Additional Results and Discussion

The WT-V129_1 asymmetric unit contains one PrP molecule, exhibiting the conserved PrP fold. The C-terminal helix 3 is 3D-domain swapped with a second molecule related by crystallographic two-fold symmetry (Figure 1A), resulting in a dimer covalently-linked by disulfide bonds that is nearly identical to the WT-M129 structure (Knaus et al, 2001). A threonine-rich hinge loop (T190-F198) precedes helix 3 and contains a short β-strand that packs against its symmetry mate to form a short twostranded intermolecular antiparallel sheet. The dimer interface also includes two helices 1 which abut in a parallel fashion. Omit difference density clearly indicates the open hinge loop conformation corresponding to a swapped dimer (Figure S1A). Western blot analysis of dissolved WT-V129_1 crystals was not possible due to crystallization irreproducibility. However, Western blot analysis confirmed that the isomorphous and more easily grown WT-M129 crystals are substantially enriched in dimer (~50% of total protein) compared to the protein stock solution used for crystallization (<5%) (Figure S1C), and that the disulfide-linked dimer collapses to monomer under reducing conditions.