Notch Ligands Are Substrates for ProteinO-Fucosyltransferase-1 and Fringe

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 C2XXGG(S/T)C3 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 withO-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, anO-fucose specific β1,3-N-acetylglucosaminyltransferase, was capable of modifying O-fucose on the ligands. Indeed, O-fucose on mammalian Delta1 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 onDrosophila Serrate have been generated. Interestingly, analysis of these mutants has revealed that O-fucose modifications occur on some EGF repeats not predicted by the C2XXGGS/TC3 consensus site. A revised, broad consensus site, C2X3–5S/TC3 (whereX3–5 are any 3–5 amino acid residues), is proposed.