TPO-induced phosphorylation. Purified human platelets were stimulated with TPO (200 ng/mL, 10 minutes, 37°C) and were immunoprecipitated with an anti–GP Ibα monoclonal (LJ-P3) (A) or an anti-PI3K antibody (B). Immunoprecipitation products were electrophoresed and blotted with an antiphosphotyrosine antibody (4G10). Phosphorylated proteins in the absence (–) or presence (+) of TPO are shown. Subsequent control blots were performed on the same filter for filamin-1, GP Ibα, and the p85 subunit of PI3K.