Transcriptional activating activity of Smad4: roles of SMAD hetero-oligomerization and enhancement by an associating transactivator.

Shioda T, Lechleider RJ, Dunwoodie SL, Li H, Yahata T, de Caestecker MP, Fenner MH, Roberts AB, Isselbacher KJ
Proc Natl Acad Sci U S A. 1998 95 (17): 9785-90

PMID: 9707553 · PMCID: PMC21414 · DOI:10.1073/pnas.95.17.9785

Smad4 plays a pivotal role in signal transduction of the transforming growth factor beta superfamily cytokines by mediating transcriptional activation of target genes. Hetero-oligomerization of Smad4 with the pathway-restricted SMAD proteins is essential for Smad4-mediated transcription. We provide evidence that SMAD hetero-oligomerization is directly required for the Smad4 C-terminal domain [Smad4(C)] to show its transcriptional transactivating activity; this requirement obtains even when Smad4(C) is recruited to promoters by heterologous DNA-binding domains and in the absence of the inhibitory Smad4 N-terminal domain. Defined mutations of GAL4 DNA-binding domain fusion of Smad4(C) that disrupt SMAD hetero-oligomerization suppressed transcriptional activation. Importantly, we found that an orphan transcriptional activator MSG1, a nuclear protein that has strong transactivating activity but apparently lacks DNA-binding activity, functionally interacted with Smad4 and enhanced transcription mediated by GAL4 DNA-binding domain-Smad4(C) and full-length Smad4. Transcriptional enhancement by MSG1 depended on transforming growth factor beta signaling and was suppressed by Smad4(C) mutations disrupting SMAD hetero-oligomerization or by the presence of Smad4 N-terminal domain. Furthermore, Smad4(C) did not show any detectable transactivating activity in yeast when fused to heterologous DNA-binding domains. These results demonstrate additional roles of SMAD hetero-oligomerization in Smad4-mediated transcriptional activation. They also suggest that the transcriptional-activating activity observed in the presence of Smad4 in mammalian cells may be derived, at least in part, from endogenously expressed separate transcriptional activators, such as MSG1.

MeSH Terms (20)

3T3 Cells Animals Binding Sites DNA-Binding Proteins Humans In Vitro Techniques Mice Mutation Nuclear Proteins Protein Conformation Recombinant Fusion Proteins Saccharomyces cerevisiae Saccharomyces cerevisiae Proteins Signal Transduction Smad4 Protein Trans-Activators Transcriptional Activation Transcription Factors Transfection Transforming Growth Factor beta

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