Regulation of the Protein Kinase Activity of Shaggy by Components of the Wingless Pathway in Drosophila Cells

The protein-serine kinase Shaggy (Sgg) is the Drosophila homolog of mammalian glycogen synthase kinase-3 and has been genetically implicated in signal transduction pathways necessary for the establishment of patterning. Sgg is a putative component of the Wingless (Wg) pathway, and epistasis analyses suggest that Sgg function is repressed by Wg signaling. Here, we have investigated the biochemical consequences of Wg signaling with respect to the Sgg protein kinase in two types of Drosophila cell lines and in embryos. Our results demonstrate that Sgg activity is inhibited following exposure of cells to Wg protein and by expression of downstream components of Wg signaling, Drosophila frizzled 2 and dishevelled. Wg-dependent inactivation of Sgg is accompanied by serine phosphorylation. We also show that the level of Sgg activity regulates the stability of Armadillo protein and modulates the level of phosphorylation of D-Axin and Armadillo. Together, these results provide direct biochemical evidence in support of the genetic model of Wg signaling and provide a model for dissecting the molecular interactions between the signaling proteins.