A regulatory role for the unstructured C-terminal domain of the CtBP transcriptional corepressor

The C-terminal Binding Protein (CtBP) is a transcriptional corepressor that plays critical roles in development, tumorigenesis, and cell fate. CtBP proteins are structurally similar to alpha hydroxyacid dehydrogenases and additionally feature an unstructured C-terminal domain (CTD). The role of a possible dehydrogenase activity has been postulated for the corepressor, although substrates are unknown, but the functional significance of the CTD is unclear. In the mammalian system, CtBP proteins lacking the CTD are able to function as transcriptional regulators and oligomerize, putting into question the significance of the CTD for gene regulation. Yet, the presence of an unstructured CTD of ∼100 residues, including some short motifs, is conserved across Bilateria, indicating the importance of this domain. To study the functional significance of the CTD, we turned to the system, which naturally expresses isoforms with the CTD (CtBP(L)), and isoforms lacking the CTD (CtBP(S)). We used the CRISPRi system to test dCas9-CtBP(S) and dCas9-CtBP(L) on diverse endogenous genes, to directly compare their transcriptional impacts . Interestingly, CtBP(S) was able to significantly repress transcription of the and genes, while CtBP(L) had minimal impact, suggesting that the long CTD modulates CtBP's repression activity. In contrast, in cell culture, the isoforms behaved similarly on a transfected reporter. Thus, we have identified context-specific effects of these two developmentally-regulated isoforms, and propose that differential expression of CtBP(S) and CtBP(L) may provide a spectrum of repression activity suitable for developmental programs.