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.Top and side view projection maps of the largest PSII-LHCII supercomplex structurally characterised to date, are used as the framework for the two possible models of subunit organization.These contoured projection maps are very similar to those presented in Boekema et al (33) but improved in that they are the sums of larger data sets (1,925 vs.500 top views; 2213 vs 80 side views) and have a higher resolution (approximately 20Ã…).They also differ in that they contain the densities of two 23 kDa subunits in addition to those of the other core (CP47, CP43, D1, D2, the 33 kDa subunit, cytb559 and the antenna (Lhcb1, Lhcb2, Lhcb4 and Lhcb5) (33, 176) proteins.Both models are identical in terms of their depiction of the extrinsic and Lhcb protein components.They differ only in the attributed locations of the PSII core components, CP47, D1, D2 and cyt b559.Localization of D1-D2-Cyt b559-CP47 Complex and CP43Regions within each of the projection maps are shaded in dark, mid and light gray.The dark gray regions represent aligned monomeric D1-D2-cytb559-CP47 complexes (and associated low molecular weight subunits) of the type reported by Dekker et al (52) and Nakazato et al (172).Together, the two dark and two mid gray regions depict the shape of the PSII core dimer consisting of the integral membrane protein components D1, D2, cyt b559, CP47 and CP43 and associated low molecular weight subunits.By elimination, it follows that the two mid gray regions each contain CP43 (205).Localization of the 33 kDa Extrinsic SubunitsTop and side view projection maps of PSII complexes (+/-33 kDa extrinsic subunit) were used to produce subunit difference maps (33, 50).Each monomeric portion of the dimer is associated with a region of density attributed to the 33 kDa subunit and these densities overlap to form a single central protrusion in the side view.The model implies that the 33 kDa subunit: PSII core monomer stoichiometry is 1:1.There are reports which indicate the presence of two copies of the 33 kDa subunit per reaction centre (133, 274, 277).However, the dimensions of the 33 kDa subunit protrusions shown in Fig.5 are consistent with single copies of the protein monomer (110).Localization of the 23 kDa Extrinsic SubunitsWhen isolated in the presence of glycine betaine PSII-LHCII supercomplexes additionally bind the 23 kDa subunit (Boekema, Nield, Hankamer, Barber Eur J Biochem., 1998).Difference mapping experiments, suggested that the two lumenal protrusions in the side view projection map, located on either side of the 33 kDa components, each contain a 23 kDa subunit.Their positions in the top views also identified by difference mapping.These proposed positions are in agreement with analysis of crystalline PSII arrays containing the 23 kDa subunit (142).Freeze-etching studies of the lumenal surface of the grana regions of thylakoid membranes (ESs surfaces), also show four lumenal protrusions (217) which could correspond to the four densities (see section entitled Heterogeneity of PSII in vivo).If the PSII complexes studied, before and after the removal of extrinsic proteins by Ford et al (66), are interpreted as dimers the positions attributed to the extrinsic subunits would be consistent with both models.Localization of the Lhcb ProteinsWestern blot analyses of PSII-LHCII supercomplexes showed them to be enriched in Lhcb1, Lhcb2, Lhcb4 (CP29) and Lhcb5 (CP26), but depleted of Lhcb3 and Lhcb6 (CP24) (176).Crosslinking studies have shown that CP47 and CP43 are both in close contact with CP29 (Lhcb4) and that CP43 is also in close contact with CP26 (Lhcb5) (46, 202).By superimposing high resolution electron density map of the Lhcb1/Lhcb2 heterotrimer (267) upon the top view projection maps shown in Fig.5a and 5b, the LHCII complex can be seen to fit snugly into the two tips of the PSII-LHCII supercomplex.Furthermore as Lhcb4 and 5 are similar in mass and have strong sequence homology with the LHCII proteins, two monomeric electron density maps based on the data of (267) were positioned in the PSII-LHCII supercomplex (in both Models 1 and 2) between the PSII core and the LHCII heterotrimer.There appears to be insufficient space to fit any additional monomeric Lhcb proteins.This conclusion is in agreement with pigment analyses of the isolated PSII-LHCII supercomplex which showed it to be associated with 156 Chl a and 46 Chl b molecules (see Fig.2; refs.88, 89, 176).In Boekema et al (33) it was suggested that Lhcb6 (CP24) might also be present in these isolated PSII-LHCII supercomplexes.This was a precautionary conclusion based on the detection of low levels of the protein present in early and less pure preparations of the complex.This conclusion has been amended in the light of the western blot data presented in (89, 176)
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