Photosystem II: Assembly and Turnover of the D1 Protein

Photosystem II (PSII), a light-driven water:plastoquinone oxidoreductase, is an integral membrane protein complex of ~ 25 subunits. PSII resides in the thylakoid membrane of organisms performing oxygenic photosynthesis, that is, in chloroplasts of higher plants, mosses, ferns, and algae, as well as in prokaryotic cyanobacteria. The synthesis and assembly of a new PSII complex in eukaryotic organisms require cooperation of both chloroplast and nuclear genomes. Besides structural protein subunits, PSII contains pigments and cofactors involved in the photochemistry of PSII. Most of the cofactors are ligated to the reaction center proteins D1 and D2. Such an arrangement together with the strongly oxidizing photochemistry renders the PSII reaction center to photodamage and accelerates the turnover rate of PSII subunits. The primary target of photodamage and the most rapidly turning-over subunit of PSII is the D1 protein. This protein has to be constantly replaced in order to maintain the PSII complex in an active state. This dynamic process, known as the PSII photoinhibition repair cycle, occurs in the thylakoid membrane and its rate increases with increasing light intensity.