DISRUPTION OF TRANSFORMING GROWTH-FACTOR-BETA SIGNALING BY A MUTATIONTHAT PREVENTS TRANSPHOSPHORYLATION WITHIN THE RECEPTOR COMPLEX

T beta R-II (transforming growth factor beta [TGF-beta] type II recept or) is a transmembrane serine/threonine kinase that acts as the primar y TGF-beta receptor. Ligand binding to T beta R-II leads to the recrui tment and phosphorylation of TPR-I, a distantly related transmembrane kinase that acts as a downstream signaling component, T beta R-I phosp horylation by T beta R-II is shown here to be essential for signaling, A mutant T beta R-II that binds ligand hut lacks signaling activity w as identified. This mutant was identified by screening with a TGF-beta -inducible vector a series of mink lung epithelial cell clones that ha ve normal TGF-beta binding activity but have lost antiproliferative an d transcriptional responses to TGF-beta. When transiently cotransfecte d with T beta R-II, one of these cell lines, S-21, recovered TGF-beta responsiveness. cDNA cloning and sequencing of T beta R-II from S-21 c ells revealed a point mutation that changes proline 525 to leucine in kinase subdomain XI. A recombinant receptor containing this mutation, T beta R-II(P525L), is similar to wild-type T beta R-II in its abiliti es to bind ligand, support ligand binding to T beta R-I, and form a co mplex with T beta R-I in vivo, T beta R-II(P525L) has autophosphorylat ing activity in vitro and in vivo; however, unlike the wild-type recep tor, it fails to phosphorylate an associated T beta R-I. These results suggest that T beta R-II(P525L) is a catalytically active receptor th at cannot recognize T beta R-I as a substrate, The close link between T beta R-I transphosphorylation and signaling activity argues that tra nsphosphorylation is essential for signal propagation via T beta R-I.