In vivo role of different domains and of phosphorylation in the transcription factor Nkx2-1

Mouse strains expressing variants of the transcription factor Nkx2-1 deleted of either activation domain have been constructed. Phenotypic analysis shows for each mutant a distinct set of defects demonstrating that distinct portions of the protein endow diverse developmental functions of Nkx2-1. Furthermore, a mouse strain expressing a Nkx2-1 protein mutated in the phosphorylation sites shows a thyroid gland with deranged follicular organization and gene expression profile demonstrating the functional role of phosphorylation in Nkx2-1.The pleiotropic functions of Nkx2-1 are not all due to the protein as a whole since some of them can be assigned to separate domains of the protein or to specific post-translational modifications. These results have implication for the evolutionary role of mutations in transcription factors.Transcription factors (TFs) bind to DNA and regulate mRNA synthesis in response to different stimuli via multiple protein domains endowing separate functions such as the binding to small ligands, the recognition of specific DNA sequences and the ability to activate or repress transcription. For the latter function it is frequently observed that more than one activation or repression domain can be present in a single TF [1]. In addition, it is well known that transcription factors can be regulated by post-translational modifications, chiefly phosphorylation[2]. However, the function of the diverse activation domains included in a single transcription factor or the role of post-translational modification has been assessed largely in cultured cells. This approach has obvious limitations since many TFs play important roles in diverse cell types and at different stages of development. Thus, whether the diverse functions of a TF could be assigned to separate protein domains or to post-translational modifications is a question that has been rarely addressed and it requires to carry out structure-function relationships studies in whole organisms expressing