Phosphorylation/dephosphorylation of D1 protein alone has no effect on the electron transport activity of photosystem II in soybean leaves

Abstract

D1 protein, as a core component of the photosystem II (PSII) reaction center complex, can be reversibly phosphorylated/dephosphorylated in higher plants. The phosphorylated D1 protein (D1*) and non-phosphorylated D1 protein (D1), having slightly different mobilities on denaturing polyacrylamide gels, can be resolved by using SDS-PAGE and Western Blotting [Callahan et al, J Biol Chem 1990, 265: 15357-15360]. The relationship between the phosphorylation of D1 protein and the electron transport activity of PSII was investigated by changing the phosphorylation level of D1 proteins with an inhibitor of protein kinase, 5¿-p-fluorosulfonylbenzoyl adenosine (FSBA). The following results were obtained. (1) D1* could be up to 74% of total D1 proteins in fully dark-adapted soybean leaves and could be completely dephosphorylated by FSBA (1 mM) treatment for 2 hours. (2) Dephosphorylation of D1* resulted in neither substantial net loss of D1 proteins nor the significant changes of chlorophyll a fluorescence parameters. (3) The electron transport activity of PS II (H2O ® 1,4-BQ) had no significant change after D1* was completely dephosphorylated. Based on these results, it is concluded that phosphorylation/dephosphorylation of D1 protein alone has no significant effect on the function of PS II reaction centers in soybean leaves.