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. 2002 Jul 16;99(16):10641–10646. doi: 10.1073/pnas.162360499

Fig 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.