Crystals of PSIIcc monomer. Left, light microscopic pictures of PSIIcc-monomer crystals, the black bar represents 100 μm. The shortest dimension of the crystals corresponds to the crystal axis with the largest cell constant (c-axis). Right, orientation of the membrane normal vector in PSIIcc-monomer crystals (pink arrow, corresponding to the local C2-axis of the dimer) relative to the crystal axes. For clarity, only one of the four possible orientations is shown. For other orientations, the membrane normal vector has to be rotated by 180° around the a, b, and c axes. In any orientation, the angle between the membrane normal and the b-axis is nearly 90°.

Schematic representation of PSIIcc monomer (A) and one monomer of PSIIcc dimer (B) looking at the dimerization surface (or the monomer-monomer interface). For clarity we show only the four major membrane intrinsic subunits (D1, yellow; D2, orange; CP43, violet; CP47, red), the small subunits at the dimerization surface (PsbT, PsbM, and PsbL), the three extrinsic subunits (PsbO, green; PsbU, light blue; PsbV, gray), and the non-heme iron cofactor (blue sphere). Lipids and detergent molecules are shown as indicated (SQDG, MGDG, DGDG, and DDM) and numbered as in the main text. A and B, black filled pictograms represent molecules found in both (PSIIcc monomer and homodimer) structures. Molecules (or changed headgroups) found only in the monomer (A) are shown in red. In B, the white pictograms show molecules present only in the model of PSIIcc dimer. The pictograms are contoured black or blue; indicating that molecules are assigned to one (contoured black) or the other (blue, also indicated by prime) monomer of the homodimer. For molecules present in both A and B the same numbers were taken as defined in supplemental Table S2. Lipids and DDM only found in homodimer are numbered according to the PDB file 3BZ1 as in supplemental Table S2.

Preparation of monomeric PSIIcc. Chromatograms of analytical FPLC runs (Toyopearl-DEAE650S) at different pH values, using a linear salt gradient for elution. The detector wavelengths were 280 (black line) and 205 nm (blue line). For clarity, the intensities corresponding to the two wavelengths were normalized to the peak of the PSIIcc monomer.

Arrangement of the PSIIcc monomers in the unit cell. A, view along the b-axis showing all four orientations of the PSIIcc monomer within the crystal lattice. All protein subunits are in schematic representation, for clarity only subunits that contribute to crystal contacts are colored, all other subunits are shown in gray. The violet circles (A and B) and the ellipse (C) mark crystal contacts described in the text. Protein subunits that contribute to crystal contacts are colored as follows: D1 (yellow), CP43 (red), PsbE (magenta), PsbF (cyan), PsbH (orange), PsbV (blue), and PsbO (green). B, view along the c-axis showing three PSIIcc monomers in one of the four orientations within the crystal lattice. The PSIIcc monomers are shown in schematic representation and are colored yellow, pink, and light blue. The membrane planes (drawn as transparent trapeziums) are oriented parallel to the crystallographic b-axis. The angle between the membrane normal and the b-axis is 87° (see Fig. 2). C, two monomers with their dimerization surfaces oriented toward each other. The monomers are related by the crystallographic C2-symmetry axis (a-axis) located between their PsbM subunits (colored white in space-filling representation). For clarity, only protein subunits, lipids, and detergent molecules are shown; protein subunits in schematic representation are colored as follows: D1 (yellow), D2 (orange), CP43 (magenta), CP47 (red), cytochrome b₅₅₉ (PsbF (cyan) and PsbE (green)), PsbU (pink), PsbV (blue), PsbO (green), and the remaining small subunits (gray). Lipids (cyan) and detergent molecules (brown) are drawn in space filling representation. The red circle marks a lipid/detergent cluster located at the dimerization surface.