Comparison of the Functional Properties of the Monomeric and Dimeric Forms of the Isolated CP47-Reaction Center Complex

Abstract

. radical but also of radicals generated on the donor side. Difference analyses indicated that one of these radicals is likely to be due to a D1 tyrosine 161 or D2 tyrosine 161. In contrast, the monomeric CP47-RC complex did not show similar EPR-detectable radicals and instead was dominated by a high yield of the spin-polarized triplet signal generated by recombination reactions between the oxidized primary reductant, pheophytin, and the primary donor, P680. It is also concluded from EPR analyses that both the monomeric and dimeric forms of the CP47-RC subcore complex contain one cytochrome b 559 per reaction center.Overall the results suggest that photosystem II normally functions as a dimer complex and that monomerization at the level of the CP47-RC subcore complex leads to destabilization of the bound plastoquinone, which functions as Q A .More than 25 different protein subunits make up the photosystem II (PSII) 1 complex of oxygenic photosynthetic organisms (1). At the heart of this complex is the reaction center (RC) consisting of the D1 and D2 proteins, where primary charge separation occurs (2). Closely associated with the D1 and D2 proteins are two similar chlorophyll a-binding proteins, CP43 and CP47 (3). These proteins serve as an "inner antennae" system that is linked to a secondary light-harvesting system. In higher plants and green algae, the chlorophyll a/bbinding proteins (encoded by nuclear located cab genes) act as the secondary light-harvesting system, while phycobilisomes serve the same purpose in other types of oxygenic photosynthetic organisms, such as red algae and cyanobacteria (4). CP43 and CP47 are also distinguished by having a large hydrophilic loop linking putative membrane-spanning regions 5 and 6 (3). These loops are almost certainly located on the luminal surface of the complex and may function in water splitting in some way (5). Treatments with detergents can peel away the various subunits, and it has been shown that during such manipulations CP43 is more readily removed than CP47 (6, 7). It is therefore possible to isolate a CP47-RC complex. Recently a method was described for spinach that yielded a preparation of the CP47-RC complex consisting of a mixture of monomeric and dimeric forms (8). Analyses using mass spectrometry showed that both forms of this subcore complex contained the products of the psbE, psbF, psbI, psbT c , and psbW genes as well as the D1 and D2 proteins and CP47. However, the CP47-RC dimer contained, in addition, the products of the psbL and psbK genes. Also of significance was the finding that the dimer and not the monomer contained about one molecule of plastoquinone-9 per RC. Overall, the findings suggest that the monomeric form of the isolated CP47-RC complex was derived by dissociation of the dimer and that the latter conformation is likely to be the in vivo state. Moreover, the finding that the CP47-RC complexes contain several small proteins with putative one transmembrane helices is relevant to de-

Dimeric and Monomeric Organization of Photosystem II

“…5). Earlier, the monomeric forms of PSII were reported to mainly originate from the nonappressed regions of the stroma lamellae (21,24,25,69), and our analysis extends this model to a higher resolution.…”

Photoassembly of the manganese cluster and oxygen evolution from monomeric and dimeric CP47 reaction center photosystem II complexes

“…Recently, a PSII subcore complex lacking CP43 and the three extrinsic proteins but containing CP47 as well as the reaction center (CP47RC) has been isolated in both monomeric and dimeric forms (30), and their electron transport properties have been characterized (31). In this work we show that the CP47RC dimers as well as the monomers can be photoactivated, i.e., reassembling the Mn 4 Ca 1 Cl x cluster and evolving oxygen with a high degree of conversion.…”

“…The dimeric form of CP47RC was found to be more amenable to photoactivation of the Mn cluster although the CP47RC monomer also showed some capacity to catalyze water oxidation by using similar reconstitution procedures. In the case of the dimer, the presence of bound plastoquinone in the subcomplex (30) helps to reduce the frequency of recombination reactions (31), which, in turn, not only aids electron donation to the added ferricyanide acceptor, but also lowers the yield of P680 triplet formation. This triplet is not quenched by the ␤-carotene in the RC but, instead, interacts with oxygen to form singlet oxygen (34).…”

Biogenesis of water splitting by photosystem II during de‐etiolation of barley (Hordeum vulgare L.)

“…Moreover, from in vitro studies, we know that the minimal structural composition of the isolated PSII core capable of water splitting is composed of the D1, D2, Cyt b 559 and CP47 subunits (Bianchetti et al . ; Büchel et al . ).…”