Stabilizes the structure of photosystem II oxygen-evolving complex (OEC), the ion environment of oxygen evolution and protects the OEC against heat-induced inactivation. The oxygen-evolving complex may be composed of PsbO, PsbQ', PsbV and PsbU. 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 transit peptide cleavages have not been experimentally proven. Belongs to the PsbU family.