Abstract
β-Catenin is the nuclear effector of the Wnt signaling cascade. The mechanism by which nuclear activity of β-catenin is regulated is not well defined. Therefore, we used the nuclear marker RanGTP to screen for novel nuclear β-catenin binding proteins. We identified a cofactor of chromosome region maintenance 1 (CRM1)–mediated nuclear export, Ran binding protein 3 (RanBP3), as a novel β-catenin–interacting protein that binds directly to β-catenin in a RanGTP-stimulated manner. RanBP3 inhibits β-catenin–mediated transcriptional activation in both Wnt1- and β-catenin–stimulated human cells. In Xenopus laevis embryos, RanBP3 interferes with β-catenin–induced dorsoventral axis formation. Furthermore, RanBP3 depletion stimulates the Wnt pathway in both human cells and Drosophila melanogaster embryos. In human cells, this is accompanied by an increase of dephosphorylated β-catenin in the nucleus. Conversely, overexpression of RanBP3 leads to a shift of active β-catenin toward the cytoplasm. Modulation of β-catenin activity and localization by RanBP3 is independent of adenomatous polyposis coli protein and CRM1. We conclude that RanBP3 is a direct export enhancer for β-catenin, independent of its role as a CRM1-associated nuclear export cofactor.
Highlights
The Wnt signaling pathway regulates a variety of processes during homeostasis and development, including cellular proliferation, cell fate decision, axis formation, and organ development (Nusse, 1999)
When we coexpressed Wnt1 and Ran binding protein 3 (RanBP3) short hairpin RNAs (shRNAs), we observed significant increases in T cell factor (TCF)/lymphocyte enhancer binding factor (LEF) reporter activity compared with the GFP–RNA interference (RNAi) control
We show that RanBP3 binds directly to -catenin and that the interaction is increased in the presence of RanGTP
Summary
The Wnt signaling pathway regulates a variety of processes during homeostasis and development, including cellular proliferation, cell fate decision, axis formation, and organ development (Nusse, 1999). Wnt binding to the Frizzled/LRP (low-density lipoprotein receptor–related protein) receptors results in inhibition of the APC–Axin–GSK3 complex by activation of Dishevelled (Boutros and Mlodzik, 1999; Wharton, 2003) and by recruitment of Axin to the plasma membrane by LRP (Mao et al, 2001; Tolwinski et al, 2003) This results in an increase in nonphosphorylated -catenin that forms active transcriptional complexes in the nucleus with T cell factor (TCF)/lymphocyte enhancer binding factor (LEF) transcription factors (Behrens et al, 1996; Molenaar et al, 1996; Staal et al, 2002)
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