TGF-β signalling is mediated by two autonomously functioning TβRI:TβRII pairs

Transforming growth factor (TGF)-beta s are dimeric polypeptides that have vital roles in regulating cell growth and differentiation. They signal by assembling a receptor heterotetramer composed of two T beta RI:T beta RII heterodimers. To investigate whether the two heterodimers bind and signal autonomously, one of the TGF-beta protomers was substituted to block receptor binding. The substituted dimer, TGF-beta 3 WD, bound the TbRII extracellular domain and recruited the TbRI with affinities indistinguishable from TGF-beta 3, but with one-half the stoichiometry. TGF-beta 3 WD was further shown to retain one-quarter to one-half the signalling activity of TGF-beta 3 in three established assays for TGF-beta function. Single-molecule fluorescence imaging with GFP-tagged receptors demonstrated a measurable increase in the proportion of TbRI and TbRII dimers upon treatment with TGF-beta 3, but not with TGF-beta 3 WD. These results provide evidence that the two T beta RI:T beta RII heterodimers bind and signal in an autonomous manner. They further underscore how the TGF-beta s diverged from the bone morphogenetic proteins, the ancestral ligands of the TGF-beta superfamily that signal through a RI:RII:RII heterotrimer. The EMBO Journal (2011) 30, 1263-1276. doi: 10.1038/emboj.2011.54; Published online 18 March 2011