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Figure 3.
Fig. 3. Structure of the dimer of N-domains. (A) Surface
representation of the N-domain dimer indicating the
wedge-shaped^ groove and the dimerization interface. Shown are
two monomers of a dimer with the left one rotated 90° around
the vertical axis away from the original position in the dimer.
Note the hook-like^ appearance of the monomer with the
coiled-coil of helices  6 and^
7 pointing
out of the planar surface formed by the ring-shaped^ element
comprising the NH[2]-terminal 40 residues. Residues from three
separate regions of the N-domain make direct or water-mediated^
contacts in the dimer and are color-coded according to their
position. Interface residues at the NH[2]-terminus are in green,
those in helices 3 and 4 are in
blue, and amino acids located in helix 6 are
yellow. The position of the critical W37 is highlighted^ in red.
The figure was created using GRASP (29). (B) A view at the
dimerization interface with amino acids represented^ as
ball-and-stick models and the c backbone
as a ribbon. The^ monomer is in the same orientation as the one
on the right side^ of (A). Side chains are colored as in (A);
the backbone ribbon is colored as in Fig. 2B, with the first 40
residues highlighted^ in red. L33 makes a backbone carbonyl
group contact, and its position is represented by the filled
circle. In the STAT-4 recombinant N-domain used for
crystallization, M1 was replaced with G plus four additional
small amino acids, one of which (G1) is visible^ in the electron
density map. In the crystals, the NH[2]-terminus of G1 is part
of the dimer interface, possibly substituting for the native M1.
(C) Close-up stereoview of the intermolecular hydrogen-bonding
network in the dimer. Selected side chains surrounding the
conserved W37 (magenta) in helices 4 and 6 of two
monomers (green and gray) are shown. W37 makes direct (E66  ) and
water-mediated^ contacts (Q63 ). Water
molecules are depicted as blue spheres.
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