One of the components of the core complex of photosystem II (PSII), required for the stability, but not for the assembly of dimeric PSII and PSII-outer antenna supercomplexes (PSII-LHCII), and for assembly of the functional Q(A)-binding site (PubMed:16920705). PSII is a light-driven water:plastoquinone oxidoreductase that uses light energy to abstract electrons from H(2)O, generating O(2) and a proton gradient subsequently used for ATP formation. It consists of a core antenna complex that captures photons, and an electron transfer chain that converts photonic excitation into a charge separation. 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, at least 3 peripheral proteins of the oxygen-evolving complex and a large number of cofactors. It forms dimeric complexes. Able to grow under greenhouse conditions, increased sensitivity to high light. Able to assemble dimeric PSII and PSII-LHCII complexes, but they are not stable. Almost complete loss of phosphorylation of D1, D2 and CP43 (PsbA, PsbD and PsbC respectively), altered phosphorylation of LHCII. Belongs to the PsbI family.