HS1 associates with Syk and PI3Ks after receptor stimulation. Aspirin-treated and washed human platelets were stimulated with agonist at 37°C for 30 seconds. (A-B) Syk kinase (A) or HS1 (B) was immunoprecipitated (IP) as described in “Materials and methods, Immunoprecipitation and Western blot analysis,” and the samples were analyzed for HS1 (A) or Syk (B) by immunoblotting (IB). Rabbit IgG (rIgG) or mouse IgG (mIgG) was used as a control. (C,D) PI3K subunits p85 and p110 (C) or HS1 (D) were immunoprecipitated as described, and the samples were analyzed for HS1 (C) or p85 and p110 (D) by immunoblotting. A negative control with immunoprecipitated normal IgG (mouse or rabbit) was analyzed for comparison. The Western blots shown are representative of experiments performed using platelets from 3 different donors.