A 22-kDa protein that interacts with the extrinsic 23- and 33-kDa proteins has been reported. It can be partially released from PSII by treatment with high salt concentration in the presence of small amounts of detergent. It has been suggested that the 22-kDa protein sits on top of the D1/D2 complex and and has a function in electron transport on the acceptor side of PSII.
PSII has been reported to contain at least one copper ion bound to an unknown protein. Copper deficiency results in the disappearance of a 29-kDa protein which suggests that the copper ion may be bound to a 29-kDa protein. It has also been suggested that the copper ion may be bound to a subform of LHCII, while others have suggested that the copper is bound to a 28-kDa protein.
A 10-kDa phosphoprotein and about eight other proteins with molecular masses below 10-kDa have also been reported. The 10-kDa phosphoprotein is a product of the chloroplast psbH gene. A 4.8-kDa protein, a product of the psbI gene copurifies with the PSII reaction center. A 5-kDa product of the chloroplast psbL gene has also been reported.