Unexpectedly robust assembly of the Axin destruction complex regulates Wnt/Wg signaling in Drosophila as revealed by analysis in vivo

Secreted proteins in the Writ family regulate gene expression in target cells by causing the accumulation of the transcriptional activator beta-catenin. In the absence of Writ, a Protein complex assembled around the scaffold protein Axin targets beta-catenin for destruction, thereby preventing it from transducing inappropriate signals. Loss of Axin or its binding partners APC and GSK3 results in aberrant activation of the Writ signaling response. We have analyzed the effects of mutant forms of Drosophila Axin with large internal deletions when expressed at physiological levels in vivo, either in the presence OF absence of wild type Axin. Surprisingly, even deletions that completely remove the binding sites for fly APC, GSK3 or beta-catenin, though they fail to rescue to viability, these mutant forms of Axin cause only mild developmental defects, indicating largely retained Axin function. Furthermore, two lethal Axin deletion constructs, Axin Delta RGS and Axin Delta beta cat (Delta Arm), can complement each other and restore viability. Our findings support a model in which the Axin complex is assembled through cooperative tripartite interactions among the binding partners, making the assembly of functional complexes surprisingly robust. (C) 2008 Elsevier Inc. All rights reserved.