Self-association of the APC tumor suppressor is required for the assembly, stability, and activity of the Wnt signaling destruction complex.
Bottom Line: The tumor suppressor adenomatous polyposis coli (APC) is an essential negative regulator of Wnt signaling through its activity in the destruction complex with Axin, GSK3β, and CK1 that targets β-catenin/Armadillo (β-cat/Arm) for proteosomal degradation.The destruction complex forms macromolecular particles we termed the destructosome.These results suggest that APC proteins are required not only for the activity of the destructosome, but also for the assembly and stability of this macromolecular machine.
Affiliation: Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213.Show MeSH
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Mentions: Because APC2 self-association is necessary for proper β-cat regulation in cultured cells, we asked whether APC2 self-association is also necessary for destructosome activity and the negative regulation of Wnt signaling in the more physiologically relevant context of the Drosophila embryo. We expressed GFP-tagged APC2-FL or APC2-∆ASAD in the embryo under the native APC2 promoter (McCartney et al., 2006) and found that the two tagged proteins are expressed at levels comparable to that of the endogenous wild-type protein (Figure 6A). As previously shown, APC2-FL protein expressed in APC2- (APC2g10) embryos is enriched at the cell cortex of embryonic epithelia similar to endogenous APC2 (McCartney et al., 1999; Zhou et al., 2011). However, APC2-∆ASAD exhibited limited enrichment at the cortex (Figure 6B), consistent with our observations in S2 cells (Supplemental Figure S4). We previously demonstrated that the localization of APC2 to the cell cortex requires both the N-terminal region (aa 1–490) and the most-C-terminal 30 amino acids (C30; Figure 1A; Zhou et al., 2011). Because we have now demonstrated that the function of C30 requires APC2 dimerization (McCartney and Molinar, unpublished data), it is not surprising that the ASAD is necessary for cortical localization. We previously showed that cortical localization of APC2 is not required to regulate Wnt signaling (Zhou et al., 2011); therefore, lack of APC2-∆ASAD cortical localization will not affect its destructosome activity.
Affiliation: Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213.