DEGRADATION OF THE D1-PROTEIN AND D2-PROTEIN OF PHOTOSYSTEM-II IN HIGHER-PLANTS IS REGULATED BY REVERSIBLE PHOSPHORYLATION

The effects of protein phosphorylation and dephosphorylation upon high -light-induced degradation of the photosystem II, reaction center prot eins D1 and D2 have been studied in isolated thylakoid membranes and p hotosystem II core complexes. The rate of photoinactivation of photosy stem II electron transport is not affected by thylakoid membrane phosp horylation. However, the degradation rate of the D1-protein in its pho sphorylated form is drastically reduced under conditions which induce either acceptor-or donor-side photoinhibition of photosystem II. The d egradation rate of the D2-protein is also reduced following protein ph osphorylation. The stability of the phosphorylated D1-protein is furth er increased under conditions of reduced phosphatase activity, suggest ing that phosphorylated and damaged D1-protein has to be dephosphoryla ted prior to proteolytic degradation. Our results expand on experiment s performed in vivo, which suggest that following photoinhibition the controlled repair of damaged photosystem II centers requires not only proteolytic enzymes but also kinase and phosphatase activities. It is suggested that the phosphorylation of the D1- and D2-proteins allows t ight coordination of the degradation of damaged proteins with insertio n of new copies of proteins into photosystem II.