The N-terminal sequence of the extrinsic PsbP protein modulates the redox potential of Cyt b 559 in photosystem II

The PsbP protein, an extrinsic subunit of photosystem II (PSII) in green plants, is known to induce a conformational change around the catalytic Mn 4 CaO 5 cluster securing the binding of Ca 2+ and Cl – in PSII. PsbP has multiple interactions with the membrane subunits of PSII, but how these affect the structure and function of PSII requires clarification. Here, we focus on the interactions between the N-terminal residues of PsbP and the α subunit of Cytochrome (Cyt) b 559 (PsbE). A key observation was that a peptide fragment formed of the first N-terminal 15 residues of PsbP, ‘pN15’, was able to convert Cyt b 559 into its HP form. Interestingly, addition of pN15 to NaCl-washed PSII membranes decreased PSII’s oxygen-evolving activity, even in the presence of saturating Ca 2+ and Cl – ions. In fact, pN15 reversibly inhibited the S 1 to S 2 transition of the OEC in PSII. These data suggest that pN15 can modulate the redox property of Cyt b 559 involved in the side-electron pathway in PSII. This potential change of Cyt b 559 , in the absence of the C-terminal domain of PsbP, however, would interfere with any electron donation from the Mn 4 CaO 5 cluster, leading to the possibility that multiple interactions of PsbP, binding to PSII, have distinct roles in regulating electron transfer within PSII.