Involved in the binding and/or turnover of quinones at the Q(B) site of Photosystem II. 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. Authors suggest the N-terminus may result from processing of a signal peptide. Authors suggest the N-terminus may result from processing of a signal peptide. Belongs to the PsbX family. Type 1 subfamily. Extended N-terminus. Extended N-terminus.