Stabilizes the structure of photosystem II oxygen-evolving complex (OEC), the ion environment of oxygen evolution and protects the OEC against heat-induced inactivation (By similarity). The oxygen-evolving complex in red algae is composed of PsbO (OEC33), PsbQ', cytochrome c-550 and PsbU. Associated with photosystem II at the lumenal side of the thylakoid membrane. Predicted to be translocated into the thylakoid lumen by the Tat system. Belongs to the PsbU family.