TRANSCRIPTIONAL ACTIVATING ACTIVITY OF SMAD4 - ROLES OF SMAD HETERO-OLIGOMERIZATION AND ENHANCEMENT BY AN ASSOCIATING TRANSACTIVATOR

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 p athway-restricted SMAD proteins is essential for Smad4-mediated transc ription. We provide evidence that SMAD hetero-oligomerization is direc tly required for the Smad4 C-terminal domain [Smad4(C)] to show its tr anscriptional transactivating activity; this requirement obtains even when Smad4(C) is recruited to promoters by heterologous DNA binding do mains and in the absence of the inhibitory Smad4 N-terminal domain. De fined mutations of GAL4 DNA-binding domain fusion of Smad4(C) that dis rupt SMAD hetero-oligomerization suppressed transcriptional activation , Importantly, we found that an orphan transcriptional activator MSG1, a nuclear protein that has strong transactivating activity but appare ntly lacks DNA-binding activity, functionally interacted with Smad4 an d 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 presenc e of Smad4 N-terminal domain. Furthermore, Smad4(C) did not show any d etectable transactivating activity in yeast when fused to heterologous DNA binding domains. These results demonstrate additional roles of SM AD hetero-oligomerization in Smad4-mediated transcriptional activation . They also suggest that the transcriptional-activating activity obser ved in the presence of Smad4 in mammalian cells may be derived, at lea st in part, from endogenously expressed separate transcriptional activ ators, such as MSG1.