Newly synthesized PsbO is reversibly attached to the thylakoid membrane and released from the membrane under light stress

The 33 kDa protein (PsbO) is an important component of the photosystem II reaction center and located at the lumenal side of the thylakoid membrane. In vivo labelling assays of tobacco seedlings and subsequent immunoprecipitation of PsbO revealed that newly synthesized PsbO is present in the lumen as soluble and membrane-associated forms. The latter form appears to be associated with photosystem II. Since membrane association also occurs in darkness and in the presence of chloramphenicol, no newly-formed photosystem II particles are required. Comparable features with regard to the membrane association and membrane release of PsbO were obtained with an in vitro reconstitution system. In contrast to the D1 protein, the overall amount of PsbO does not change significantly under light stress; however, the polypeptide appears to be released from the membranes and accumulates in the thylakoid lumen. Re-association with the membrane does not occur under light-stress conditions. The conclusion is that membrane association of PsbO is sensitive to high light intensitites.