Homotypic VN interactions occur through the amino-terminal and carboxyl-terminal portions. (A) Homotypic VN interaction was measured after incubation of increasing amounts of biotinylated multimeric VN on immobilized multimeric VN (▵) or bovine serum albumin (○). Half-maximal binding occurred at 28 nM. (B) Homotypic VN binding was characterized using a competition ELISA in which increasing amounts of unfractionated heparin (▵), VN peptide containing residues 348 to 361 (▴) and residues 47 to 64 (○), or control peptide 105Y (⋄) or PAI-1 (•) was added to a fixed concentration of biotinylated VN (28 nM). Results are shown as percentage of binding in the absence of competitors.