A truncated mutant of the extrinsic 23-kDa protein that absolutely requires the extrinsic 17-kDa protein for Ca2+ retention in photosystem II.

One function of the extrinsic 23-kDa protein in photosystem II (OEC23) is to retain Ca(2+ )and Cl(-), two essential cofactors for photosynthetic oxygen evolution. A truncated mutant of OEC23 (OEC23 Delta19) revealed that 19 residues of the N-terminus of OEC23 were necessary for Ca(2+ )retention but not for its proper interaction with OEC17, the extrinsic 17-kDa protein in photosystem II. The lost ability of OEC23 Delta19 to reconstitute the oxygen-evolving activity was partially restored by OEC17 binding, suggesting the involvement of OEC17 in Ca(2+ )retention in photosystem II.