Fig. 3. Models of how ligand endocytosis promotes N signaling. Three models have been proposed to resolve the paradox that N activation in the signal receiving cell requires the endocytosis of Dl and Ser/Jagged in the signal-sending cell, which removes the ligands from the cell surface where they have to reside to interact with N. (1) The endocytosis of Notch-bound DSL ligands might create pulling forces on N that induce conformational changes associated with the unmasking of the S2 cleavage site. (2,3) Newly synthesized inactive DSL ligands become active upon being trafficked through endosomal compartments. In model 2, internalized DSL ligands transit via the recycling endosomes (RE, see Box 1) where they would be activated by an as yet unknown post-translational modification (Wang and Struhl, 2004). In model 3, endocytosed DSL ligands are targeted inside the lumen of MVBs (see Box 2), leading either to their degradation upon maturation of the MVBs into lysosomes, or to the extracellular release of secreted vesicles, called exosomes, on the fusion of the MVBs with the plasma membrane. These models are not mutually exclusive. The pH gradient of vesicles is color-coded from neutral (pale yellow) to pH 5 (orange). CCP, clathrin-coated pit; CCV, clathrin-coated vesicle; ECV, endosomal carrier vesicles; LE, late endosome; Lys, lysosome; MVBs, multivesicular bodies; NICD, N intracellular domain; RE, recycling endosome; SE, sorting endosomes; U, ubiquitin.