InterPro domain: IPR002644

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 ].

This family represents PsbZ (Ycf9), which is a core low molecular weight transmembrane protein of photosystem II in thylakoid-containing chloroplasts of cyanobacteria and plants. It is thought to be located at the interface of PSII and LHCII (light-harvesting complex II) complexes, the latter containing the light-harvesting antenna. PsbZ appears to act as a structural factor, or linker, that stabilises the PSII-LHCII supercomplexes, which fail to form in PsbZ-deficient mutants. This may in part be due to the marked decrease in two LHCII antenna proteins, CP26 and CP29, found in PsbZ-deficient mutants, which result in structural changes, as well as functional modifications in PSII [ 5 ]. PsbZ may also be involved in photo-protective processes under sub-optimal growth conditions.

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. The chloroplast gene ycf9 encodes a photosystem II (PSII) core subunit, PsbZ, that participates in PSII supramolecular architecture. Plant Cell 13, 1347-67