Molecular and systemic functions of the vertebrate-specific TATA-binding protein N terminus
The invertebrate/vertebrate transition and associated innovations can be regarded as a major event in evolution. Recent molecular progresses invite to an analysis of the events leading to the apparition of vertebrates and the underlying embellishment in gene regulation. In eukaryotes, the TATA-Binding Protein (TBP) has a central role in transcription initiation of most genes. TBP is comprised of a highly conserved DNA binding domain and, in vertebrates, it also contains a novel region: the N-terminal TBP protein coding sequence. The role of the TBP-N is largely unknown, but previous studies suggest that it is important for fetal survival. Most animals lacking the TBP-N (tbp Delta N/Delta N) die before weaning. The goal of the present work was to establish a deeper knowledge of the vertebrate-specific TBP-N. It was hypothesized that TBP-N could be involved in protein-protein interactions and that the high degree of similarity of TBP-N protein sequences in different species could correlate with similar functions. To test those hypotheses, two independent approaches were taken: (1) Protein-protein interactions involving the TBP-N via unbiased screens were characterized. (2) The mouse TBP-N was replaced by a similar and homologous TBP-N, in vivo, through homologous recombination. The TBP-N-replacement mutation was characterized through pathway analyses, bioinformatics, and whole-animal physiology. Screens for proteins interacting with the TBP-N of hagfish (hf), a basal vertebrate, uncovered hfPitxA. The Pitx family of transcription factors are proteins important in vertebrate development. The mouse paralogs of hfPitxA, Pitx1 and Pitx2, were found to interact with the mouse TBP-N. Moreover, the interaction appeared functional as it regulated the expression of nppa, a known target gene of Pitx2. In vivo replacement of the mouse TBP-N with the similar hfTBP-N did not affect the survival. Gene expression analysis indicated that lipid metabolism pathways were affected in animals lacking the TBP-N or when the hfTBP-N was present. Further analyses pointed toward a potential defect in insulin response and an abnormal hepatic fat storage. The data presented here argues in favor of an important role for TBP-N in vertebrate-specific gene regulation. More specifically, it is likely involved in heart development and in regulation of lipid metabolism.