Stabilizes the structure of photosystem II oxygen-evolving complex (OEC), the ion environment of oxygen evolution and protects the OEC against heat-induced inactivation. Part of the oxygen-evolving complex of photosystem II. Associated with photosystem II at the lumenal side of the thylakoid membrane. Might be translocated into the thylakoid lumen by the Tat system. The position of the signal and transit peptide cleavages have not been experimentally proven. Belongs to the PsbU family.