Terminating Wnt signals: a novel nuclear export mechanism targets activated (beta)-catenin

Abstract

Nuclear targeting of beta-catenin is an obligatory step in Wnt signal transduction, but the factors that control import and export remain to be clarified. In this issue, Hendriksen et al. (p. 785) show that the RanBP3 export factor antagonizes beta-catenin/T cell factor (TCF) transcription by targeting the signaling-competent form of beta-catenin. We speculate that cells may use multiple export mechanisms to inhibit beta-catenin signaling in different ways.

Figure 1.

RanBP3 promotes export of activated β-catenin. During Wnt signaling, the Axin–GSK3β–APC phosphorylation/predestruction complex is inactivated and leads to cytosolic accumulation of a form of β-catenin that is unphosphorylated at GSK3β-dependent serines 37 and 41. This “activated” form is then available to enter the nucleus and participate as a coactivator for the transcription of TCF target genes. Data presented by Hendriksen et al. (2005) supports a model in which this activated form of β-catenin is exported from the nucleus by RanBP3 in a RanGTP-dependent manner. Cytosolic Ran GTPase-activating proteins promote GTP hydrolysis and presumably release the active form of β-catenin in the cytosol.

Figure 2.

Distinct modes of β-catenin nuclear export. APC-directed export of β-catenin may be coupled to degradation, whereas RanBP3-directed export may allow for cycles of export and reimport. See text for details.