Phosphorylation of the Tumor Suppressor Fat Is Regulated by Its Ligand Dachsous and the Kinase Discs Overgrown

Abstract

Summary The Drosophila tumour suppressor gene fat encodes a large cadherin that regulates growth and a form of tissue organization known as planar cell polarity (PCP). Fat regulates growth via the Hippo kinase pathway [1–4], which controls expression of genes promoting cell proliferation and inhibiting apoptosis (reviewed in [5–11]). The Hippo pathway is highly conserved and is implicated in the regulation of mammalian growth and cancer development [12–18]. Genetic studies suggest that Fat activity is regulated by binding to another large cadherin Dachsous (Ds) [19–25]. The tumour suppressor, discs overgrown (dco)/Casein Kinase I δ/ε, also regulates Hippo activity and PCP [1, 26, 27]. The biochemical nature of how Fat, Ds and Dco interact to regulate these pathways is poorly understood. Here we demonstrate that Fat is cleaved to generate 450kDa and 110kDa fragments (Fat450 and Fat110). Fat110 contains the cytoplasmic and transmembrane domain. The cytoplasmic domain of Fat binds Dco, and is phosphorylated by Dco at multiple sites. Importantly, we show Fat forms cis-dimers, and that Fat phosphorylation is regulated by Dachsous and Dco in vivo. We propose that Ds regulates Dco-dependent phosphorylation of Fat and Fat-associated proteins to control Fat signaling in growth and PCP.

Signal transduction by the Fat cytoplasmic domain

“…In wild type, a fraction of Fat is phosphorylated on its ICD. This phosphorylation is visible as a mobility shift that results in the 95 kDa cleavage product appearing as a smeared doublet, with the reduced mobility of the upper band dependent upon phosphorylation (Feng and Irvine, 2009;Sopko et al, 2009). A similar smeared doublet was observed for all of the Fat deletion constructs that provide significant Fat-Hippo activity (∆A, ∆B, ∆C, ∆E and ∆F) (Fig.…”

“…An antimorphic allele, dco 3 , specifically impairs Fat-Hippo signaling Feng and Irvine, 2009). A portion of Fat is phosphorylated on its intracellular domain (ICD); this phosphorylation depends upon both Ds and Dco, suggesting that Fat ICD phosphorylation is a key step in Fat-Hippo signal transduction (Feng and Irvine, 2009;Sopko et al, 2009). Dachs is a myosin that downregulates Wts, and is required for the influence of fat or dco mutations on Hippo signaling (Cho and Irvine, 2004;Cho et al, 2006).…”

The Hippo pathway regulates stem cell proliferation, self-renewal, and differentiation

“…Removing one copy of yki dramatically suppressed the fat mutant overgrowth phenotype (Bennett and Harvey, 2006;Silva et al, 2006;Willecke et al, 2006), indicating that yki is an important mediator of fat function. Fat activity is regulated by binding to another protocadherin, Dachsous (Ds) (Matakatsu and Blair, 2006), and is modulated by several proteins such as the casein kinase Discs overgrown (Dco) (Feng and Irvine, 2009;Sopko et al, 2009), the Golgi-resident kinase Four-jointed (Fj) (Rogulja et al, 2008;Willecke et al, 2008;Simon et al, 2010), and the Fat/Ds-interacting protein Lowfat (Lft) (Mao et al, 2009). A LIM domain-containing protein Zyxin (Zyx) also inhibits Hippo signaling possibly by physical interaction with Wts and Dachs downstream of Fat (Rauskolb et al, 2011).…”

When pathways collide: collaboration and connivance among signalling proteins in development

“…Transcription of fat, dachsous, four-jointed, expanded and merlin are enhanced by loss of Hippo pathway activity, providing negative feedback, and also allowing changes in Hippo activity to be transmitted to adjacent cells. Fat also binds Casein Kinase Iε, which promotes Hpo pathway activity 69,70 . There are multiple interactions between the upstream components.…”