Structure of the subset 169 P6540 case BcR Fab fragment and its homotypic interactions. (A) Superposition of the heavy chain variable domains of P6540 and the subset 2 P11475, showing a different, more open conformation for the VH CDR3 of the P6540 Fab. A cartoon of the P6540 Fab is shown, with heavy chain (blue), light chain (orange), and CDRs labeled accordingly. The subset 2 P11475 Fab is also shown in a cartoon in white. The 2mFo-DFc electron density map is shown from the VH CDR3 loop (blue) and at a contour level of 1.0σ. (B) Superposition of the light chain variable domains of subset 169 P6540 and subset 2 P11475, showing a minor difference in the VL CDR3 conformation caused by a hinged motion of the loop. (C) The homotypic interactions between subset 169 P6540 BCR Fab molecules are revealed by the mutual orientation of 2 interacting Fab molecules in the crystal. Heavy and light chains are shown in the cartoon and are blue and orange, respectively. The interacting region is marked with an arrow. (D) Close-up of the interface between the 2 receptor molecules. The residues involved in the interaction are labeled and shown as sticks. Contacts are dominated by the IGLV3-21 light chain residues through the VL CDR2. All residues shown are also conserved and involved in the homotypic interactions between subset 2 P11475 receptor molecules leading to cell-autonomous signaling.¹⁷  Figures were generated with Pymol (http://www.pymol.org).