Solution structure of the VWF C4 domain. (A) Schematic presentation of the VWF domain structure with the C4 domain highlighted. The C4 domain has a specific role in binding to the heterodimeric platelet integrin subunits α_IIb/β₃, as indicated. Prodomain dissociation is indicated by dashed lines. (B) Ability of the VWF C4 domain to bind to α_IIbβ₃ platelet integrin: wt, Y2561 variant, glutamates (EGE) (R2507ED2509E), and alanines (AGA) (R2507AD2509A). (C) Ribbon diagram of the lowest-energy solution structure of the C4 domain of VWF, with its subdomain structure SD1 (residues 2497-2546) shown in magenta, SD2 (residues 2548-2577) in blue, and the N-terminal cloning tag in light gray. All β-strands are labeled with Roman numbers. Disulfide bridges (compare Table 2) are numbered and colored in yellow. The side chains of other VWF C4 residues described in the text are shown in stick representation with atom-specific colors: V2501 and W2521, magenta; RGD motif (residues 2507-2509), lavender; the SD1/SD2 hinge residue V2547, green; F2561, changed to tyrosine in a clinical variant,¹⁵  blue. The extended loop connecting SD1 strands βII and βIII is indicated with an arrow (compare Figure 4). Insets show stick representation of the local environment of F2561. A view rotated by 90° demonstrates that the side chain of F2561 is presented on the surface (right). (D) The 20 lowest-energy NMR conformers are superimposed on SD1 (left) and SD2 (right), respectively, demonstrating substantial SD1/SD2 hinge variability. The flexible RDG motif is indicated.