Requirements for different combinations of the extrinsic proteins in specific cyanobacterial photosystem II mutants

The extrinsic proteins of Photosystem II

In this review we examine the structure and function of the extrinsic proteins of Photosystem II. These proteins include PsbO, present in all oxygenic organisms, the PsbP and PsbQ proteins, which are found in higher plants and eukaryotic algae, and the PsbU, PsbV, CyanoQ, and CyanoP proteins, which are found in the cyanobacteria. These proteins serve to optimize oxygen evolution at physiological calcium and chloride concentrations. They also shield the Mn(4)CaO(5) cluster from exogenous reductants. Numerous biochemical, genetic and structural studies have been used to probe the structure and function of these proteins within the photosystem. We will discuss the most recent proposed functional roles for these components, their structures (as deduced from biochemical and X-ray crystallographic studies) and the locations of their proposed binding domains within the Photosystem II complex. This article is part of a Special Issue entitled: Photosystem II.

The lipoproteins of cyanobacterial photosystem II

Photosystem II (PSII) complexes from cyanobacteria and plants perform water splitting and plastoquinone reduction and yet have a different complement of lumenal extrinsic proteins. Whereas PSII from all organisms has the PsbO extrinsic protein, crystal structures of PSII from cyanobacteria have PsbV and PsbU while green algae and higher plants instead contain the extrinsic PsbP and PsbQ subunits. Proteomic studies in Synechocystis sp. PCC 6803 identified three further extrinsic proteins in the thylakoid lumen that are associated with cyanobacterial PSII and these are predicted to attach to the thylakoid membrane via a lipidated N-terminus. These proteins are cyanobacterial homologues to the PsbP and PsbQ subunits as well as to Psb27, an additional extrinsic protein associated with "inactive" photosystems that lack the other extrinsic polypeptides. The PsbQ homologue is not present in Prochlorococcus species but otherwise these proteins have been identified in most cyanobacteria although our phylogenetic analyses identified some strains that lack an apparent motif for lipidation in one or other of these subunits. Over the past decade the physiological function of these additional lipoproteins has been investigated in several cyanobacterial strains and recently the structures for each have been solved. This review will evaluate the physiological and structural results obtained for these lipid-attached extrinsic proteins and in silico protein docking of these proteins to PSII centers will be presented.

Structural and functional aspects of the MSP (PsbO) and study of its differences in thermophilic versus mesophilic organisms

The Manganese Stabilizing Protein (MSP) of Photosystem II (PSII) is a so-called extrinsic subunit, which reversibly associates with the other membrane-bound PSII subunits. The MSP is essential for maximum rates of O(2) production under physiological conditions as stabilizes the catalytic [Mn(4)Ca] cluster, which is the site of water oxidation. The function of the MSP subunit in the PSII complex has been extensively studied in higher plants, and the structure of non-PSII associated MSP has been studied by low-resolution biophysical techniques. Recently, crystal structures of PSII from the thermophilic cyanobacterium Thermosynechococcus elongatus have resolved the MSP subunit in its PSII-associated state. However, neither any crystal structure is available yet for MSP from mesophilic organisms, higher plants or algae nor has the non-PSII associated form of MSP been crystallized. This article reviews the current understanding of the structure, dynamics, and function of MSP, with a particular focus on properties of the MSP from T. elongatus that may be attributable to the thermophilic ecology of this organism rather than being general features of MSP.

The PsbP-like protein (sll1418) of Synechocystis sp. PCC 6803 stabilises the donor side of Photosystem II

The PsbP-like protein of the cyanobacterium Synechocystis sp. PCC 6803 is a peripheral component of Photosystem II, located at the lumenal side of the thylakoid membrane. Removal of this protein leads to decreased competitive potential of a PsbP-like deletion mutant when grown in a mixture with wild-type cells. Flash-induced oxygen evolution traces of the mutant show a higher probability of misses, correlated with increased amplitudes of the S-states decay in the dark. Thermoluminescence emission traces demonstrate a changed charge recombination pattern in the mutant, the S(3)Q(B)(-) couple becoming the major species instead of the S(2)Q(B)(-). Our data suggest a possible role of the PsbP-like protein in stabilisation of the charge separation in Photosystem II of cyanobacteria through interaction with the Mn cluster.

Expression, assembly and auxiliary functions of photosystem II oxygen-evolving proteins in higher plants

The oxygen-evolving complex (OEC) of higher plant photosystem II (PSII) consists of an inorganic Mn(4)Ca cluster and three nuclear-encoded proteins, PsbO, PsbP and PsbQ. In this review, we focus on the assembly of these OEC proteins, and especially on the role of the small intrinsic PSII proteins and recently found "novel" PSII proteins in the assembly process. The numerous auxiliary functions suggested during the past few years for the OEC proteins will likewise be discussed. For example, besides being a manganese-stabilizing protein, PsbO has been found to bind calcium and GTP and possess a carbonic anhydrase activity. In addition, specific roles have been suggested for the two isoforms of the PsbO protein in Arabidopsis thaliana. PsbP and PsbQ seem to play an additional role in the formation of PSII supercomplexes and in grana stacking, besides their originally recognized role in providing a proper calcium and chloride ion concentration for water splitting.

The extrinsic proteins of Photosystem II

Years of genetic, biochemical, and structural work have provided a number of insights into the oxygen evolving complex (OEC) of Photosystem II (PSII) for a variety of photosynthetic organisms. However, questions still remain about the functions and interactions among the various subunits that make up the OEC. After a brief introduction to the individual subunits Psb27, PsbP, PsbQ, PsbR, PsbU, and PsbV, a current picture of the OEC as a whole in cyanobacteria, red algae, green algae, and higher plants will be presented. Additionally, the role that these proteins play in the dynamic life cycle of PSII will be discussed.