The separation of cells into populations that do not intermix, termed compartments, is a fundamental organizing principle during development. Dorsal-ventral compartmentalization of the Drosophila wing is regulated downstream of the apterous (ap) gene, which encodes a transcription factor that specifies dorsal wing fate. fringe (fng) is normally expressed by dorsal cells downstream of ap; here we show that fng plays a key role in dorsal-ventral compartmentalization. Loss of fng function causes dorsal cells to violate the compartment boundary, and ectopic expression of the Fng protein causes ventral cells to violate thecompartment boundary. Fng modulates signalling through the Notch receptor. Notch and its ligands are essential for formation of the dorsal-ventral compartment border, and repositioning the stripe of Notch activation that is normally established there appears to reposition the compartment border. However, activation of Notch does not itself confer either dorsal or ventral cell location, and fng can influence compartmentalization even within regions of ubiquitous Notch activation. Our results indicate that the primary mechanism by which fng establishes a compartment border is by positioning a stripe of Notch activation, but also that fng may exert additional influences on compartmentalization.

Fringe proteins are ␤1,3-N-acetylglucosaminyltransferases that modulate signaling through Notch receptors by modifying Olinked fucose on epidermal growth factor domains. Fringe is highly conserved, and comparison among 18 different Fringe proteins from 11 different species identifies a core set of 84 amino acids that are identical among all Fringes. Fringe is only distantly related to other glycosyltransferases, but analysis of the predicted Drosophila proteome identifies a set of four sequence motifs shared among Fringe and other putative ␤1,3-glycosyltransferases. To gain functional insight into these conserved sequences, we genetically and molecularly characterized 14 point mutations in Drosophila fringe. Most nonsense mutations act as recessive antimorphs, raising the possibility that Fringe may function as a dimer. Missense mutations identify two distinct motifs that are conserved among ␤1,3-glycosyltransferases, and that can be modeled onto key motifs in the crystallographic structures of bovine ␤1,4-galactosyltransferase 1 and human glucuronyltransferase I. Other missense mutations map to amino acids that are conserved among Fringe proteins, but not among other glycosyltransferases, and thus may identify structural motifs that are required for unique aspects of Fringe activity. Although the Notch pathway mediates a wide range of cell fate decisions, the influence of fringe ( fng) genes on Notch signaling has been best studied during the development of the Drosophila wing. In the developing wing imaginal disk, FNG inhibits the ability of Serrate to activate Notch, and potentiates the ability of Delta to activate Notch. These effects of FNG position a stripe of Notch activation along the border between dorsal and ventral cells, which is essential for the subsequent patterning, growth, and compartmentalization of the wing (reviewed in refs. 5 and 6).Drosophila and vertebrate FNG proteins are highly conserved and possess an unusual ␤1,3-N-acetylglucosaminyltransferase activity that elongates O-linked fucose within EGF domains (7,8). Although FNG is not closely related to other glycosyltransferases, it does share a few short sequence motifs with certain other glycosyltransferases (9), and we report here that it is most closely related to members of a ␤1,3-glycosyltransferase (␤3GT) superfamily. These enzymes use a variety of sugar donors and acceptors, but all transfer the sugar from a UDP-sugar donor, creating a ␤ linkage between the 1 carbon of the donor and the 3 carbon of the acceptor. With the exception of site-directed mutations in a DDD motif of FNG (7,8,10,38), the functional requirements for conserved sequence motifs in FNG or other ␤3GTs have not been assessed. Moreover, the only crystallographic structure for a ␤3GT determined to date is that of human glucuronyltransferase I (GlcAT-I), which does not share significant primary sequence similarity with FNG.Here, we employ two complementary approaches to identify functionally important amino acids within FNG: sequence conservation, and genetic and mole...