Novel Transcriptional Potentiation of BETA2/NeuroD on the Secretin Gene Promoter by the DNA-Binding Protein Finb/RREB-1

Abstract

The basic helix-loop-helix protein BETA2/NeuroD activates transcription of the secretin gene and is essential for terminal differentiation of secretin-producing enteroendocrine cells. However, in heterodimeric complexes with its partner basic helix-loop-helix proteins, BETA2 does not appear to be a strong activator of transcription by itself. Mutational analysis of a proximal enhancer in the secretin gene identified several cis-acting elements in addition to the E-box binding site for BETA2. We identified by expression cloning the zinc finger protein RREB-1, also known to exist as a longer form, Finb, as the protein binding to one of the mutationally sensitive elements. Finb/RREB-1 lacks an intrinsic activation domain and by itself did not activate secretin gene transcription. Here we show that Finb/RREB-1 can associate with BETA2 to enhance its transcription-activating function. Both DNA binding and physical interaction of Finb/RREB-1 with BETA2 are required to potentiate transcription. Thus, Finb/RREB-1 does not function as a classical activator of transcription that recruits an activation domain to a DNA-protein complex. Finb/RREB-1 may be distinguished from coactivators, which increase transcription without sequence-specific DNA binding. We suggest that Finb/RREB-1 should be considered a potentiator of transcription, representing a distinct category of transcription-regulating proteins.

ACKNOWLEDGMENTS

This work was supported in part by NIH grants DK43673 and DK52870 to A.B.L. and by GRASP Digestive Disease Center grants P30-DK34928 and T32-DK07542.

We thank Barry Nelkin (Johns Hopkins University, Baltimore, Md.), Tadashi Yamamoto (University of Tokyo, Tokyo, Japan), and Tom Curran (St. Jude Children's Research Hospital, Memphis, Tenn.) for generously providing plasmids as noted under Materials and Methods.