Receptor-associated Mad homologues synergize as effectors of the TGF-beta response

TRANSFORMING growth factor-beta TGP-Beta is the prototype for a family of extracellular proteins that affect cell proliferation and tissue differentiation(1-3). TGF-beta-related factors, including BMP-2/4, Dpp and activin, act through two types of serine/threonine kinase receptors which can form a heteromeric complex(3,4). However, the mechanism of signal transduction by these receptors is largely unknown. In Drosophila, Mad is required for signalling by Dpp(5). We have isolated complementary DNAs for four human Mad homologues, one of which, hMAD-4, is identical to DPC-4, a candidate tumour suppressor(6). hMAD-3 and -4 synergized to induce strong ligand-independent TGF-beta-like responses. When truncated at their carboxy termini, hMAD-3 and -4 act as dominant-negative inhibitors of the normal TGF-beta response. The activity of hMAD-3 and -4 was regulated by the TGF-beta receptors, and hMAD-3 but not hMAD-4 was phosphorylated and associated with the ligand-bound receptor complex. These results define hMAD-3 and -4 as effecters of the TGF-beta response and demonstrate a function for DPC-4/hMAD-4 as a tumour suppressor.