(A) Immunoprecipitation of WT-SMAD4 or SMAD4-I500V with anti-FLAG and detection of phospho-SMAD1,5,9. (B) Immunoprecipitation of WT-SMAD4 or SMAD4-I500V with anti-FLAG and detection of phospho-SMAD2. FLAG-WT-SMAD4, FLAG-SMAD4-I500V, or FLAG-WT-SMAD4/FLAG-SMAD4-I500V were immunoprecipitated from cell lysates at basal, TGFβ1-treated, or BMP4-treated conditions, and blotted for phosphorylated SMAD1,5,9 or phosphorylated SMAD2.