Interactome of transforming growth factor-β type I receptor (TβRI):: Inhibition of TGFβ signaling by Epac1

Transforming growth factor-beta (TGF beta) is a potent regulator of cell growth, differentiation, and apoptosis. Type I TGF beta receptor (T beta RI) is the key receptor for initiation of intracellular signaling by TGF beta. Here we report proteomics-based identification of proteins that form a complex with T beta RI. Using 2D-GE and MALDI TOF mass spectrometry, we identified 16 proteins that specifically interacted with a GST-fused T beta RI Thr204Asp construct with constitutively active serine/threonine kinase. We confirmed interactions of the receptor with cAMP regulated guanine nucleotide exchange factor 1 (Epac1), alpha-spectrin, PIASy, and alpha-catenin proteins using immunoblotting. Interaction of the receptor with Epac1 required intact kinase activity of T beta RI but was not affected by deletion of cAMP-binding domain of Epac1. TGF beta 1-induced C-terminal phosphorylation of Smad2 was inhibited in vivo and in vitro in the presence of Epac1. Epac1 inhibited also TGF beta 1/T beta RI-dependent transcriptional activation, as evaluated by luciferase reporter assays. We observed that expression of Epac1 counteracted TGF beta/T beta RI-dependent decrease of cell adhesion and TGF beta/T beta RI-induced stimulation of cell migration. Thus, we have reported novel T beta RI-interacting proteins and have shown that Epac1 inhibited TGF beta-dependent regulation of cell migration and adhesion.