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Biosynthesis of the type I and type II TGF-beta receptors. Implications for complex formation
R G Wells1, H Yankelev, H Y Lin
1Whitehead Institute, Cambridge, Massachusetts 02142, USA.
Abstract:
The TGF-beta type I and type II receptors (TbetaRI and TbetaRII) are signaling receptors that form heteromeric cell surface complexes with the TGF-betas as one of the earliest events in the cellular response to these multifunctional growth factors. Using TGF-beta-responsive mink lung epithelial cells (Mv1Lu), we have determined the half-lives of the endoplasmic reticulum (ER) and mature forms of these receptors. In metabolically labeled cells, approximately 90% of newly synthesized type II receptor undergoes modification of N-linked sugars in the Golgi, with a half-life of 30-35 min; the Golgi-processed form of the receptor has a relatively short metabolic half-life of 2.5 h. In contrast, only 50% of pulse-labeled type I receptor is converted to the Golgi-processed and therefore endoglycosidase H-resistant form, and the endoglycosidase H-sensitive ER form has a half-life of 2.8-3 h. Addition of 100 pM TGF-beta1 causes the Golgi-processed type II receptor to become less stable, with a half-life of 1.7 h, and also destabilizes the Golgi-processed type I receptor. TGF-beta1 binding and cross-linking experiments on cells treated with tunicamycin for various times confirm different ER to cell surface processing times for TbetaRI and TbetaRII. Our results, which suggest that stable complexes between type I and II TGF-beta receptors do not form until the proteins reach a post-ER compartment (presumably the cell surface), have important implications for our understanding of complex formation and receptor regulation.
Insights
Transforming growth factor-beta (TGF-β) type I and type II receptors (TbetaRI and TbetaRII) have different stabilities and processing times. TGF-β1 binding destabilizes both receptor forms, impacting complex formation.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Transforming growth factor-beta (TGF-β) signaling is crucial for cellular processes.
- TGF-β type I and type II receptors (TbetaRI and TbetaRII) form cell surface complexes.
- Understanding receptor processing and stability is key to TGF-β pathway regulation.
Purpose of the Study:
- To determine the half-lives of endoplasmic reticulum (ER) and mature TbetaRI and TbetaRII.
- To investigate the effect of TGF-β1 on receptor stability and processing.
- To elucidate the timing of TbetaRI and TbetaRII complex formation.
Main Methods:
- Metabolic labeling of mink lung epithelial cells (Mv1Lu).
- Analysis of N-linked sugar modification in the Golgi.
- Endoglycosidase H (Endo H) sensitivity assays.
- TGF-β1 binding and cross-linking experiments.
- Tunicamycin treatment to assess ER to cell surface processing.
Main Results:
- TbetaRII undergoes extensive Golgi modification with a short half-life (2.5 h).
- TbetaRI shows partial Golgi processing, with ER and mature forms having half-lives of ~3 h and ~2.5 h, respectively.
- TGF-β1 binding significantly reduces the half-life of both Golgi-processed receptors (TbetaRII to 1.7 h).
- TbetaRI and TbetaRII exhibit distinct ER to cell surface processing times.
Conclusions:
- Stable TbetaRI and TbetaRII complexes likely form at the cell surface, not in post-ER compartments.
- TGF-β1 binding destabilizes mature receptors, suggesting a regulatory mechanism.
- Differential receptor processing and stability influence TGF-β signaling complex formation.