Ligand-induced recruitment and phosphorylation of reduced TGF-beta type I receptor.

Receptors I (T beta R-I) and II (T beta R-II) of transforming growth factor-beta (TGF-beta) are components of a heteromeric complex in which receptor I requires receptor II to bind ligand, and receptor II requires receptor I to signal. We show that pretreatment of cells with low concentrations of dithiothreitol, which is known to disrupt ligand binding to T beta R-I, does not prevent interaction and complex formation between T beta R-II and T beta R-I. Nevertheless, our results demonstrate that ligand interaction with T beta R-I is able to induce high-affinity convertion to the complex formed. We also demonstrate that transphosphorylation of T beta R-I through bound T beta R-II can occur independently of ligand binding to T beta R-I.