Minor Constituents of Photosystem II Core Complexes: Possible Regulators of Photosystem II Protein Kinase

We recently identified a 58-kDa protein kinase, PS II-PK, closely associated in substoichiometric abundance with a core complex of photosystem II and capable of phosphorylating both the photosystem and its associated light-harvesting proteins. Oxidizing species, produced during aerobic illumination, inhibited the kinase, and the irreversibility of this process is now demonstrated. Other substoichiometric protein constituents of the core preparation were identified as probably originating in the grana margins. These include the cytochrome b6/f complex, the apocytochrome f precursor, and membrane-bound ferredoxin:NADP+oxidoreductase. PS II-PK was successfully resolved from photosystem II cores and shown to be active in the absence of cytochrome complex.