The Drosophila tumor suppressors fat and discs overgrown (dco) function within an intercellular signaling pathway that controls growth and polarity. fat encodes a transmembrane receptor, but post-translational regulation of Fat has not been described. We show here that Fat is subject to a constitutive proteolytic processing, such that most or all cell surface Fat comprises a heterodimer of stably associated N- and C-terminal fragments. The cytoplasmic domain of Fat is phosphorylated, and this phosphorylation is promoted by the Fat ligand Dachsous. dco encodes a kinase that influences Fat signaling, and Dco is able to promote the phosphorylation of the Fat intracellular domain in cultured cells and in vivo. Evaluation of dco mutants indicates that they affect Fat's influence on growth and gene expression but not its influence on planar cell polarity. Our observations identify processing and phosphorylation as post-translational modifications of Fat, correlate the phosphorylation of Fat with its activation by Dachsous in the Fat-Warts pathway, and enhance our understanding of the requirement for Dco in Fat signaling.

The conserved Drosophila tumor suppressors Fat and Expanded have both recently been implicated in regulating the activity of the Warts tumor suppressor. However, there has been disagreement as to the nature of the links among Fat, Expanded, and Warts and the significance of these links to growth control. We report here that mutations in either expanded or fat can be rescued to viability simply by overexpressing Warts, indicating that their essential function is their influence on Warts rather than reported effects on endocytosis or other pathways. These rescue experiments also separate the transcriptional from the planar cell polarity branches of Fat signaling and reveal that Expanded does not directly affect polarity. We also investigate the relationship between expanded and fat and show, contrary to prior reports, that they have additive effects on imaginal disk growth and development. Although mutation of fat can cause partial loss of Expanded protein from the membrane, mutation of fat promotes growth even when Expanded is overexpressed and accumulates at its normal subapical location. These observations argue against recent proposals that Fat acts simply as a receptor for the Hippo signaling pathway and instead support the proposal that Fat and Expanded can act in parallel to regulate Warts through distinct mechanisms.