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Figure 4.
Figure 4. Gain-of-interaction in Zn -H46R SOD1 giving rise to
water-filled helical filaments. (a) One-half of the helical
Zn -H46R filament, shown in i and ii, is represented by the two
dimers shown from top to bottom in green and gold. Image ii is
related to the left half of iii by a rotation of 90°. A
schematic diagram of the tubular filament is shown in iv. In
iii, the approximate location of the crystallographic two-fold
axis that runs along the diagonal in the tetragonal unit cell is
indicated by a black line without arrows and a 180° rotation
symbol. Application of this two-fold operator generates one
complete turn of the helical filament. The double-headed black
arrow indicates the diameter of the helical filament, and the
blue arrow indicates the diameter of the central cavity. The GOI
interface between Zn -H46R dimers is boxed. In iii,  -strands
1, 2, 3 and 6, which form one-half of each SOD1 -barrel,
are shown in red. The zinc loop forms a short -strand
(blue) that reciprocally adds to this -sheet
in neighboring Zn -H46R dimers, stabilizing the GOI interface.
Zinc ions are shown as purple spheres. (b) Stereo view of the
GOI interface in the Zn -H46R helical filament. Residues 78 -81
of the zinc loop from one Zn -H46R dimer (orange) interact with
an exposed edge of a -strand
in a neighboring Zn -H46R dimer (green) in the crystal and vice
versa. Zinc ions are represented as purple spheres. The 2.15 Å
 [A]^55-weighted
electron density, with coefficients 2mF[o] - DF[c], is contoured
at 1.0 .
Water molecules have been omitted for clarity. (c) Stereo view
of two turns of the helical filament generated by repetition of
the GOIs in the Zn -H46R structure. This view of the helical
filament is rotated 90° around a horizontal axis relative to
images iii and vi of panel a. Successive Zn -H46R dimers (green,
yellow, blue and red) comprise one turn of the helical filament
with a pitch of  35
Å.
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