InterPro domain: IPR002683

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 [ 1 , 2 , 3 ]. 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 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 [ 4 ].

In PSII, the oxygen-evolving complex (OEC) is responsible for catalysing the splitting of water to O(2) and 4H+. The OEC is composed of a cluster of manganese, calcium and chloride ions bound to extrinsic proteins. In cyanobacteria there are five extrinsic proteins in OEC (PsbO, PsbP-like, PsbQ-like, PsbU and PsbV), while in plants there are only three (PsbO, PsbP and PsbQ), PsbU and PsbV having been lost during the evolution of green plants [ 5 ].

This entry represents the C-terminal domain found in PSII OEC protein PsbP. Both PsbP and PsbQ ( IPR008797 ) are regulators that are necessary for the biogenesis of optically active PSII. PsbP increases the affinity of the water oxidation site for chloride ions and provides the conditions required for high affinity binding of calcium ions [ 6 , 7 ]. The crystal structure of PsbP from Nicotiana tabacum (Common tobacco) revealed a two-domain structure, where domain 1 may play a role in the ion retention activity in PSII, the N-terminal residues being essential for calcium and chloride ion retention activity [ 8 ]. PsbP is encoded in the nuclear genome in plants.

1. Crystal structure of oxygen-evolving photosystem II from Thermosynechococcus vulcanus at 3.7-A resolution. Proc. Natl. Acad. Sci. U.S.A. 100, 98-103
2. The evolutionary development of the protein complement of photosystem 2. Biochim. Biophys. Acta 1655, 133-9
3. Thylakoid membrane lipid sulfoquinovosyl-diacylglycerol (SQDG) is required for full functioning of photosystem II in Thermosynechococcus elongatus. J Biol Chem 293, 14786-14797
4. The low molecular mass subunits of the photosynthetic supracomplex, photosystem II. Biochim. Biophys. Acta 1608, 75-96
5. Homologs of plant PsbP and PsbQ proteins are necessary for regulation of photosystem ii activity in the cyanobacterium Synechocystis 6803. Plant Cell 16, 2164-75
6. Nucleotide sequence of the psbP gene encoding precursor of 23-kDa polypeptide of oxygen-evolving complex in Arabidopsis thaliana and its expression in the wild-type and a constitutively photomorphogenic mutant. DNA Res. 3, 277-85
7. Photoactivation and photoinhibition are competing in a mutant of Chlamydomonas reinhardtii lacking the 23-kDa extrinsic subunit of photosystem II. J. Biol. Chem. 271, 28918-24
8. Crystal structure of the PsbP protein of photosystem II from Nicotiana tabacum. EMBO Rep. 5, 362-7