PSII purified from thermophilic cyanobacteria exists as a dimer both in solution and in crystallized form. The overall structure of PSII dimer at 3.0 À resolution is shown in Figure 4.1 . The two monomers in PSII are related by a local twofold symmetry axis in its center; each monomer is composed of 36 transmembrane helices (TMHs). Among them, 10 are assigned to the D1, D2 subunits located in the center of the monomer (Figure 4.1b). The D1 and D2 subunits are surrounded by the CP47 and CP43 subunits in each side, each with 6TMHs. In addition, 10 TMHs are attributed to the LMM subunits of PsbE, F, H, I, J, K, L, M, Tc, and PsbY subunits , and 2 TMHs to PsbZ. The remaining two TMHs, designated X1 and X3 (Figure 4.1b), are not assigned in the current structure due to the limited resolution.
A large extra-membrane region, found at the lumenal side of PSII, is formed by the three extrinsic proteins PsbO, PsbU, and PsbV, together with the large, hydrophilic loops of D1, D2, CP47, and CP43 protruding from the membrane surface (Figure 4.1a). The Mn4Ca cluster is located at the immediate surface of the membrane facing the lumenal side and is thus deeply covered by the large extramembrane region. In contrast, a relatively small extra-membrane region is found at the cytoplasmic side (stromal side in the case of chloroplasts) of PSII, which may be related to the fact that no distinct function has been assigned in the peripheral region of the stromal side except the location of QA and QB, two plastoquinone electron acceptors immediately underneath the cytoplasmic membrane surface (see 4.3.3). The lack of a large extra-membrane region in the cytoplasmic side may facilitate the association of phycobilisome at the membrane surface required for collecting the light energy in cyanobacteria. In higher plants, phycobilisome is replaced by light-harvesting complex II, a transmembrane protein complex. The lack of an extra-membrane region in the stromal side may, therefore, facilitate the stacking of multiple thylakoid membranes. This phenomenon is known to occur in green algal and higher plants containing light-harvesting complex II, but not in cyanobacteria.
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