Figure 2.

(A) Four hundred microliters of platelet cytosol (9 mg/ml) was incubated with the indicated volume of p85 antibody overnight at 4°C. Each immunoprecipitation was made to contain a total of 10 μl of serum using nonimmune serum. Immunoprecipitates were captured on protein A-Sepharose and washed. 4-Phosphatase enzyme activity was determined by measuring the hydrolysis of [³H]Ins(1,3,4)P₃ to form [³H]Ins(1,3)P₂. (B) One milliliter of platelet cytosol was immunoprecipitated with (1) 5 μl nonimmune serum, (2) 5 μl of p85 antiserum, (3) 5 μl of antiserum to the N terminus of the 4-phosphatase, or (4) 5 μl of the antiserum to the C terminus of the 4-phosphatase. Washed immunoprecipitates were assayed for PI 3-kinase activity by using PtdIns as a substrate.