversion of dimeric PSII to its monomeric form and in this way regulates the rate of D1 protein degradation. Therefore changes in phosphorylation of PSII core proteins induced by Pb ions may be a crucial regulation step in protection mechanism important for stabilization the dimeric PSII complex in stress conditions. Our results show that acclimation to Pb ions was achieved in both types of maize chloroplasts in the same way. However, these processes are obviously more complex because of different metabolic status in M and BS chloroplasts.