TβRIII independently binds type I and type II TGF-β receptors to inhibit TGF-β signaling

Transforming growth factor-beta (TGF-beta) receptor oligomerization has important roles in signaling. Complex formation among type I and type II (T beta RI and T beta RII) TGF-beta receptors is well characterized and is essential for signal transduction. However, studies on their interactions with the type III TGF-beta coreceptor (T beta RIII) in live cells and their effects on TGF-beta signaling are lacking. Here we investigated the homomeric and heteromeric interactions of T beta RIII with T beta RI and T beta RII in live cells by combining IgG-mediated patching/immobilization of a given TGF-beta receptor with fluorescence recovery after photobleaching studies on the lateral diffusion of a coexpressed receptor. Our studies demonstrate that T beta RIII homo-oligomerization is indirect and depends on its cytoplasmic domain interactions with scaffold proteins (mainly GIPC). We show that T beta RII and T beta RI bind independently to T beta RIII, whereas T beta RIII augments T beta RI/T beta RII association, suggesting that T beta RI and T beta RII bind to T beta RIII simultaneously but not as a complex. T beta RIII expression inhibited TGF-beta-mediated Smad2/3 signaling in MDA-MB-231 cell lines, an effect that depended on the T beta RIII cytoplasmic domain and did not require T beta RIII ectodomain shedding. We propose that independent binding of T beta RI and T beta RII to T beta RIII competes with T beta RI/T beta RII signaling complex formation, thus inhibiting TGF-beta-mediated Smad signaling.