Maintenance of functional photosystem II by D1 protein turnover

Abstract

Water splitting of oxygenic photosynthesis produces various radicals and active oxygen species with harmful effects on PSII. Such photodamage to PSII occurs at all light intensities. Damaged PSII centers, however, do not usually accumulate in the thylakoid membrane due to a rapid and efficient repair mechanism. A grant design of PSII protects most of the protein components with the damage targeted to only one protein, the reaction center D1 protein. Repair of PSII via turnover of the D1 protein is a complex process. Central for such turnover and repair is a regulated reversible phosphorylation of PSII proteins, changes between dimer/monomer organization of PSII, migration of PS II between grana and stroma-exposed thylakoid domains, partial PSII disassembly and a multistep highly specific proteolysis of the D1 protein. Replacement of the damaged D1 with a new copy requires targeting of ribosome psbA mRNA complexes to the thylakoid membrane and subsequent light-regulated translation elongation. Concomitantly with D1 elongation, the protein is inserted into the thylakoid membrane, probably via a cpSecY-related translocation channel. Before termination of translation the nascent D1 chain starts interacting with other PSII proteins, particularly with the D2 protein, the presence of which seems to be a prerequisite for D1 elongation to be completed. CP47 is next integrated into the complex and finally, after termination of translation and maturation of the D1 protein, the CP43 protein is associated. Various assembly steps of PSII are highly regulated and require proper redox conditions and maintenance of the trans-thylakoid proton gradient.