Activated Notch1 associates with a presenilin-1/gamma-secretase docking site.

Presenilin-1 (PS1), implicated as the active component of the gamma-secretase enzymatic complex, is known to cleave the cell surface receptor Notch1 after ligand binding. Here we directly visualize Notch1-PS1 interactions using a novel fluorescence lifetime imaging microscopy assay to monitor fluorescence resonance energy transfer. We demonstrate that endogenous Notch1 and PS1 move into close proximity at the cell surface after activation of Notch1 by the Delta1 ligand. A constitutively active N-terminally truncated form of Notch1, an immediate substrate of the gamma-secretase complex, similarly is found in close proximity to PS1. Interestingly, this interaction remains in the presence of a potent gamma-secretase active site inhibitor. Thus ligand binding to Notch1 appears to result in access of truncated Notch1 to a putative docking site on the PS1-gamma-secretase complex. These results suggest a novel mechanism of ligand binding-mediated signal transduction of Notch1.