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, Ca(2+), 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 Ca(2+) retention by PSII in steady-state assays of activity, nor is it required for Ca(2+) to protect the Mn cluster against reductive inhibition in darkness. In contrast to the results with Ca(2+), 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.
Popelkova
Hana
H
Department of Molecular, Cellular and Developmental Biology, The University of Michigan, Ann Arbor, Michigan 48109-1048, USA.
Commet
Alan
A
Kuntzleman
Thomas
T
Yocum
Charles F
CF
D003067:Coenzymes / Q000378:metabolism*; D003624:Darkness; D007287:Inorganic Chemicals / Q000378:metabolism*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D017384:Sequence Deletion; D018724:Spinacia oleracea / Q000201:enzymology*; D013379:Substrate Specificity; D014508:Urea / Q000378:metabolism
D003067:Coenzymes; D007287:Inorganic Chemicals; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; C052389:oxygen-evolving enhancer protein 1, plant; D014508:Urea; D010100:Oxygen