Characterization of the interaction of FKBP12 with the transforming growth factor-beta type I receptor in vivo

The type I transforming growth factor-beta receptor (T beta R-I) is the efferent component of the receptor complex, which presumably phosphorylates intracellular targets. FKBP12, a binding protein for FK506 and rapamycin, is shown to associate with the cytoplasmic region of T beta R-I in vitro. In this report, we investigated the interaction of FKBP12 with T beta R-I in vivo. FKBP12 interacts with T beta R-I in mammalian cells as well as in yeast. Ligand addition does not affect the interaction, and both constitutively active and kinase-negative mutants of T beta R-I bind FKBP12. FKBP12 dissociates from T beta PR-I in the presence of a high concentration of FK506. The juxtamembrane region of T beta R-I, containing the major phosphorylation sites by the type II receptor, is required for the interaction. One of the deletion mutants in this region, which was shown to mediate transcriptional response, does not bind FKBP12, suggesting that FKBP12 is not directly involved in TGF-beta signaling. Furthermore T beta R-I does not phosphorylate FKBP12 in vitro, FKBP12 may not be a direct substrate of T beta R-I but possibly modulates the T beta R-I function through its interaction with the regulatory domain of the kinase.