Glycosphingolipids keep signaling in top-Notch condition

C ell surface proteins are absolutely essential to activating the Notch signaling pathway, but the lipids that surround them in the plasma membrane have a signifi cant infl uence too, say Hamel et al. (1). Notch receptor ligands (Delta and Serrate in fl ies) are transmembrane proteins that bind receptors on the surface of neighboring cells. Mysteriously, the ligands must be endocytosed before they can activate their receptors—whether this happens before or after their interaction is unclear. One possibility is that the ligand is internalized after binding, pulling the receptor with it to induce a conformational change that triggers downstream signaling. Alternatively, the ligand may by endocytosed before receptor binding, only to be recycled back to the cell surface in a modifi ed form capable of stimulating the Notch receptor (2). Either way, Delta and Serrate endo-cytosis is essential, and is promoted by E3 ubiquitin ligases called Mindbomb and Neuralized, both of which add ubiquitin to the ligands' intracellular tails. Mutations in either of these two genes block Notch signaling (3). To understand more about Notch ligand endocytosis, Sophie Hamel and Fran-çois Schweisguth, from the Pasteur Institute in Paris, France, screened for new regulators of the process. They looked for genes that, when overexpressed, restored normal Notch sig-naling to fl ies carrying a dominant-negative version of Mindbomb (1). Using this approach, the two researchers found that increased amounts of an enzyme called ␣1,4-N-acetylgalactosaminyltransferase-1 (␣4GT1) rescued the block in Delta and Serrate endocytosis caused by the Mind-bomb mutant. The enzyme also counteracted defects in the Neuralized ubiquitin ligase. ␣4GT1 is a Golgi-localized protein involved in the synthesis of glycosphin-golipids (GSLs) that are transported to the cell surface where they may promote the formation of lipid raft membrane domains (4). Overexpressing ␣4GT1 increased the amounts of a particular GSL called N5 on the surface of Drosophila cells. But how would this activity promote Delta and Serrate endocytosis? Hamel and Schweis-guth turned to a collaborator—Jacques Fantini from the University of Aix-Mar-seille, France—who determined that GSLs like N5 bind to a specifi c domain in the extracellular portions of the Notch ligands. " You might imagine that the ligands interact with membrane patches rich in glycosphin-golipids, " says Schweisguth, which would cluster the proteins into raft-like domains, facilitating their endo-cytosis and subsequent signaling activity. It remains to be seen whether this is how increased N5 production by ␣4GT1 promotes Notch ligand internal-ization. But unlike …