RECONSTITUTION AND TRANSPHOSPHORYLATION OF TGF-BETA RECEPTOR COMPLEXES

Transforming growth factor-beta (TGF-beta) signals by contacting two d istantly related transmembrane serine/threonine kinases called recepto rs I (T beta R-I) and II (T beta R-II). TGF-beta binds to T beta R-II, which is a constitutively active kinase and this complex recruits T b eta R-I, causing its phosphorylation and signal propagation to downstr eam substrates. The biochemical properties of this interaction were an alyzed with reconstituted receptor systems. T beta R-I and T beta R-II baculovirally expressed at high levels in insect cells have the ligan d binding properties of receptors expressed in mammalian cells, and fo rm a complex in which T beta R-I phosphorylation is dependent on the k inase activity of T beta R-II. Furthermore, T beta R-I and T beta R-II can form a complex in vitro, and their cytoplasmic domains can specif ically interact in a yeast two-hybrid system. In vitro complex formati on with catalytically active T beta R-II is necessary and sufficient f or T beta R-I phosphorylation, which within this complex does not requ ire the catalytic activity of T beta R-I, thus mimicking T beta R-I ph osphorylation in intact cells. In addition, T beta R-I phosphorylated in vitro remains associated with T beta R-II. These results suggest th at T beta R-I and T beta R-II have affinity for each other, however, t he ligand is required for stable complex formation under physiological conditions, Once formed, this complex is sufficient for T beta R-I ph osphorylation by T beta R-II.