The large atypical cadherin Fat is a receptor for both Hippo and planar cell polarity (PCP) pathways. Here we investigate the molecular basis for signal transduction downstream of Fat by creating targeted alterations within a genomic construct that contains the entire fat locus, and by monitoring and manipulating the membrane localization of the Fat pathway component Dachs. We establish that the human Fat homolog FAT4 lacks the ability to transduce Hippo signaling in Drosophila, but can transduce Drosophila PCP signaling. Targeted deletion of conserved motifs identifies a four amino acid C-terminal motif that is essential for aspects of Fat-mediated PCP, and other internal motifs that contribute to Fat-Hippo signaling. Fat-Hippo signaling requires the Drosophila Casein kinase 1_ encoded by discs overgrown (Dco), and we characterize candidate Dco phosphorylation sites in the Fat intracellular domain (ICD), the mutation of which impairs Fat-Hippo signaling. Through characterization of Dachs localization and directed membrane targeting of Dachs, we show that localization of Dachs influences both the Hippo and PCP pathways. Our results identify a conservation of Fat-PCP signaling mechanisms, establish distinct functions for different regions of the Fat ICD, support the correlation of Fat ICD phosphorylation with Fat-Hippo signaling, and confirm the importance of Dachs membrane localization to downstream signaling pathways.

The Notch family of transmembrane receptor proteins mediate developmental cell-fate decisions, and mutations in mammalian Notch genes have been implicated in leukaemia, breast cancer, stroke and dementia. During wing development in Drosophila, the Notch receptor is activated along the border between dorsal and ventral cells, leading to the specification of specialized cells that express Wingless (Wg) and organize wing growth and patterning. Three genes, fringe (fng), Serrate (Ser) and Delta (Dl), are involved in the cellular interactions leading to Notch activation. Ser and Dl encode transmembrane ligands for Notch, whereas fng encodes a pioneer protein. We have investigated the relationship between these genes by a combination of expression and coexpression studies in the Drosophila wing. We found that Ser and Dl maintain each other's expression by a positive feedback loop. fng is expressed specifically by dorsal cells and functions to position and restrict this feedback loop to the developing dorsal-ventral boundary. This is achieved by fng through a cell-autonomous mechanism that inhibits a cell's ability to respond to Serrate protein and potentiates its ability to respond to Delta protein.

O-Fucose has been identified on epidermal growth factor-like (EGF) repeats of Notch, and elongation of O-fucose has been implicated in the modulation of Notch signaling by Fringe. O-Fucose modifications are also predicted to occur on Notch ligands based on the presence of the C(2)XXGG(S/T)C(3) consensus site (where S/T is the modified amino acid) in a number of the EGF repeats of these proteins. Here we establish that both mammalian and Drosophila Notch ligands are modified with O-fucose glycans, demonstrating that the consensus site was useful for making predictions. The presence of O-fucose on Notch ligands raised the question of whether Fringe, an O-fucose specific beta 1,3-N-acetylglucosaminyltransferase, was capable of modifying O-fucose on the ligands. Indeed, O-fucose on mammalian Delta 1 and Jagged1 can be elongated with Manic Fringe in vivo, and Drosophila Delta and Serrate are substrates for Drosophila Fringe in vitro. These results raise the interesting possibility that alteration of O-fucose glycans on Notch ligands could play a role in the mechanism of Fringe action on Notch signaling. As an initial step to begin addressing the role of the O-fucose glycans on Notch ligands in Notch signaling, a number of mutations in predicted O-fucose glycosylation sites on Drosophila Serrate have been generated. Interestingly, analysis of these mutants has revealed that O-fucose modifications occur on some EGF repeats not predicted by the C(2)XXGGS/TC(3) consensus site. A revised, broad consensus site, C(2)X(3-5)S/TC(3) (where X(3-5) are any 3-5 amino acid residues), is proposed.

In addition to the core components of the Notch pathway, a number of proteins have been identified that exert positive or negative influences on Notch signaling. These include extracellular modulators, which may influence binding or activation of Notch by its ligands, cytoplasmic modulators, which presumably influence signal transduction steps after receptor activation, and nuclear modulators, which may influence the transcriptional activity of a Notch-CSL protein complex. Many of the cytoplasmic and nuclear modulators appear to bind directly to discrete domains within the intracellular domain of Notch. Genetic studies indicate that distinct modulators are deployed during distinct modes of Notch signaling.

The development of the Drosophila eye has served as a model system for investigations of tissue patterning and cell-cell communication; however, early eye development has not been well understood. The results presented here indicate that specialized cells are established along the dorsal-ventral midline of the developing eye by Notch-mediated signaling between dorsal and ventral cells, and that Notch activation at the midline plays an essential role both in promoting the growth of the eye primordia and in regulating eye patterning. These observations imply that the developmental homology between Drosophila wings and vertebrate limbs extends to Drosophila eyes.