Formation of the S₂ state and structure of the Mn complex in photosystem II lacking the extrinsic 33 kilodalton polypeptide

Electron paramagnetic resonance (EPR) spectroscopy and O₂ evolution assays were performed on photosystem II (PSII) membranes which had been treated with 1 M CaCl₂ to release the 17, 23 and 33 kilodalton (kDa) extrinsic polypeptides. Manganese was not released from PSII membranes by this treatment as long as a high concentration of chloride was maintained. We have quantitated the EPR signals of the several electron donors and acceptors of PSII that are photooxidized or reduced in a single stable charge separation over the temperature range of 77 to 240 K. The behavior of the samples was qualitatively similar to that observed in samples depleted of only the 17 and 23 kDa polypeptides (de Paula et al. (1986) Biochemistry 25, 6487-6494). In both cases, the S₂ state multiline EPR signal was observed in high yield and its formation required bound Ca²⁺. The lineshape of the S₂ state multiline EPR signal and the magnetic properties of the manganese site were virtually identical to those of untreated PSII membranes. These results suggest that the structure of the manganese site is unaffected by removal of the 33 kDa polypeptide. Nevertheless, in samples lacking the 33 kDa polypeptide a stable charge separation could only be produced in about one half of the reaction centers below 160 K, in contrast to the result obtained in untreated or 17 and 23 kDa polypeptide-depleted PSII membranes. This suggests that one function of the 33 kDa polypeptide is to stabilize conformations of PSII that are active in secondary electron transfer events.