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Figure 4.
Fig 4. Changes in CD21 structure upon interaction with its
ligand C3d. (A) Superposition of nonliganded CD21 SCR1-SCR2
(orange) with the CD21 SCR1-SCR2-C3d complex (gray) (17).
Binding of C3d introduces a main-chain shift in the B-B' loop of
SCR2, displacing Ser-85 and unlocking the Arg-83 side chain. The
interaction primarily involves the base of C3d helix H5. The
C3d/CD21 interface contains a nonphysiologic zinc ion (cyan),
which may distort the interaction between CD21 and C3d somewhat
because the zinc-coordinating C3d residue Glu-117 might
otherwise be available to form a salt bridge with Arg-83 or
interact with other CD21 residues. W denotes water molecules.
(B) Differences in interdomain orientation between the
unliganded (orange) and liganded (gray) forms of CD21. The view
is the same as in A; the tracing for C3d has been omitted for
clarity. The small changes at the C3d binding site lead to a
different interdomain orientation. Hydrogen bonds are indicated
with dashed lines. Arrows indicate the directions of main-chain
movements.
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