One of the components of the core complex of photosystem II (PSII). 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. This subunit is found at the monomer-monomer interface. Probably involved in dimerization of PSII; at the monomer-monomer interface the only protein-protein contacts observed are between the 2 PsbM subunits. Lipids, chlorophylls and carotenoids contribute strongly to PSII dimerization. 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, at least 3 peripheral proteins PsbO, PsbU, PsbV and a large number of cofactors. It forms dimeric complexes. Decreased yields of PSII dimers with increased PSII monomers, slightly slower photoautotrophic growth, about 20% less oxygen evolution. Formed PSII dimers are less stable than wild-type. Belongs to the PsbM family.