Loosely associated component of the core 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. PSII binds multiple chlorophylls, carotenoids and specific lipids. Cyanobacterial PSII is composed of 1 copy each of membrane proteins PsbA, PsbB, PsbC, PsbD, PsbE, PsbF, PsbH, PsbI, PsbJ, PsbK, PsbL, PsbM, PsbT, PsbX, PsbY, PsbZ, Ycf12, 3 peripheral proteins PsbO, PsbU, PsbV and a large number of cofactors. It forms dimeric complexes. This protein is only loosely associated with PSII, and is not often found in crystals. Belongs to the PsbY family.