PTF1 Is an Organ-Specific and Notch-Independent Basic Helix-Loop-Helix Complex Containing the Mammalian Suppressor of Hairless (RBP-J) or Its Paralogue, RBP-L

Abstract

PTF1 is a trimeric transcription factor essential to the development of the pancreas and to the maintenance of the differentiated state of the adult exocrine pancreas. It comprises a dimer of P48/PTF1a (a pancreas and neural restricted basic helix-loop-helix [bHLH] protein) and a class A bHLH protein, together with a third protein that we show can be either the mammalian Suppressor of Hairless (RBP-J) or its paralogue, RBP-L. In mature acinar cells, PTF1 exclusively contains the RBP-L isoform and is bound to the promoters of acinar specific genes. P48 interacts with the RBP subunit primarily through two short conserved tryptophan-containing motifs, similar to the motif of the Notch intracellular domain (NotchIC) that interacts with RBP-J. The transcriptional activities of the J and L forms of PTF1 are independent of Notch signaling, because P48 occupies the NotchIC docking site on RBP-J and RBP-L does not bind the NotchIC. Mutations that delete one or both of the RBP-interacting motifs of P48 eliminate RBP-binding and are associated with a human genetic disorder characterized by pancreatic and cerebellar agenesis, which indicates that the association of P48 and RBPs is required for proper embryonic development. The presence of related peptide motifs in other transcription factors indicates a broader Notch-independent function for RBPJ/SU(H).

We thank Jane Johnson for helpful discussions; Susan Hayward for gifts of CBF1 and Notch1 plasmids and advice; Shinichiro Sawada for the anti-HEB serum; Jamie Zoloty for Drosophila RNA; Y. Megan Kong for advice and assistance; and Ken Zaret for advice on ChIP.

This study was supported by Public Health Service grants DK61220 and DK55266 from the National Institute of Diabetes and Digestive and Kidney Diseases.