Identification of two regulatory elements within the high mobility group box transcription factor XTCF-4

Some members of the Wnt family of extracellular glycoproteins regulate targ et gene expression by inducing stabilization and nuclear accumulation of be ta -catenin, which functions as a transcriptional activator after binding t o transcription factors of the T-cell factor! lymphoid enhancer factor (TCF /LEF) family. Three different members of this family have been identified i n Xenopus laevis thus far that differ in their ability to influence mesoder mal differentiation and to activate expression of the Wnt target gene fibro nectin, Here we report on the isolation and characterization of additional variants of XTCF-4. We:show that the differential ability of these proteins and other members of the TCF family to activate target genes is neither du e to preferential interaction with transcriptional cofactors of the groucho family or SMAD4 nor to different DNA binding affinities, Expression of the se proteins in an epithelial cell line reveals differences in their ability to form a ternary complex with DNA and beta -catenin. Interestingly, forma tion of this ternary complex was not sufficient to activate target gene exp ression as previously thought. Our experiments identify two amino acid sequ ence motifs, LVPQ and SFLSS, in the central domain of XTCF-4 that regulate the formation of the DNA-TCF-beta -catenin complex or activation of target genes, respectively Biochemical studies reveal that the phosphorylation sta te of these XTCF-4 variants correlates with their ability to form a ternary complex with beta -catenin and DNA but not to activate target gene express ion, The described variants of XTFC-4 with their different properties in co mplex formation provide strong evidence that in addition to the regulation of beta -catenin stability the isoforms of TCF/LEF transcription factors an d their posttranslational modifications define the cellular response of a W nt/wingless signal.