Multifunctional role of the Pitx2 homeodomain protein C-terminal tail

Pitx2 is a newly described bicoid-like homeodomain transcription factor tha t is defective in Rieger syndrome and shows a striking leftward development al asymmetry. We have previously shown that Pitx2 (also called Ptx2 and RIE G) transactivates a reporter gene containing a bicoid enhancer and synergis tically transactivates the prolactin promoter in the presence of the POU ho meodomain protein Pit-1. In this report, we focused on the C-terminal regio n which is mutated in some Rieger patients and contains a highly conserved 14-amino-acid element. Deletion analysis of Pitx2 revealed that the C-termi nal 39-amino-acid tail represses DNA binding activity and is required for P itx2-Pit-1 interaction and Pit-1 synergism. Pit-1 interaction with the Pitx 2 C terminus masks the inhibitory effect and promotes increased DNA binding activity. Interestingly, cotransfection of an expression vector encoding t he C-terminal 39 amino acids of Pitx2 specifically inhibits Pitx2 transacti vation activity. In contrast, the C-terminal 39-amino-acid peptide interact s with Pitx2 to increase its DNA binding activity. These data suggest that the C-terminal tail intrinsically inhibits the Pitx2 protein and that this inhibition can be overcome by interaction with other transcription factors to allow activation during development.