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GS domain mutations that constitutively activate T beta R-I, the downstream signaling component in the TGF-beta
R Wieser1, J L Wrana, J Massagué
1Howard Hughes Medical Institute, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA.
Abstract:
The TGF-beta type II receptor (T beta R-II) is a transmembrane serine/threonine kinase that, upon ligand binding, recruits and phosphorylates a second transmembrane kinase, T beta R-I, as a requirement for signal 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 conserved in type I receptors for other TGF-beta-related factors. The functional role of seven serines and threonines in the T beta R-I GS domain was investigated by mutational analysis. Five of these residues are clustered (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 inhibits receptor phosphorylation and signaling activity. Replacement of one of these residues, Thr204, with aspartic acid yields a product that has elevated in vitro kinase activity and signals anti-proliferative and 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 down-stream signaling component in the TGF-beta receptor complex, and its activation by T beta R-II or by mutation is necessary and sufficient for propagation of anti-proliferative and transcriptional responses.
Insights
Mutational analysis of the TGF-beta type I receptor (T beta R-I) GS domain revealed key serine and threonine residues essential for phosphorylation and signaling. Constitutively active T beta R-I forms were identified, confirming its role in TGF-beta pathway signal transduction.
Area of Science:
- Cellular signaling pathways
- Molecular biology
- Receptor tyrosine kinases
Background:
- Transforming growth factor-beta (TGF-beta) signaling is crucial for cellular processes.
- The TGF-beta receptor complex involves type II (T beta R-II) and type I (T beta R-I) transmembrane kinases.
- T beta R-II phosphorylates T beta R-I in its GS domain for signal transduction.
Purpose of the Study:
- To investigate the functional significance of serine and threonine residues within the T beta R-I GS domain.
- To identify mutations that confer constitutive activity to T beta R-I.
- To confirm the role of T beta R-I as a downstream signaling component.
Main Methods:
- Site-directed mutagenesis of serine and threonine residues in the T beta R-I GS domain.
- Analysis of receptor phosphorylation and signaling activity in response to mutations.
- In vitro kinase assays and assessment of anti-proliferative and transcriptional responses.
Main Results:
- Mutation of clustered GS domain residues impairs phosphorylation and signaling.
- Mutation of threonine residues near the kinase domain strongly inhibits activity.
- A specific mutation (Thr204 to Asp) resulted in constitutively active T beta R-I with elevated kinase activity.
- Constitutively active T beta R-I signaled anti-proliferative and transcriptional responses independently of ligand and T beta R-II.
Conclusions:
- Specific serine and threonine residues in the T beta R-I GS domain are critical for TGF-beta receptor complex assembly and signaling.
- T beta R-I activation, either by T beta R-II or by specific mutations, is sufficient for downstream signal propagation.
- The findings validate T beta R-I as a key downstream effector in the TGF-beta signaling pathway.