GPIb-associated kinase activity induced by botrocetin-vWF. Platelets (10⁹ cells/mL) suspended in a buffer containing 1 mmol/L Ca²⁺ were activated with 3 μg/mL of botrocetin and 10 μg/mL of vWF for the indicated time intervals. Reactions were then terminated with lysis buffer, and GPIb was isolated by immunoprecipitation with anti-GPIb MoAbs, WGA3. In vitro kinase assays were performed using enolase as exogenous substrate. The proteins were separated under reducing conditions by 8% SDS-PAGE and quantified with a BAS 2000 Phosphorimager. The arrow represents the band presumably derived from IgG heavy chains. The data are representative of three experiments.