The type I TGF beta receptor (TbR-I) is activated by phosphorylation of the
GS region, a conserved
juxtamembrane segment located just N-terminal to the kinase domain. We have studied the molecular
mechanism of receptor activation using a homogeneously tetraphosphorylated form of TbR-I, prepared using
protein semisynthesis. Phosphorylation of the GS region dramatically enhances the specificity of TbR-I for the
critical C-terminal serines of Smad2. In addition, tetraphosphorylated TbR-I is bound specifically by Smad2 in a
phosphorylation-dependent manner and is no longer recognized by the inhibitory protein FKBP12. Thus,
phosphorylation activates TbR-I by switching the GS region from a binding site for an inhibitor into a binding
surface for substrate. Our observations suggest that phosphoserine/phosphothreonine-dependent localization is a
key feature of the TbR-I/Smad activation process.
Most protein kinases are activated by phosphorylation on a centrally located loop known as the "activation loop". This is illustrated below for insulin receptor tyrosine kinase, based on crystallographic results from Stevan Hubbard's Lab at NYU:
The Type-I TGF-beta receptor is a transmembrane receptor containing a cytoplasmic catalytic Ser/Thr kinase domain. We had previously determined the structure of the catalytic domain in complex with the inhibitor protein FKBP-12. The type-I receptor, when activated, phosphorylates Smad proteins, transcription factors that then proceed to the nucleus and turn on gene transcription. Interestingly, the type-I TGF-beta receptor does not become activated by phosphorylation of the activation loop. Instead, multiple phosphorylations of Ser and Thr residues in a segment known as the GS-loop, result in receptor activation. We have studied how this happens by producing a multiply phosphorylated form of the Type-I receptor by linking a chemically synthesized phosphopeptide to the rest of the catalytic domain. A schematic diagram of the receptor is shown below:
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