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Figure 2
(a) The subunits of the His-YIIIM4AII tetramer are connected via calcium ions. Two chains are sketched as ribbons and coloured as described in Fig. 1[link](b). Two chains are shown as grey surfaces. Calcium ions are indicated as spheres. Calcium ions binding only to internal repeats are in yellow, those involving the N-cap in light blue and those at the twofold axis in salmon. (b) Calcium-binding site between internal repeats viewed along the axial direction (from the direction of Pro23# O, which was omitted for clarity). Residues from different chains are shown as sticks with blue and salmon C atoms. Calcium ions and water molecules are depicted as grey and red spheres with reduced atomic radii, respectively. Polar interactions in the pentagonal plane involving the calcium ion are shown as dashed lines in orange. Additional interactions are in yellow. The 2FoFc and anomalous difference electron-density maps are contoured at 1.3σ (light blue) and 4σ (green), respectively. (c) Calcium-binding site involving the N-cap. (d) Calcium-binding site at the twofold axis. Colour coding is as described for (b).

Journal logoSTRUCTURAL
BIOLOGY
ISSN: 2059-7983
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