Crystal structures of GPVI mutant ΔPAVS-ΔPAPYKN and GPVI collagen peptide complexes. (A) Superimposition of the crystal structures of loop truncation mutant ΔPAVS-ΔPAPYKN (dark blue loops) and previously solved WT GPVI ectodomain (PDB-ID: 2GI7; light blue loops)²⁷ showing significant conformational differences limited to the region around the truncated loops (red) in its D2 domain. The insets show an overlay of the WT loops (red) and the mutant loops (blue) in sticks representation, together with the 2Fo-Fc electron density map of the mutant (blue, 1.2σ contour level). See also supplemental Figures 1 and 2. (B) Crystal structure of the GPVI-(GPO)₅ complex showing two GPVI molecules in cartoon and surface representations that bind the collagen peptide with their D1 domains. Chains of the collagen triple helix are shown in pink (leading; L), magenta (middle; M), and orange (trailing; T). Molecule A (blue) binds the M + T combination of chains; molecule B (salmon) binds the L + M chain combination. Also indicated (red) are the truncated D2 loops that are situated outside the region involved in collagen binding. See also supplemental Figure 3. (C) Crystal structure of the GPVI-(GPO)₃ complex showing the same molecular arrangement as observed in crystals of the GPVI-(GPO)₅ complex, but with shifted collagen chain stagger, such that the GPVI molecules bind the L + M and T + L combinations of chains, respectively. The two generic GPP repeats at either end of the (GPO)₃ peptide are shown in white sticks. See also supplemental Figure 5. (D) Back-to-back D2–D2 interactions between two symmetry-related GPVI-(GPO)₅ complexes. The interhelix angle of about 80° is incompatible with a parallel helix orientation as in collagen fibrils,³⁴ and the back-to-back dimer is therefore not expected to be of functional relevance for in vivo binding to fibrillar collagen. See also supplemental Figure 4. (E-F) Schematic representation of the GPVI contact sites on the (GPO)₅ (E) and (GPO)₃ (F) peptides, also illustrating collagen chain stagger and GPO triplets. Collagen residues located within 4.5 Å of the first (blue) and second (salmon) GPVI molecule are highlighted, and the corresponding sequence motifs are depicted below the figure. Three out of 4 sites are formed by a POGPOGPO sequence; the fourth, the second site on (GPO)₃, overlaps the generic C-terminal (GPP)₂ extension of the peptide and is formed by the longer POGPOGPPGPP sequence due to the collagen chain stagger of the T + L chain combination.