Nuclear pore complexes (NPCs) traverse the nuclear envelope (NE), providing a channel through which nucleocytoplasmic transport occurs. Nup358/RanBP2, Nup214/CAN, and Nup88 are components of the cytoplasmic face of the NPC. Here we show that Nup88 localizes midway between Nup358 and Nup214 and physically interacts with them. RNA interference of either Nup88 or Nup214 in human cells caused a strong reduction of Nup358 at the NE. Nup88 and Nup214 showed an interdependence at the NPC and were not affected by the absence of Nup358. These data indicate that Nup88 and Nup214 mediate the attachment of Nup358 to the NPC. We show that localization of the export receptor CRM1 at the cytoplasmic face of the NE is Nup358 dependent and represents its empty state. Also, removal of Nup358 causes a distinct reduction in nuclear export signal-dependent nuclear export. We propose that Nup358 provides both a platform for rapid disassembly of CRM1 export complexes and a binding site for empty CRM1 recycling into the nucleus.

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.

The standard model of Wnt signaling specifies that after receipt of a Wnt ligand at the membranous receptor complex, downstream mediators inhibit a cytoplasmic destruction complex, allowing β-catenin to accumulate in the cytosol and nucleus and co-activate Wnt target genes. Unexpectedly, shortly after Wnt treatment, we detected the dephosphorylated form of β-catenin at the plasma membrane, where it displayed a discontinuous punctate labeling. This pool of β-catenin could only be detected in E-cadherin–/– cells, because in E-cadherin+/+ cells Wnt-induced, membranous β-catenin was concealed by a constitutive junctional pool. Wnt-signaling-dependent dephosphorylated β-catenin colocalized at the plasma membrane with two members of the destruction complex – APC and axin – and the activated Wnt co-receptor LRP6. β-catenin induced through the Wnt receptor complex was significantly more competent transcriptionally than overexpressed β-catenin, both in cultured cells and in early Xenopus embryos. Our data reveal a new step in the processing of the Wnt signal and suggest regulation of signaling output beyond the level of protein accumulation.