One of the components of the core complex of photosystem II (PSII). Essential for PSII function, it may be important for unidirectionality of electron flow (PubMed:11546758, PubMed:12655396, PubMed:14979726) and/or assembly of the oxygen evolving complex (PubMed:11827973). 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 assemble functional PSII, however electron flow is deregulated; forward flow from Q(A) to plastoquinone is slower while reduction of the oxygen evolving complex is faster, loss of light-activated, redox-dependent phosphorylation of LCHII, plants senesce quickly (PubMed:11546758, PubMed:12655396, PubMed:14979726). Plants are hypersensitive to light, cannot grow photoautotrophically and have drastically reduced oxygen-evolving capacity. Dramatic reduction in PSI subunits, almost complete loss of PsbP, a subunit of the oxygen-evolving complex (PubMed:11827973). Belongs to the PsbJ family.