Identification of the 95-kDa protein as the serine-phosphorylated endoglin with an antibody agonist phospho–NF-κB p65 (Ser-536). Lysates from HAECs stimulated with thrombin (3 U/mL) for 30 minutes (A), HAEC cytosolic and membrane fractions (B), or lysates from HAECs treated for 3 hours with phenylarsine oxide (PAO; 1μM) (C) were subjected to immunoblotting (IB) with an antibody against phospho–NF-κB p65 (Ser-536). (D) HAEC lysates were immunoprecipitated (IP) with polyclonal antibodies against phospho–NF-κB p65 (Ser-536) (p-p65) or PI3K, or control immunoglobulin G (IgG) from normal rabbit serum and subjected to immnunoblotting with a monoclonal antibody against NH₂-terminus of endoglin. (E) HAEC lysates were immunoprecipitated with a monoclonal antibody (SN6h) against full-length endoglin followed by immunoblotting with the polyclonal antibody against Ser-536–phosphorylated p65. (F) Amino acid sequence of the COOH-terminus of NF-κB p65 or endoglin. (G) Immunoblotting of HAEC cytosolic and membrane fractions with a monoclonal antibody against NH₂-terminus of endoglin.