Location of charged amino acid residues involved in interactions with Ln511/521 on the surface of Lu gp. (A) Lu gp domains D1D2 (dark gray) and D3 (light gray) are shown in an orientation consistent with the SAXS envelope. Indicated are the positions of acidic residues that have been mutated to alanine. The residues are color coded according to their effect on Ln511/521 binding: blue indicates no effect; yellow, slight effect; orange, marked effect; and red, severe effect. (B) Lu D1D2D3 in the same orientation, shown as an electrostatic surface.