The members of the Smad protein family are intracellular mediators of transforming growth factor ␤ (TGF-␤) signaling. Smad1 transduces bone morphogenetic protein signals, inducing formation of ventral mesoderm in Xenopus embryos, whereas Smad2 transduces activin/ TGF-␤ signals, generating dorsal mesoderm. Calmodulin directly binds to many Smads and was shown to downregulate Smad2 activity in a cell culture system (Zimmerman, C. M., Kariapper, M. S. T., and Mathews, L. S. (1997) J. Biol. Chem.

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... 3, 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, . (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-␤ pathways.