Pfam PF00283: Cytochrome b559, alpha (gene psbE) and beta (gene psbF)subunits (family)
Oxygenic photosynthesis uses two multi-subunit photosystems (I and II) located in the cell membranes of cyanobacteria and in the thylakoid membranes of chloroplasts in plants and algae. Photosystem II (PSII) has a P680 reaction centre containing chlorophyll 'a' that uses light energy to carry out the oxidation (splitting) of water molecules, and to produce ATP via a proton pump. Photosystem I (PSI) has a P700 reaction centre containing chlorophyll that takes the electron and associated hydrogen donated from PSII to reduce NADP+ to NADPH. Both ATP and NADPH are subsequently used in the light-independent reactions to convert carbon dioxide to glucose using the hydrogen atom extracted from water by PSII, releasing oxygen as a by-product .
PSII is a multisubunit protein-pigment complex containing polypeptides both intrinsic and extrinsic to the photosynthetic membrane . Within the core of the complex, the chlorophyll and Î²-carotene pigments are mainly bound to the antenna proteins CP43 (PsbC) and CP47 (PsbB), which pass the excitation energy on to the reaction centre proteins D1 (Qb, PsbA) and D2 (Qa, PsbD) that bind all the redox-active cofactors involved in the energy conversion process. The PSII oxygen-evolving complex (OEC) oxidises water to provide protons for use by PSI, and consists of OEE1 (PsbO), OEE2 (PsbP) and OEE3 (PsbQ). The remaining subunits in PSII are of low molecular weight (less than 10kDa), and are involved in PSII assembly, stabilisation, dimerisation, and photo-protection .
Cytochrome b559, which forms part of the reaction centre core of PSII is a heterodimer composed of one alpha subunit (PsbE), one beta (PsbF) subunit, and a haem cofactor. Two histidine residues from each subunit coordinate the haem. Although cytochrome b559 is a redox-active protein, it is unlikely to be involved in the primary electron transport in PSII due to its very slow photo-oxidation and photo-reduction kinetics. Instead, cytochrome b559 could participate in a secondary electron transport pathway that helps protect PSII from photo-damage. Cytochrome b559 is essential for PSII assembly .

This domain occurs in both the alpha and beta subunits of cytochrome B559. In the alpha subunit it occurs together with a lumenal domain (IPR013082), while in the beta subunit it occurs on its own.
Wikipedia: Cytochrome b559 is an important component of Photosystem II (PSII) is a multisubunit protein-pigment complex containing polypeptides both intrinsic and extrinsic to the photosynthetic membrane. Within the core of the complex, the chlorophyll and beta-carotene pigments are mainly bound to the antenna proteins CP43 (PsbC) and CP47 (PsbB), which pass the excitation energy on to chlorophylls in the reaction centre proteins D1 and D2 that bind all the redox-active cofactors involved in the energy conversion process. The PSII oxygen-evolving complex (OEC) provides electrons to re-reduce the PSII reaction center, and oxidizes 2 water molecules to recover its reduced initial state. It consists of OEE1 (PsbO), OEE2 (PsbP) and OEE3 (PsbQ). The remaining subunits in PSII are of low molecular weight, and are involved in PSII assembly, stabilisation, dimerization, and photoprotection.