CB3s mimics BAFF binding to BR3. Structure of the CB3s-Fab/BR3 complex (top) compared with the BAFF/BR3 complex (bottom; 1OTZ/1P0T³⁵ ). BR3 (magenta) is shown in the same orientation in both complexes. The tip of the BR3 β-hairpin (L28/V29) points into a cavity at the junction of light (green) and heavy (blue) chains on CB3s (transparent surface models, selected side chain shown), or at the junction of 2 BAFF monomers (light and dark gold), with BR3 D26 forming a salt bridge with an arginine side chain in each complex. CB3s contact residues are labeled in black, light chain residues are in italic; key BAFF-binding residue side chains of BR3 are shown and labeled in white.