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Development December 1, 2007; 134 (23): 4209-18.

Nuclear accumulation of Smad complexes occurs only after the midblastula transition in Xenopus.

Saka Y , Hagemann AI , Piepenburg O , Smith JC .

Activin and the Nodal-related proteins induce mesendodermal tissues during Xenopus development. These signals act through specific receptors to cause the phosphorylation, at their carboxyl termini, of Smad2 and Smad3. The phosphorylated Smad proteins form heteromeric complexes with Smad4 and translocate into the nucleus to activate the transcription, after the midblastula transition, of target genes such as Xbra and goosecoid (gsc). In this paper we use bimolecular fluorescence complementation (BiFC) to study complex formation between Smad proteins both in vivo and in response to exogenous proteins. The technique has allowed us to detect Smad2-Smad4 heteromeric interactions during normal Xenopus development and Smad2 and Smad4 homo- and heteromers in isolated Xenopus blastomeres. Smad2-Smad2 and Smad2-Smad4 complexes accumulate rapidly in the nuclei of responding cells following Activin treatment, whereas Smad4 homomeric complexes remain cytoplasmic. When cells divide, Smad2-Smad4 complexes associate with chromatin, even in the absence of ligand. Our observation that Smad2-Smad4 complexes accumulate in the nucleus only after the midblastula transition, irrespective of the stage at which cells were treated with Activin, may shed light on the mechanisms of developmental timing.

PubMed ID: 17959720
PMC ID: PMC2435607
Article link: Development
Grant support: [+]

Species referenced: Xenopus
Genes referenced: gsc nodal nodal1 smad2 smad3 smad4 smad4.2 tbxt

References [+] :
Armes, The ALK-2 and ALK-4 activin receptors transduce distinct mesoderm-inducing signals during early Xenopus development but do not co-operate to establish thresholds. 1997, Pubmed, Xenbase