Transforming growth factor-β (TGF-β) signaling is mediated by a complex of type I (TBRI) and type II (TBRII) receptors. The type III receptor (TBRIII) lacks a recognizable signaling domain and has no dearly defined role in TGF-β signaling. Cardiac endothelial cells that undergo epithelial- mesenchymal transformation express TBRIII, and here TBRIII-specific antisera were found to inhibit mesenchyme formation and migration in atrioventricular cushion explants. Misexpression of TBRIII in nontransforming ventricular endothelial ceils conferred transformation in response to TGF-β2. These results support a model where TBRIII localizes transformation in the heart and plays an essential, nonredundant role in TGF-β signaling.
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