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Figure 7.
Figure 7. Terbium ion substitution in G1-G3. (a) A cartoon
representation of the structure of G1-G3/actin (PDB i.d. 1RGI).
Actin is coloured blue and the domains of gelsolin are coloured:
G1, red; G2, light green; and G3, yellow. The P3[1]21 terbium
anomalous difference electron density map, contoured at 5 s
(black) and derived from terbium-soaked crystals of G1-G3/actin,
is superimposed onto the structure. Three terbium ion positions,
characterized by the positive density in this map, are identical
with the positions of type II metal ion-binding sites in the
G1-G3/actin structure and are labeled G1:II, G2:II and G3:II. A
fourth terbium ion (G1:I) is in a position where a type I
calcium ion is found in the structure.5 An actin-ATP-associated
Tb^3+ (labelled A) also is indicated. (b) A cartoon
representation of the structure of G1-G3/actin, with a P6[5]22
Tb^3+ anomalous electron density map superimposed, as in (a). In
this case, the terbium ion-soaked crystals were backwashed with
mother liquor containing 10 mM CaCl[2] and 0.2 mM Tb(NO[3])[3].
Tb^3+ was replaced only at the type II sites in G1 and G3. The
type II site in G2, which was unoccupied in the structure of
G1-G3/actin, and the type I site sandwiched between actin and G1
remained occupied by terbium ions.
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