Stabilizes the structure of photosystem II oxygen-evolving complex (OEC), the ion environment of oxygen evolution and protects the OEC against heat-induced inactivation. With PsbO and PsbV forms the oxygen-evolving complex of photosystem II. This protein is not detected by antibodies against green algal PsbU but it is detected by antibodies against red algal PsbU (PubMed:16176274). 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.