Inhibition of binding of WT-scuPA to uPAR by variant scuPA. (A) Binding of ¹²⁵I-scuPA to CHO cells line expressing GPI-anchored uPAR was measured in the absence and presence of scuPA variants. CHO cells were prechilled for 30 minutes and washed twice with binding buffer (PBS, 1% BSA). Cells were then incubated with ¹²⁵I-scuPA (10 nM) in the presence of the indicated concentrations (0.012 to 800 nM) of GFD (□) or ATF 1-135 (▪) for 1 hour at 4°C. The cells were washed 4 times with binding buffer, and the cell-associated ligand was eluted with glycine, pH 2.8, and counted. The mean ± SD of 3 experiments is shown.