Fig. 3. The level of IGF-I receptor and its autuphosphorylation in Null and Null+ cells. Adherent Null and Null+ cells were incubated at 37°C in medium containing 10% FBS. At a confluence level of 50-70%, the cells were serum-starved by incubating overnight in medium containing 0.5% FBS. After trypsinization the cells were placed in suspension culture by plating on poly-HEMA coated dishes. IGF-I (100 ng/ml) was added as indicated and incubated at 37°C for 15 minutes. The cells were then lysed in TGH buffer, and the cell extracts were subjected to immunoprecipitation. (A) An aliquot (1 mg) of lysate was immunoprecipitated with IGF-I receptor antibody and analyzed by western blotting with anti-phosphotyrosine. (B) The phosphotyrosine blot was stripped and reprobed with IGF-I receptor antibody.