A screen for targets of the Drosophila pseudokinase Tribbles identifies Neuralized and Mindbomb, ubiquitin ligases that mediate Notch signaling

Drosophila Tribbles (Trbl) is the founding member of a family of pseudokinases with conserved roles in antagonizing cell division, tissue growth and cell differentiation. In humans, three Tribbles isoforms serve as adaptor proteins, binding targets such as Cdc25 phosphatase, Akt kinase or the transcription factor C/EBP to block their activity or direct their proteosomal degradation. Mutations in Tribbles family members are associated with susceptibility to diabetes and cancer, notably Notch-induced tumor growth. Trbl misexpression in the fly wing disk leads to a block in mitosis associated with decreased levels of String/Cdc25 and increased levels of Cyclin B leading to reduced overall wing size and reduced trichome density. We show these Trbl growth-restricting phenotypes can be suppressed by manipulating levels of known Trbl targets, and use this sensitized wing system to screen a collection of growth regulating open reading frames (ORFs) to search for enhancers and suppressors affecting cell and tissue size. By precisely measuring morphometric changes in wing phenotypes using a computer-based tool, we detected synthetic interactions with several E3 ubiquitin ligases, and focused our analysis on the Notch pathway components Neuralized (Neur) and Mindbomb1 (Mib1). In the wing, notum and egg chamber epithelia, Trbl misexpression suppressed Neur and Mib1 activities and stabilized the accumulation of both proteins. To understand these interactions, we used yeast two-hybrid assays to show Trbl physically bound to both Neur and Mib1. Our data are consistent with published reports that mammalian Tribbles3 modulates Notch responses by binding and stabilizing Mindbomb and indicate that a wing misexpression approach is useful to identify novel components in a conserved Tribbles signaling pathway.\n\nAUTHOR SUMMARYTribbles pseudokinases are adaptor molecules, binding diverse targets regulating cell differentiation, growth and proliferation and directing their proteasomal degradation. To search for novel targets of Drosophila Tribbles, we adopted a wing co-misexpression scheme and measured changes in cell/tissue size to identify enhancers and suppressors of the Tribbles phenotype. We show the Notch pathway components Neuralized and Mindbomb1 E3 ligases act as Tribbles suppressors and demonstrate that Tribbles modulates their levels and activites. Recent demonstration that mammalian Tribbles 3 binds the E3 ligase Mindbomb to promote ligand-mediated Notch activation implies a conserved role for Tribbles family members in Notch activation.