How a Novel Zinc Finger Domain Improves Target Gene Selection by TCF in WNT/ß-Catenin Signaling.

Abstract

The Wnt/beta-catenin pathway is a highly conserved signaling pathway that plays an important role in metazoan development. T-cell factors (TCF) are transcription factors that regulate the transcription of many Wnt target genes. TCF-mediated transcription is dependent on the ability of TCF to accurately locate its targets. All TCFs contain a High Mobility Group (HMG) domain that recognizes specific DNA sequences. Most invertebrate and some vertebrate TCFs contain an additional DNA binding domain called the C-clamp that binds the Helper site DNA motif. The C-clamp has been found to be important for activation of Wnt reporter genes in cell culture and enhanced in vitro DNA binding. However, not much else is known about this domain. In this study, I characterized the structural and functional properties of the C-clamp. The C-clamp was found to be a zinc-binding domain that was sufficient to bind DNA. It was also capable of inhibiting the HMG domain from binding DNA. C-clamp mutants compromised for either DNA binding or the inhibitory effect were isolated. DNA binding ability was found to be required for TCF function in cell culture and in vivo in patterning the Drosophila embryonic epidermis. This was also the first demonstration of the in vivo contribution of the C-clamp. The inhibitory effect mutant was however partially functional in cell culture and functional in vivo. The analysis of the C-clamp was extended to C.elegans, where functional Helper sites have been discovered in several worm Wnt target genes. I found that the C-clamp was necessary for enhanced DNA binding by the worm TCF, POP-1. In addition, Helper sites were found to improve the ability of a synthetic reporter to respond to Wnt/beta-catenin signaling in vivo in C.elegans, specifically in the intestinal int9 cells. Overall, these findings help highlight the important contribution of the C-clamp in Wnt/beta-catenin signaling.