JCB_201609094 265..278

A central question in developmental biology is how overall size and cell number in an organ are regulated during normal development, particularly in tissues that display endogenous size regulation. Recent studies have revealed the existence of the Hippo pathway that appears to coordinate developmental patterning, proliferation, apoptosis, and cell growth to regulate overall organ size (Halder and Johnson, 2011; Boggiano and Fehon, 2012; Staley and Irvine, 2012; Enderle and McNeill, 2013; Hariharan, 2015). At the core of this pathway are two kinases, Hippo and Warts (Wts), and the scaffold proteins Salvador (Sav) and Mats. Downstream of these core components is the transcriptional coactivator Yorkie (Yki), which promotes expression of cell proliferation and anti-apoptotic genes. Phosphorylation by Wts inactivates Yki, thereby restricting tissue growth. Thus, loss of any of the core components leads to increased proliferation, reduced apoptosis, and tissue overgrowth. It is still not entirely clear how input from upstream of the core components regulates Hippo pathway activity and growth. Three proteins, Expanded (Ex), Merlin, and Kibra, are proposed to form a complex at the apical junctional region (AJR) to regulate activity of the core kinases (Hamaratoglu et al., 2006; Baumgartner et al., 2010; Genevet et al., 2010; Yu et al., 2010). Ex also associates directly with Yki and is believed to sequester Yki at the AJR (Badouel et al., 2009). The transmembrane proteins Echinoid and Crumbs bind Sav and Ex, respectively (Ling et al., 2010; Yue et al., 2012). Loss of any of these proteins causes nuclear accumulation of Yki, transcriptional up-regulation of Yki target genes, and overgrowth. Another transmembrane protein that is believed to function upstream in the Hippo pathway is Fat (Ft), which encodes a giant protocadherin (Bennett and Harvey, 2006; Cho et al., 2006; Silva et al., 2006; Willecke et al., 2006). Ft binds in a heterophilic manner to another giant protocadherin, Dachsous (Ds), and ft mutants display not only imaginal tissue overgrowth, but also disruption in planar cell polarity (PCP; Blair, 2012; Thomas and Strutt, 2012; Lawrence and Casal, 2013; Matis and Axelrod, 2013; Carvajal-Gonzalez and Mlodzik, 2014). Loss of Ft leads to accumulation of the atypical myosin Dachs at the AJR, and genetic epistasis data suggest that ft controls growth through dachs, suggesting that Ft restricts tissue growth by regulating the amount of Dachs at the AJR (Mao et al., 2006; Rogulja et al., 2008). How Dachs regulates growth is not yet entirely clear, although it is known to alter the conformation of Wts and promote its destabilization (Cho et al., 2006; Vrabioiu and Struhl, 2015). Other upstream pathway components are thought to work with Ft to regulate the localization or accumulation of Dachs at the AJR. Fbxl7 encodes an F-box domain protein, which binds to the Ft intracellular domain (ICD; Bosch et al., 2014; Rodrigues-Campos and Thompson, 2014). Although the function of Fbxl7 is still not clear, Fbxl7 might destabilize Dachs by The large protocadherin Fat functions to promote Hippo pathway activity in restricting tissue growth. Loss of Fat leads to accumulation of the atypical myosin Dachs at the apical junctional region, which in turn promotes growth by inhibiting Warts. We previously identified Approximated (App), a DHHC domain palmitoyltransferase, as a negative regulator of Fat signaling in growth control. We show here that App promotes growth by palmitoylating the intracellular domain of Fat, and that palmitoylation negatively regulates Fat function. Independently, App also recruits Dachs to the apical junctional region through protein–protein association, thereby stimulating Dachs’s activity in promoting growth. Further, we show that palmitoylation by App functions antagonistically to phosphorylation by Discs-overgrown, which activates Fat. Together, these findings suggest a model in which App promotes Dachs activity by simultaneously repressing Fat via posttranslational modification and recruiting Dachs to the apical junctional region, thereby promoting tissue growth. The palmitoyltransferase Approximated promotes growth via the Hippo pathway by palmitoylation of Fat