FIGURE 3.

Signaling of C-terminal Notch truncations. A and B, ligand-induced activation of C-terminal N1gal4 truncations. Luciferase assays were performed as described in the legend to Fig. 2. A, Notch C-terminal truncations activated with OP9-Dll1 cells. Activity comparable with full-length N1gal4A is observed in C-terminal truncations up to EGF15, and truncating from both ends resulted in the identification of the EGF repeat 6–15 fragment, the smallest Notch1 fragment identified with activity comparable with the full-length ectodomain. B, Notch C-terminal truncations activated with MS5-Dll4 cells. The observed activation pattern is similar to OP9-Dll1 results. All data are shown normalized to co-culture with non-ligand-bearing OP9 or MS5 cells, and treatment with 1 μm GSI inhibits activation in all receptor constructs tested. C, surface expression of truncated receptors. Surface expression was detected via flow cytometry, using an antibody to the Notch1 NRR (WC-75) and a FITC-conjugated secondary antibody. Percentage of Notch1-positive cells is indicated on the y axis.