Receptors for the TGF-beta superfamily: multiple polypeptides and serine/threonine kinases

Members of the transforming growth factor beta (TGF-beta) superfamily of peptide growth factors have profound effects on the growth and differentiation of many cell types. Insights into the poorly understood mechanisms of action of these ligands have come from the recent molecular cloning of two types of high-affinity receptors - type II and type III - for TGF-beta superfamily members. The cell surface expression of the type III receptor, a membrane-bound proteoglycan, appears to modulate the binding of ligand to the type II receptor, which is a transmembrane serine/threonine kinase. These results provide evidence for interactions between different receptor types, and suggest that serine/threonine phosphorylation is an important element in TGF-beta-induced signalling.