Inorganic Cofactor Stabilization and Retention: The Unique Functions of the Two PsbO Subunits of Eukaryotic Photosystem II

Eukaryotic PsbO, the photosystem II (PSII) manganese-stabilizing protein, has two N-terminal sequences that are required for binding of two copies of the protein to PSII [Popelkova, H., et al. (2002) Biochemistry 41, 10038-10045; Popelkova, H., et al. (2003) Biochemistry 42, 6193-6200]. In the work reported here, a set of selected N-terminal truncation mutants of PsbO that affect subunit binding to PSII were used to determine the effects of PsbO stoichiometry on the Mn, Ca2+, and Cl- cofactors and to characterize the roles of each of the PsbO subunits in PSII function. Results of the experiments with the PsbO-depleted PSII membranes reconstituted with the PsbO deletion mutants showed that the presence of PsbO does not affect Ca2+ retention by PSII in steady-state assays of activity, nor is it required for Ca2+ to protect the Mn cluster against reductive inhibition in darkness. In contrast to the results with Ca2+, PsbO increases the affinity of Cl- for the active site of the O-2-evolving complex (OEC) as expected. These results together with other data on activity retention suggest that PsbO can stabilize the Mn cluster by facilitating retention of Cl- in the OEC. The data presented here indicate that each of two copies of PsbO has a distinctive function in PSII. Binding of the first PsbO subunit fully stabilizes the Mn cluster and enhances Cl- retention, while binding of the second subunit optimizes Cl- retention, which in turn maximizes O-2 evolution activity. Nonspecific binding of some PsbO truncation mutants to PSII has no functional significance.