Structural characterization of the C-Terminal binding protein family

C-terminal binding proteins are members of a recently discovered protein family (isoforms CtBP1, 2, and 3) showing two distinct functions according to their cellular localization. In the nucleus, CtBP acts as a transcriptional co-repressor protein, targeting DNA-binding regulatory proteins, in the Golgi apparatus, at least for the CtBP3 isoform, it has been shown that CtBP is involved in intracellular membrane trafficking. CtBPs are composed of three distinct domains. The substrate-binding domain (mainly localized a the N-terminus) is involved in the binding of transcription repressors, relying on the specific recognition of a five-residue motif, -Pro-X-Asp-Leu-Ser- (PXDLS), present in the DNA-binding factors and in histone deacetylases. The nucleotide-binding domain, or central domain, binds a NAD(H) molecule responsible for modulation of co-repression activity. The C-terminal domain is thought to play a regulatory function, being the site of post-transcriptional modifications, like phosphorylation and SUMOylation. In the past, we solved the 3D structure of a truncated form of mouse CtBP3, comprising the substrate- and the nucleotide-binding domains. On the contrary, the structure of the CtBP C-terminal segment, or that of the full-length protein, have remained elusive and uncharacterized so far. Here, we investigate the structural features of full-length CtBPs using complementary biophysical approaches, including bioinformatics, NMR, and small angle X-ray scattering (SAXS), applied to mouse CtBP3.