The X-ray structure of AVR2. (A) Multiple sequence alignment of the avidin family. The secondary structure elements are numbered according to the AVR2 structure. The residues mutated in this study (AVR2, Ile-109; AVR4, Lys-109; AVR6, Cys-58) are coloured green. Residue Cys-122 in AVR4, which was mutated to serine in a previous study , is indicated in blue. (B) Two tetramers found in the asymmetric unit of the AVR2-b crystal. (C) Monomer A of AVR2-b. The weighted difference Fo – Fc electron density map (blue), calculated in the absence of biotin, is drawn with a 1.5 Å radius around the atoms of D-biotin of the final structure of AVR2-b. Contours are shown at 3.0σ. The secondary structure elements of AVR2-b are numbered. (D) A close-up view of (C) focused on biotin.