Protein Domain : Photosystem II PsbR IPR006814

Type	Family
Description	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 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 []. This family represents the low molecular weight intrinsic protein PsbR found in PSII, which is also known as the 10kDa polypeptide. The PsbR gene is found only in the nucleus of green algae and higher plants. PsbR may provide a binding site for the extrinsic oxygen-evolving complex protein PsbP to the thylakoid membrane. PsbR has a transmembrane domain to anchor it to the thylakoid membrane, and a charged N-terminal domain capable of forming ion bridges with extrinsic proteins, allowing PsbR to act as a docking protein. PsbR may be a pH-dependent stabilising protein that functions at both donor and acceptor sides of PSII [ ].
Short Name	PSII_PsbR