Regulation of Wnt signaling by the tumor suppressor APC does not require the ability to enter the nucleus nor a particular cytoplasmic localization. David M. Roberts¹, Mira I. Pronobis², John S. Poulton², Eric G. Kane¹, Mark Peifer². 1) Department of Biology, Franklin & Marshall College, Lancaster, PA; 2) Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC.

   Wnt signaling plays key roles in both development and disease. The tumor suppressor Adenomatous polyposis coli (APC) is an essential negative regulator of Wnt signaling that is inactivated in over 80% of all colon cancer cases. APC regulates Wnt signaling by contributing to a multi-protein complex (the destruction complex) that targets the Wnt effector protein catenin for phosphorylation and subsequent proteasomal destruction. However, several studies have suggested additional roles for APC in negatively regulating Wnt signaling, postulating that APC can also act in the nucleus to either modify activity of Wnt-responsive promoters or to actively export catenin out of the nucleus to facilitate its destruction. In addition, we previously suggested that an additional function of APC might be to position the destruction complex at the appropriate subcellular location. Here, we directly test these models by generating APC variants with localization tags that force APC to different cytoplasmic locations while simultaneously preventing its nuclear entry. These APC localization variants were then assessed for function in human colon cancer cells and Drosophila embryos. Strikingly, all tethered APC variants rescued catenin destruction and down-regulated Wnt target genes in colon cancer cells, and most restored Wg/Wnt regulation in Drosophila embryos null for APC. These data suggest that APC does not have required nuclear functions, nor does it position the destruction complex to a precise subcellular location to function in Wnt signaling.