Effect of charged mutations on modulator-induced VWF binding. Cells expressing either wild-type or mutant GP Ibα were incubated with increased concentration of VWF in the presence of (A) 2.0 μg/mL botrocetin or (B) 1.5 mg/mL ristocetin. The binding of VWF to mutant GP Ibα was assessed as described in the legend for Figure 4. Error bars indicate SEM.