GS DOMAIN MUTATIONS THAT CONSTITUTIVELY ACTIVATE T-BETA-R-I, THE DOWNSTREAM SIGNALING COMPONENT IN THE TGF-BETA RECEPTOR COMPLEX

The TGF-beta type II receptor (T beta R-II) is a transmembrane serine/ threonine kinase that, upon ligand binding, recruits and phosphorylate s a second transmembrane kinase, T beta R-I, as a requirement for sign al transduction, T beta R-I is phosphorylated by T beta R-II in the GS domain, a 30 amino acid region preceding the kinase domain and conser ved in type I receptors for other TGF-beta-related factors, The functi onal role of seven serines and threonines in the T beta R-I GS domain was investigated by mutational analysis, Five of these residues are cl ustered (TTSGSGSG) in the middle of the GS domain, Mutation of two or more of these residues impairs phosphorylation and signaling activity, Two additional threonines are located near the canonical start of the kinase domain, and their individual mutation to valine strongly inhib its receptor phosphorylation and signaling activity. Replacement of on e of these residues, Thr204, with aspartic acid yields a product that has elevated in vitro kinase activity and signals anti-proliferative a nd transcriptional responses in the absence of ligand and T beta R-II, The identification of constitutively active T beta R-I forms confirms the hypothesis that this kinase acts as a downstream signaling compon ent in the TGF-beta receptor complex, and its activation by T beta R-I I or by mutation is necessary and sufficient for propagation of antipr oliferative and transcriptional responses.