Part 2

Fig. 2a shows the 3D map for a single supercomplex with dimensions of 165 Å x 330 Å x 110 Å overall width (x), length (y) and height (z), respectively, and the volume enclosed by the surface is approximately 3.5 x 106 Å cubed. The thickness of the supercomplex is approximately 60 Å at its edges, as expected for a membrane-spanning region, and the protrusions extend maximally to 50 Å above the lumenal surface. The protrusions marked A and A` have been shown previously to be the 33 kDa manganese stabilizing protein of the OEC based on earlier studies (7,8). As previously suggested (4) the protrusions labelled B and B` are in part due to the 23 kDa protein and, given that the isolated supercomplex used for the present study contained, in addition, the 17 kDa protein, we conclude that this OEC subunit is also located in this protrusion. Indeed, biochemical studies have indicated that the 17 kDa protein binds to the 23 kDa protein (9). The structure of the B/B` protrusion, with a height of 50 Å, is more extensive than that of the protrusion assigned to the 33 kDa protein which has a height of about 40 Å, and could therefore accommodate both the 23 kDa and 17 kDa proteins. It has been suggested that there may be two copies of the 33 kDa protein (10) although others have argued for one (11). From considerations of molecular mass and volume, our 3D map indicates a single copy of this protein per reaction centre. The centre-to-centre distances, as judged from Fig. 2e, between A and A` and B and B` are 68 Å and 109 Å, respectively, and that between A and B or A` and B` of each monomer is 65 Å. To further aid the interpretation of the 3D map, three 10 Å thick projections at different regions of the supercomplex are presented in Fig. 2. On the stromal side (Fig. 2c) the distribution of features is relatively even throughout the section but, in contrast, on the lumenal side (Fig. 2d) the region assigned to the surface of the dimeric core, has markedly higher density as compared to the peripheral regions containing LHCII, CP29 and CP26 (7, 8, 12). The dimeric core contains the D1, D2, CP47 and CP43 proteins (13) that possess substantial lumenal loops joining their transmembrane helices. It is likely that these lumenal domains are responsible for the high density observed in this region, particularly the extensive loops belonging to the CP43 and CP47 proteins (14). Fig. 2e shows the 33 and 23/17 kDa proteins and clearly demonstrates their tetrameric organisation on the surface of the dimer.