Psb28 is a 13 kDa soluble protein that is directly assembled in dimeric PSII supercomplexes. The negatively charged N-terminal region is essential for this process [1]. This protein was formerly known as PsbW, but PsbW is now reserved for Pfam:PF07123.

1: Novel approach reveals localisation and assembly pathway of the PsbS and PsbW proteins into the photosystem II dimer. Thidholm E, Lindstrom V, Tissier C, Robinson C, P\tSchroder W, Funk C; FEBS Lett 2002;513:217-222. PMID:11904154