Identification of Domains Responsible for Ubiquitin-Dependent Degradation of dMyc by Glycogen Synthase Kinase 3β and Casein Kinase 1 Kinases

In the present study, we report that ubiquitin-mediated degradation of dMyc, the Drosophila homologue of the human c-myc proto-oncogene, is regulated in vitro and in vivo by members of the casein kinase 1 (CK1) family and by glycogen synthase kinase 3 beta (GSK3 beta). Using Drosophila S2 cells, we demonstrate that CK1 alpha promotes dMyc ubiquitination and degradation with a mechanism similar to the one mediated by GSK3 beta in vertebrates. Mutation of ck1 alpha or -epsilon or sgg/gsk3 beta in Drosophila wing imaginal discs results in the accumulation of dMyc protein, suggesting a physiological role for these kinases in vivo. Analysis of the dMyc amino acid sequence reveals the presence of conserved domains containing potential phosphorylation sites for mitogen kinases, GSK3 beta, and members of the CK1 family. We demonstrate that mutations of specific residues within these phosphorylation domains regulate dMyc protein stability and confer resistance to degradation by CK1 alpha and GSK3 beta kinases. Expression of the dMyc mutants in the compound eye of the adult fly results in a visible defect that is attributed to the effect of dMyc on growth, cell death, and inhibition of ommatidial differentiation.