Movie S1.

Morphing movie to illustrate the epitope shift between C0020187 and C0021158. A monomer of ARG2 (white) bound to Fab C0020187 (VH dark gray, VL light gray) is morphed into ARG2 (blue) bound to Fab C0021158 (VH orange, VL light orange, and CDRs yellow and light yellow, respectively). The morph was produced in PyMOL by coordinate interpolation and 3 subsequent rounds of geometry refinement. Fab C0020187 causes a slight extension of the N-terminal to ARG2 residue 88 and interacts with the residues 78-86 via its hydrophobic cleft between VH and VL. Fab C0021158 is rotated by approximately 120°, changing the epitope significantly, while still interacting with the same residues in its hydrophobic cleft between VH and VL. In this case, the Fab induces the formation of a short helix (ARG2 residues 81-85), which lies deep in the cleft.