Binding of sDI to DII+DIII-uPAR requires GFD and kringle domains. Binding of ¹²⁵I-sDI (50 nM) to 293 cells expressing GPI-anchored uPAR DII+DIII was measured in the presence of the indicated (0 to 600 nM) concentrations of WT-scuPA (□), GFD (○), ATF 1-135 (×), ΔGFD-scuPA (•), and ΔK-scuPA (▴). Cells were incubated at 4°C for 1 hour, washed 4 times with binding buffer, and the cell-bound ligand was eluted with glycine, pH 2.8, and counted. The mean ± SD of 3 experiments is shown.