Polypeptides of PS II

The polypeptides of the light-harvesting complex are most easily resolved by polyacrylamide gel electrophoresis of thylakoid membranes solubilized with sodium or lithium dodecyl sulphate. This produces the green chlorophyll-protein band, CP II, which on staining for protein reveals up to four polypeptides of 24-27 kDa [2], each of which is believed to bind Chi a and Chi b. In most plants two polypeptides predominate, but other minor polypeptides may be resolved [2]. 

A hydrophobic 10 kDa protein is also associated with PS II preparations [14], but its function is obscure. The protein is phosphorylated by a membrane-bound protein kinase [15] and the identity of this phosphoprotein has been the subject of much speculation [16]. It is clearly not the 9 kDa polypeptide of Cyt 6-559 or the 8 kDa proteolipid subunit of ATP synthase as shown by N-terminal amino acid sequence [14]. 

The oxygen-evolving complex, responsible for the transfer of electrons from 

Approximately 20 polypeptides have therefore been identified as associated with PS II preparations. These are all presumably required in a particular stoichiometry for the optimal functioning of PS II. The stoichiometry of the subunits may be controlled by the synthesis and/or degradation of these subunits. 

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The regulation of the synthesis of the polypeptides of PS II appears to be particularly complex, with evidence for regulation at transcriptional, translational and post-translational levels. The synthesis of the individual polypeptides does not appear to be tightly coordinated, with certain polypeptides accumulating in the absence of other PS II polypeptides under a variety of experimental conditions. The synthesis of a functional PS II complex is strongly dependent on light, both for its effect on the transcription of PS II genes and for its absolute requirement for Chi synthesis. 

Several polypeptide components of PS II and OEC have been isolated from thy-lakoids and PS II preparations capable of O2 evolution, after the initial isolation by Kuwabara and Murata [31] of a 33-34 kDa polypeptide (see, for a review. Ref. 10). On the basis of several criteria, such as the extraction by different reagents and the accessibility to antibodies in thylakoids or in inside-out vesicles prepared from thylakoids, a tentative and certainly incomplete picture has been proposed... 

This review is intended to summarize the PS II/OEC unit in terms of its polypeptide and electron transfer cofactor composition, its electron transfer pathways and its mode of operation in producing oxygen. The photochemical aspects of its operation wil be dealt with only cursorily as these are treated in detail in Chapter 4 of this volume. There has been considerable review activity recently on specific aspects of PS II/OEC function, including articles on polypeptide composition [7-9], manganese function [10-12], electron transfer and Oj-evolving properties [13-18] and the chloride requirement [19]. 

The fourth area of activity in which Mn valence and organization are addressed involves quantitation of the amount of Mn released from the PS II/OEC following perturbation. This method led to the early estimates of manganese stoichiometry [138,139] and has been refined and used to study the effects of a number of PS II inhibitory treatments [112]. With the realization of the role of the peripheral polypeptides in maintaining PS II/OEC integrity, these studies have continued and a clearer picture of the factors which control Mn binding is emerging. 

A number of PS II inhibitor treatments, such as Tris-washing and NH2OH extraction, had been well established as releasing Mn from the OEC [109]. Aker-lund, Andersson and co-workers showed that these treatments also released the peripheral polypeptides [71]. Mathis and co-workers showed that the ns phases in P-680 reduction that occur in untreated preparations are replaced by fxs components in inhibited samples [180]. The predominant decay phase is pH dependent, =2 ijs at pH 8 and =45 /is at pH 5, and is attributed to P-680 reduction by a secondary donor, usually designated D in optical work, which is identical to the EPR-detectable Z species [180,194,195]. The bulk pH and salt concentration dependencies of this phase implicate local membrane pH in influencing its time course... 

A full discussion of the preparation of PS II complexes is given elsewhere [1]. The polypeptides associated with these complexes and their genetics and synthesis are discussed below. 

Cyt b-559, detected spectroscopically, also accumulates in dark-grown plants [93] and its regulation therefore appears to be different from that of the other core polypeptides. The genes, psbE and psbF, for the 9 and 4 kDa polypeptides of Cyt b-559 are co-transcribed [56] and, due to the close spacing of the genes, may also be translationally coupled. This may ensure the synthesis of equal amounts of the two polypeptides, but as yet the stoichiometry of these two polypeptides in PS II preparations has not been established. 

Trypsin is a water-soluble protease that cleaves polypeptide chains preferentially at arginine and lysine residues. Thus, when trypsin is applied to broken chloroplasts or PS II particles the surface exposed proteins are digested first and the fully buried proteins are initially left alone. A number of studies (.10,22,29, 30,38,41,42,43,67) have been conducted to determine the effects of digestion of the surface-exposed polypeptides on the function of the reducing side of PS II. Trypsin treatment impairs the ability of PS II herbicides to inhibit the reduction of water-soluble... 

Two families of inhibitors interfere with the plastoquinone or herbicide binding site on the D-1 polypeptide, i.e. on one of the reaction center subunits of PS II. The phenol and urea/triazinone family of PS II inhibitors are different in their functional inhibitory pattern (reviewed in [1]), although they both bind to the D-1 polypeptide and displace each other from the binding site (1). Both QSAR studies (2) and - more refined - quantum mechanical calculations... 

Another class of PS II inhibitors are phenol-derivatives, liXe the herbicides ioxynil and dinoseb where the QSAR is governed by steric parameters (2), as well as hydroxypyridines (7,8) and Xetoni-triles (9). The two inhibitory families may also be designated as a serine and a histidine families, named after the amino acid in the D-1 polypeptide to which the inhibitor is predominantly oriented (though not necessarly bound) (10). 

In some earlier papers (3,4.) we have already emphasized the importance of quantum chemical calculations for a better understanding of the binding of PS II inhibitors to the D-1 polypeptide subunit... 

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