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Calmodulin differentially modulates Smad1 and Smad2 signaling
1Center for Developmental Biology, Department of Molecular Biology and Oncology, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9133, USA.
Abstract:
The members of the Smad protein family are intracellular mediators of transforming growth factor beta (TGF-beta) signaling. Smad1 transduces bone morphogenetic protein signals, inducing formation of ventral mesoderm in Xenopus embryos, whereas Smad2 transduces activin/TGF-beta signals, generating dorsal mesoderm. Calmodulin directly binds to many Smads and was shown to down-regulate Smad2 activity in a cell culture system (Zimmerman, C. M., Kariapper, M. S. T., and Mathews, L. S. (1997) J. Biol. Chem. 273, 677-680). Here, we extend those data and demonstrate that calmodulin alters Smad signaling in living embryos, increasing Smad1 activity while inhibiting Smad2 function. To characterize this regulation, we undertook a structure-function analysis and found that calmodulin binds to two distinct and conserved regions in both Smad1 and Smad2. Receptor tyrosine kinase signaling also modifies Smad activity (Kretzschmar, M., Doody, J., and Massagué, J. (1997) Nature 389, 618-622; Kretzschmar, M., Doody, J., Timokhina, I., and Massagué, J. (1999) Genes Dev. 13, 804-816; de Caestecker, M. P., Parks, W. T., Frank, C. J., Castagnino, P., Bottaro, D. P., Roberts, A. B., and Lechleider, R. J. (1998) Genes Dev. 12, 1587-1592). We show that calmodulin binding to Smads inhibits subsequent Erk2-dependent phosphorylation of Smads and vice versa. These observations suggest the presence of a cross-talk between three major signaling cascades as follows: Ca(2+)/calmodulin, receptor tyrosine kinase, and TGF-beta pathways.
Insights
Calmodulin modulates Smad protein activity in embryos, enhancing Smad1 and inhibiting Smad2 signaling. This reveals cross-talk between Ca(2+)/calmodulin, receptor tyrosine kinase, and TGF-beta pathways.
Area of Science:
- Molecular Biology
- Developmental Biology
- Cell Signaling
Background:
- Smad proteins are key intracellular mediators of transforming growth factor beta (TGF-beta) signaling pathways.
- Smad1 and Smad2 play distinct roles in embryonic development, regulating mesoderm formation in Xenopus.
- Calmodulin is known to interact with Smad proteins, potentially regulating their activity.
Purpose of the Study:
- To investigate the role of calmodulin in Smad signaling within living embryos.
- To characterize the structural basis of calmodulin-Smad interactions.
- To explore the cross-talk between Ca(2+)/calmodulin, receptor tyrosine kinase (RTK), and TGF-beta signaling pathways.
Main Methods:
- In vivo studies using Xenopus embryos to assess Smad signaling.
- Structure-function analysis to identify calmodulin binding sites on Smad proteins.
- Investigating the interplay between Smad phosphorylation by Erk2 and calmodulin binding.
Main Results:
- Calmodulin alters Smad signaling in vivo, increasing Smad1 activity and inhibiting Smad2 function.
- Calmodulin binds to two conserved regions in both Smad1 and Smad2.
- Calmodulin binding to Smads inhibits Erk2-dependent phosphorylation, and vice versa, indicating pathway cross-talk.
Conclusions:
- Calmodulin directly regulates Smad1 and Smad2 activity in a developmental context.
- A significant cross-talk exists between the Ca(2+)/calmodulin, RTK, and TGF-beta signaling cascades.
- These findings provide new insights into the complex regulation of developmental signaling pathways.