Protein involved in photosystem II (PSII), a complex that uses light energy to mediate electron flows in the chloroplast thylakoid membrane of plants and in cyanobacteria. PSII splits water, releasing hydrogen ions and molecular oxygen. PSII cannot use photons of a wavelength greater than 680 nm, and is thus sometimes called P680. PSII is a quinone type rection center (RC), sharing a common ancestor with the RCs of purple bacteria.