Structural organization of proteins on the oxidizing side of photosystem II. Two molecules of the 33-kDa manganese-stabilizing proteins per reaction center
Abstract
The 33-kDa manganese-stabilizing protein stabilizes the manganese cluster in the oxygen-evolving complex. There has been, however, a considerable amount of controversy concerning the stoichiometry of this photosystem II (PS II) component. In this paper, we have verified the extinction coefficient of the manganese-stabilizing protein by amino acid analysis, determined the manganese content of oxygen-evolving photosystem II membranes and reaction center complex using inductively coupled plasma spectrometry, and determined immunologically the amount of the manganese-stabilizing protein associated with photosystem II. Oxygen-evolving photosystem II membranes and reaction center complex preparations contained 258 ± 11 and 67 ± 3 chlorophyll, respectively, per tetranuclear manganese cluster. Immunoquantification of the manganese-stabilizing protein using mouse polyclonal antibodies on 'Western blots' demonstrated the presence of 2.1 ± 0.2 and 2.0 ± 0.3 molecules of the manganese-stabilizing protein/tetranuclear manganese cluster in oxygen- evolving PS II membranes and highly purified PS II reaction center complex, respectively. Since the manganese-stabilizing protein co-migrated with the D2 protein in our electrophoretic system, accurate immunoquantification required the inclusion of CaCl2-washed PS II membrane proteins or reaction center complex proteins in the manganese-stabilizing protein standards to compensate for the possible masking effect of the D2 protein on the binding of the manganese-stabilizing protein to Immobilon-P membranes. Failure to include these additional protein components in the manganese-stabilizing protein standards leads to a marked underestimation of the amount of the manganese- stabilizing protein associated with these photosystem II preparations.