VWF A1 domain contains key residues for collagen 4 binding. Scanning alanine mutagenesis was performed for VWF A1 domain residues 1387 to 1412. (A) VWF:CB4/VWF:Ag ratios for each alanine mutant as listed on the x-axis. Error bars show mean ± 1 SD. (B) Location of key residues with absent binding (blue spheres) or reduced binding (orange spheres) to collagen 4 in a drawing of the VWF A1 domain crystal structure.³⁰  Residues that were changed in the scanning alanine mutagenesis are shown in purple. An 11-amino-acid deletion that also affected collagen 4 binding is not shown for clarity.