PS II complex

PS II and PS I, and the 28 kDa protein of LHC II, show a number of possible a-helical regions in each case (see Chapter 11). Given the size of the chlorin ring (= 15 X 15 A) and the fact that each apoprotein binds many pigment molecules, it is not surprising that these apoproteins are associated with the membrane o-helical regions, since protease digestions of intact thylakoids or vesicles do not release any of the chlorophyll of the PS II complex [27], the PS I complex [28,29] or LHC II [27,28], Moreover, as neither the chlorophyll or xanthophyll molecules of LHC II are accessible to proton attack, they are likely to be located within the hydrophobic interior of the membrane [30], 

Karlin-Neumann et al. [31] have presented a model for the main apoprotein of LHC II which has 3 a-helices (Chapter 11), consistent with the direct determination of the a-helical content of LHC II [32]. There is a large domain of surface-exposed protein (= 48%) this is also consistent with the electron microscopic pictures of reconstituted LHC II [18,19], While most of the Chi molecules are thought to reside in the hydrophobic membrane interior, there are insufficient histidine residues present for the co-ordination of all Chi a molecules, and the ligand for Chi b has not been recognized yet. The location of the carotenoids, so often ignored, but always a constituent of all Chl-proteins, is not established [30], 

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Shortly after the introduction of the triazine herbicides, it was confirmed that their target site in the photosystem II (PS II) complex was in the thylakoid membranes. Triazines displace plastoquinone at the QB-binding site on the D1 protein, thereby blocking electron flow from QA to QB. This in turn inhibits NADPH2 and ATP synthesis, preventing C02 fixation. 

Further energy transfer within the rods may be performed by the /3f-chromophores of the PBS rods, modulated by the various 30 kDa-linker polypeptides, and directed along the central rod channel towards the APC complexes of the PBS core. Excitation energy transfer in the PBS was found to occur in up to five steps with different energy transfer rates [134] within the discs (hexamers) from (s)- to (f)-chromophores and from (f)- to (f)-chromophores, from disc to disc, from the rods to the APC complexes in the core, to the APB-Lcm complex and to Chi a in the PS II complex. The disc-to-disc excitation energy transfer (20 ps) was supposed to be the rate-limiting step. 

Most of the thylakoid proteins are organized into four intrinsic protein complexes PS II complex, Cyt b/f complex, PS I complex and ATP synthetase (Fig. 1). The electron transport complexes are linked by mobile electron transport carriers, plastoquinone, plastocyanin and ferredoxin (see Chapter 10). Furthermore, chloroplasts that possess Chi b have the major light-harvesting Chi a/h-proteins of PS II (LHC II) that may represent over 50% of the thylakoid protein [13], as well... 

The three extrinsic 33 kDa, 24 kDa and 18 kDa polypeptides of the oxygen-evolving complex are associated with the PS II complex at the lumenal side of the membrane [34,35] (Fig. 1). Each of the three proteins is able to re-bind stoichiomet-rically to depleted PS II complexes. The 33 and 24 kDa proteins bind directly to the complex, with the 33 kDa protein promoting binding of the 24 kDa protein [34,36], while the 18 kDa protein binds to the 24 kDa protein only when the latter polypeptide is bound to the PS II complex [37]. The 33 and 24 kDa proteins appear to be directly associated with two uncharacterized polypeptides of 24 kDa and 10 kDa that are also present in the PS II complex [36]. 

There is little evidence as yet for the specific association of individual acyl lipids with thylakoid complexes. However, phosphatidyldiacylglycerol, esterified with 16 3-T/ AA5-hexadecenoic (a fatty acid found only in thylakoid membranes), is associated mainly with oligomeric LHC II [ +5], MGDG has been implicated also in mediating interaction between the PS II complex and LHC II [46], and ATP... 

The aqueous polymer two-phase partition technique pioneered by Albertsson et al. [11] not only provides a method to separate right-side-out from inside-out vesicles (Section 2.2), but also allows the partial separation of appressed and non-appressed membrane fractions. The inside-out vesicles which partition to the lower phase were depleted in PS II activity [59]. Significantly, they were derived from the appressed membranes of the grana stacks as judged by electron microscopy [60] and their mode of formation [61], Futhermore, analysis of the Chl-protein content revealed a substantial depletion of PS I complex, and an enrichment of PS II complex and LHC II in the appressed membrane fraction [62]. In 1980, An-dersson and Anderson postulated that PS II and PS I are mainly laterally segregated, with PS I excluded from the appressed grana partitions, where most PS II-LHC II complexes are located [62,63] (Fig. 5). 

Fig. 7. Section of a spinach leaf, fixed in glutaraldehyde and embedded in K4 M resin, which has been treated with rabbit antibody to the 68 kDa apoprotein of the P-700-Chl a-protein of the PS II complex followed by goat anti-rabbit antibody with 20-nm gold particles attached (see Refs. 84 and 88) (Good-child, D.J. and Anderson, J.M., unpublished results).

Fig. 8. Schematic diagrams adapted from Staehelin and Arntzen [48] depicting how the reversible phosphorylation of some Chi a/h-proteins of LHC II affects membrane appression and their distribution between the PS II complex-enriched appressed membranes and the non-appressed membrane regions which are enriched in PS I complex.

It is thought that membrane appression results from the localized decrease in the net negative surface charge of the many LHC II proteins surrounding each core PS II complex, thereby decreasing the overall electrostatic repulsive forces between adjacent membrane surfaces, and also increasing the van der Waals attrac-... 

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. 

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. 

Note that LHCll is a separate light-harvesting complex, which supplements the inner antennae (CP29, CP26, CP24 and CP22) associated with the PS-II complex. The presence of this separate LHC II complex has been confirmed by the results of freeze-fracture experiments obtained with thylakoid membranes of both wild-type and mutant plants. The PS-11 inner antenna complex and the LHC-II complex appear as distinctly different classes of membrane particles. On the other hand, the LHC-I proteins associated with the PS-1 complex appear to be complexed to the PS-1 reaction center, much as the inner antennae of PS II are complexed to the PS-II reaction center. 

The model in Fig. 21 (A) shows that the PS-II complexes are present mainly in the appressed region of the thylakoid membrane, while the PS-1 complexes are in the non-appressed, stromal region of the membrane. The Cyt 6 /complexes are, on the hand, distributed almost equally among the appressed and non-appressed membranes. The CFo CF ATP synthase complex is present exclusively in the stromal part ofthe thylakoid membrane. More than twenty years ago Miller and Staehelin ° found that the ATP-synthase part of protein complex was lollipop-shaped, with the knob protruding from the non-appressed thylakoid membrane into the stroma. A typical distribution of the five major complexes between the appressed and non-appressed regions is summarized in the accompanying table in Fig. 21 (A). 

Fig. 1. Left Model for the thylakoid membrane consisting of the four major complexes PS-II, Cyt b f, PS-I and ATP synthase. Right A more detailed view of the PS-II complex, where the components more intimately involved in oxygen are marked off by a dashed line.

Fig. 2. Pattern showing removal and restoration of cofactors in the oxygen-evolving complex in a PS-II particle from spinach. The PS-II complex consists of the D1/D2 proteins, as shown In Fig. 1. Legend for the extrinsic proteins is shown at upper left Manganese atoms are represented by four small dots. See text for other details. Scheme adapted from Yamamoto (1989) Molecular organization of oxygen-evolution system in chioropiasts. Bot Mag (Tokyo) 102 572.

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