Fig. 1. Structural model of Lu gp defined by X-ray crystallography, small-angle X-ray scattering and homology modeling. (A) The experimental scattering data for the D1D2D3 construct is plotted in black as a function of q (q = 4π sinθ / λ, where 2θ is the scattering vector and λ is the wavelength). The theoretical scattering from the D1D2D3 portion of a representative reconstruction is shown in red. (B) The experimental scattering for the D1D2D3D4D5 construct plotted in black with the theoretical scattering from a full reconstruction shown in red. (C) Overlay of eight rigid-body fits of the Lu gp D1D2 crystal structure and D3, D4 and D5 homology models to the SAXS data. Each reconstruction is displayed as a Cα ribbon in a different colour with the D3 glycosylation highlighted in stick representation and the residues critical for binding to Ln511/521 [2] highlighted in space-filling representation. (D) The same image rotated 90 about the y-axis. (E) The representative reconstruction which generated the theoretical scattering shown in (A) and (B) is displayed as an electrostatic surface with negative potentials coloured red and positive potentials coloured blue. (F) The same reconstruction is displayed as a Cα ribbon with the residues critical for Ln511/521 binding highlighted in stick representation. The colours reflect the effect of mutation to alanine on the affinity for Ln511/521 — red indicates severe effect; orange, marked effect and yellow, slight effect [2]. Cα atoms of blood group active residues are displayed as blue spheres. In panels (E) and (F) a black bar represents the 10 residue linker between the C-terminus of the D5 model and the putative transmembrane domain.