Notch and its ligands are modified by a protein O-fucosyltransferase (OFUT1) that attaches fucose to a Serine or Threonine within EGF domains. By using RNAi to decrease Ofut1 expression in Drosophila, we demonstrate that O-linked fucose is positively required for Notch signaling, including both Fringe-dependent and Fringe-independent processes. The requirement for Ofut1 is cell autonomous, in the signal-receiving cell, and upstream of Notch activation. The transcription of Ofut1 is developmentally regulated, and surprisingly, overexpression of Ofut1 inhibits Notch signaling. Together, these results indicate that OFUT1 is a core component of the Notch pathway, which is required for the activation of Notch by its ligands, and whose regulation may contribute to the pattern of Notch activation during development.

Notch proteins are receptors for a conserved signaling pathway that affects numerous cell fate decisions. We found that in Drosophila, Protein O-fucosyltransferase 1 (OFUT1), an enzyme that glycosylates epidermal growth factor-like domains of Notch, also has a distinct Notch chaperone activity. OFUT1 is an endoplasmic reticulum protein, and its localization was essential for function in vivo. OFUT1 could bind to Notch, was required for the trafficking of wild-type Notch out of the endoplasmic reticulum, and could partially rescue defects in secretion and ligand binding associated with Notch point mutations. This ability of OFUT1 to facilitate folding of Notch did not require its fucosyltransferase activity. Thus, a glycosyltransferase can bind its substrate in the endoplasmic reticulum to facilitate normal folding.

The O-linked-N -acetylglucosamine (O-GlcNAc) modifi cation of cytoplasmic and nuclear proteins regulates basic cellular functions and is involved in the aetiology of diabetes and neurodegeneration. This intracellular O-GlcNAcylation is catalyzed by a single O-GlcNAc transferase, OGT. Here we report a novel OGT, EOGT, responsible for extracellular O-GlcNAcylation. Although both OGT and EOGT are regulated by hexosamine fl ux, EOGT localizes to the lumen of the endoplasmic reticulum and transfers GlcNAc to epidermal growth factor-like domains in an OGT-independent manner. Loss of Eogt gives phenotypes similar to those caused by defects in the apical extracellular matrix. Dumpy (Dp), a membrane-anchored extracellular protein, is O-GlcNAcylated, and EOGT is required for Dp-dependent epithelial cell -matrix interactions. Thus, O-GlcNAcylation of secreted and membrane glycoproteins is a novel mediator of cell -cell or cell -matrix interactions at the cell surface.

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Background: O-fucosyltransferase1 (OFUT1) is a conserved ER protein essential for Notch signaling. OFUT1 glycosylates EGF domains, which can then be further modified by the Nacetylglucosaminyltransferase Fringe. OFUT1 also possesses a chaperone activity that promotes the folding and secretion of Notch. Here, we investigate the respective contributions of these activities to Notch signaling in Drosophila.

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The polymorphic carbohydrate structures of gangliosides play regulatory roles. In particular, b-series gangliosides, all of which contain ␣-2,8 sialic acids, have been considered to be critical in various biological events such as adhesion, toxin binding, neurite extension, cell growth, and apoptosis. To clarify the physiological functions of b-series gangliosides in vivo, we have established a gene knockout mouse of GD3 synthase. Although all b-series structures were deleted in the mutant mice, they showed an almost complete nervous tissue morphology with no apparent abnormal behavior. Moreover, no differences in Fas-mediated apoptotic reaction of lymphocytes between wild type and the mutant mice were detected. However, the mutant mice exhibited clearly reduced regeneration of axotomized hypoglossal nerves compared with the wild type, suggesting that b-series gangliosides are more important in the repair rather than in the differentiation of the nervous system and apoptotic process induced via Fas.Gangliosides, sialic acid-containing glycosphingolipids, are enriched in the nervous tissues of mammals and birds, and also distributed in various tissues and cells (1), playing regulatory roles with their polymorphic carbohydrate structures. In particular, b-series gangliosides including GD3, 1 GD2, GD1b, GT1b, and GQ1b have been considered to be critical as receptors for bacteria toxins (2) and are adhesion-associated (3, 4), neurite-inducing (5), cell growth-promoting (6, 7) and apoptosis-mediating molecules (8).To clarify the physiological functions of these b-series gangliosides in vivo, we have established a gene knockout mice line for ␣-2,8-sialyltransferase (9), which is responsible for the generation of all b-series gangliosides as well as c-series gangliosides. As expected, all b-series structures were deleted in the mutant mice, and a-series species were slightly increased instead. However, these b-series ganglioside-lacking mice showed an almost intact morphology of the brain and other nervous tissues, and no clear abnormal behaviors were detectable during the early period of life. Despite that GD3 has been considered to mediate Fas-induced apoptosis in lymphoid cells (8,10), no differences in the apoptotic reaction of lymphocytes between wild-type and mutant mice were detected. However, the mutant mice exhibited clearly reduced regeneration of axotomized hypoglossal nerves compared with the wild type, suggesting that b-series gangliosides are important in the repair of damaged nerves rather than in the differentiation of the nervous system. EXPERIMENTAL PROCEDURESGeneration of GD3 Synthase Gene Knockout Mice-The chromosomal GD3 synthase gene was isolated from the gt11 phage library using GD3 synthase cDNA (pD3T-31) and mapped as described previously (11). To distinguish the true GD3 synthase gene from pseudo-genes, in situ hybridization was performed, and the identity was confirmed based on the correspondence of the gene assignment between humans and mice. The neo r gene was inserted between the BalI and ...

The expression cloning of a cDNA for globotriaosylceramide (Gb3)/CD77 synthase (␣1,4-galactosyltransferase) was achieved using an anti-Gb3 antibody and mouse L cells as a recipient cell line for the transfection. The isolated cDNA clone designated pVTR1 predicted a type II membrane protein with 19 amino acids of cytoplasmic domain, 26 amino acids of transmembrane region, and a catalytic domain with 308 amino acids. Introduction of the cDNA clone into L cells resulted in the neosynthesis of Gb3/CD77, and the extracts of the transfectant cells showed ␣1,4-galactosyltransferase activity only on lactosylceramide and galactosylceramide. In Northern blotting, a 2.3-kilobase mRNA was strongly expressed in heart, kidney, spleen, and placenta and weakly in colon, small intestine, and brain. Transfection of the cDNA into L cells resulted in the constitution of sensitivity to the apoptosis with Shiga-like toxins (verotoxins). Since Gb3/CD77 synthase initiates the synthesis of globo series glycolipids, the isolation of this cDNA will make possible further investigations into the function of its important series of glycolipids.