Inhibition of Tcf3 Binding by I-mfa Domain Proteins

We have determined that I-mfa, an inhibitor of several basic helix-loop-helix (bHLH) proteins, and XIC, a Xenopus ortholog of human I-mf domain-containing protein that shares a highly conserved cysteine-rich C-terminal domain with I-mfa, inhibit the activity and DNA binding of the HMG box transcription factor XTcf3. Ectopic expression of I-mfa or XIC in early Xenopus embryos inhibited dorsal axis specification, the expression of the Tcf3/β-catenin-regulated genes siamois and Xnr3, and the ability of β-catenin to activate reporter constructs driven by Lef/Tcf binding sites. I-mfa domain proteins can regulate both the Wnt signaling pathway and a subset of bHLH proteins, possibly coordinating the activities of these two critical developmental pathways.

本研究证实，作为多种碱性螺旋-环-螺旋（basic helix-loop-helix, bHLH）蛋白抑制剂的I-mfa，以及与人类含I-mf结构域蛋白的非洲爪蟾同源物XIC（其与I-mfa共享高度保守的富含半胱氨酸的C端结构域），可抑制高迁移率族蛋白盒（HMG box）转录因子XTcf3的活性与DNA结合能力。在早期非洲爪蟾胚胎中异位表达I-mfa或XIC，可抑制背轴特化过程、Tcf3/β-连环蛋白（β-catenin）调控的靶基因siamois与Xnr3的表达，以及β-连环蛋白激活由Lef/Tcf结合位点驱动的报告基因构建体的能力。含I-mfa结构域的蛋白可同时调控Wnt信号通路与部分bHLH蛋白，或可协调这两条关键发育通路的活性。