One of the extrinsic, lumenal subunits of photosystem II (PSII). PSII is a light-driven water plastoquinone oxidoreductase, using light energy to abstract electrons from H(2)O, generating a proton gradient subsequently used for ATP formation. The extrinsic proteins stabilize the structure of photosystem II oxygen-evolving complex (OEC), the ion environment of oxygen evolution and protect the OEC against heat-induced inactivation. Low-potential cytochrome c that plays a role in the OEC of PSII