Figure 3.

 (A) Receptor–Smad association in COS-1 cells transfected with the indicated type I receptors, the corresponding type II receptors, and Flag-tagged Smad1(1–454) or Smad2(1–456). Receptors were cross-linked to ¹²⁵I-labeled TGF-β1 (left) or ¹²⁵I-labeled BMP2 (right). Smad-bound receptors were visualized by anti-Flag immunoprecipitation, SDS-PAGE, and autoradiography (top). Total cell lysates were analyzed to control for receptor expression (middle). Smad expression was controlled by immunoprecipitation from metabolically labeled cells (bottom). (B) Smad phosphorylation was determined in L17 cells transfected with Flag-tagged Smads, the indicated type I receptors, and the corresponding type II receptors. Cells were labeled with [³²P]phosphate, incubated with TGF-β1 or BMP2, and immunoprecipitated with anti-Flag.