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Determining the functional attributes of pancreatic transcription factors is essential to understand how the pancreas is specified distinct from other endodermal organs, such as liver, stomach and duodenum, and to direct the differentiation of other cell types into pancreas. Previously, we demonstrated that Pdx1-VP16 was sufficient to convert liver to pancreas. In this paper, we characterize the functional ability of another pancreatic transcription factor, Ptf1a, in promoting ectopic pancreatic fates at early stages throughout the endoderm and later during organogenesis. Using the transthyretin promoter to drive expression in the early liver region/bud of transgenic Xenopus tadpoles, we find that Ptf1a-VP16 is able to convert liver to pancreas. Overexpression of the unmodified Ptf1a on the other hand has no effect in liver but is able to convert stomach and duodenum to pancreas. When overexpressed at earlier embryonic stages throughout the endoderm, Ptf1a activity is similarly limited, whereas Ptf1a-VP16 has increased activity. Interestingly, in all instances we find that Ptf1a-VP16 is only capable of promoting acinar cell fates, whereas Ptf1a promotes both acinar and endocrine fates. Lastly, we demonstrate that, similar to mouse and zebrafish, Xenopus Ptf1a is essential for the initial specification of both endocrine and exocrine cells during normal pancreas development.
The ability of transcriptional activation domains (TADs) to signal ubiquitin-mediated proteolysis suggests an involvement of the ubiquitin-proteasome pathway in transcription. To probe this involvement, we asked how ubiquitylation regulates the activity of a transcription factor containing the VP16 TAD. We show that the VP16 TAD signals ubiquitylation through the Met30 ubiquitin-ligase and that Met30 is also required for the VP16 TAD to activate transcription. The requirement for Met30 in transcription is circumvented by fusion of ubiquitin to the VP16 activator, demonstrating that activator ubiquitylation is essential for transcriptional activation. We propose that ubiquitylation regulates TAD function by serving as a dual signal for activation and activator destruction.
The TATA box-binding protein (TBP) interacts in vitro with the activation domains of many viral and cellular transcription factors and has been proposed to be a direct target for transcriptional activators. We have examined the functional relevance of activator-TBP association in vitro to transcriptional activation in vivo. We show that alanine substitution mutations in a single loop of TBP can disrupt its association in vitro with the activation domains of the herpes simplex virus activator VP16 and of the human tumor suppressor protein p53; these mutations do not, however, disrupt the transcriptional response of TBP to either activation domain in vivo. Moreover, we show that a region of VP16 distinct from its activation domain can also tightly associate with TBP in vitro, but fails to activate transcription in vivo. These data suggest that the ability of TBP to interact with activation domains in vitro is not directly relevant to its ability to support activated transcription in vivo.