Figure 5.

 Matching receptor L45 loops and R-Smad L3 loops. (A) The L3 loop determines Smad activation by a specific receptor but not Smad interaction with Fast1. COS1 cells were transfected with Flag-tagged Smad constructs, myc-tagged Fast1, and TGF-β receptors or BMP receptors. Cells were incubated with the corresponding receptor ligands, TGF-β1 or BMP4, and Smad association with Fast1 was determined. [Ig(H)] immunoglobulin heavy chain. (B,C) TβR-I(LB) rescues the ability of TGF-β to induce Smad2(L1) association with Fast1 (B) and activation of the A3–luciferase Mix.2 reporter (C). R1B/L17 cells transfected with various constructs, as indicated, were incubated with 0.5 nm TGF-β for 20 hr, and luciferase activity was measured.