One of the components of the core complex of photosystem II (PSII), required for its stability and/or assembly. 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, numerous small proteins, at least 3 peripheral proteins of the oxygen-evolving complex and a large number of cofactors. It forms dimeric complexes. PSII is more abundant in mesophyll cells than bundle sheath cells. Phosphorylated in both bundle sheath and mesophyll cells, phosphorylation increases when cells are grown under high rather than low light regimes (70 vs 900 umol photons/m-2/s). Double phosphorylation in bundle sheath cells is only seen when cells are grown under high light regimes. Phosphorylation is a light-dependent reaction catalyzed by a membrane-bound kinase; phosphorylation occurs on Thr residue(s) in the N-terminus of the protein. Belongs to the PsbH family.