Low pH-Induced Dissociation of Three Extrinsic Proteins from O2-Evolving Photosystem II

Three extrinsic proteins involved in oxygen evolution reversibly dissociated from Photo-system II (PS II) membranes at acidic pHs showing distinctly different pH dependencies. The pHs for half dissociation of 17, 23 and 33 kDa extrinsic proteins were determined to be 5.0, 4.1 and 3.6, respectively. The half dissociation pHs of 17 and 23 kDa proteins were much lower than their respective isoelectric points, while that for 33 kDa protein was close to its isoelectric point. It was suggested that protonation of the negatively charged binding domain on membrane proteins causes dissociation of the former two extrinsic proteins, whereas protonation of the extrinsic protein itself is responsible for the dissociation of 33 kDa protein. Based on these, features of low pH-induced dissociation of extrinsic proteins and Mn from PS II were discussed.