Casein kinase I phosphorylates the Armadillo protein and induces its degradation in Drosophila

Abstract

Casein kinase I (CKI) was recently reported as a positive regulator of Wnt signaling in vertebrates and Caenorhabditis elegans. To elucidate the function of Drosophila CKI in the wingless (Wg) pathway, we have disrupted its function by double‐stranded RNA‐mediated interference (RNAi). While previous findings were mainly based on CKI overexpression, this is the first convincing loss‐of‐function analysis of CKI. Surprisingly, CKIα‐ or CKIϵ‐RNAi markedly elevated the Armadillo (Arm) protein levels in Drosophila Schneider S2R+ cells, without affecting its mRNA levels. Pulse–chase analysis showed that CKI‐RNAi stabilizes Arm protein. Moreover, Drosophila embryos injected with CKIα double‐stranded RNA showed a naked cuticle phenotype, which is associated with activation of Wg signaling. These results indicate that CKI functions as a negative regulator of Wg/Arm signaling. Overexpression of CKIα induced hyper‐phosphorylation of both Arm and Dishevelled in S2R+ cells and, conversely, CKIα‐RNAi reduced the amount of hyper‐modified forms. His‐tagged Arm was phosphorylated by CKIα in vitro on a set of serine and threonine residues that are also phosphorylated by Zeste‐white 3. Thus, we propose that CKI phosphorylates Arm and stimulates its degradation.