Gamma-secretase cleavage is the final enzymatic step generating beta-amyloid via 
intramembranous cleavage of the amyloid precursor protein (APP). Presenilin 
(PS), initially identified as a gene in which mutations account for the vast 
majority of early-onset autosomal dominant Alzheimer's disease, is a major 
component of gamma-secretase. Enzymatic activity also depends on nicastrin, 
Aph-1, and Pen-2. We propose a model in which gamma-secretase components 
assemble, interact with substrates initially at a docking site, and then cleave 
and release substrates. To test this model, we developed a novel morphological 
technique on the basis of advanced fluorescence microscopy methods, fluorescence 
lifetime imaging microscopy (FLIM). FLIM allows us to examine protein-protein 
"proximity" in intact cells. We show that, although the strongest colocalization 
of APP and PS1 is in the perinuclear area, the strongest interactions detected 
by FLIM are at or near the cell surface. We also found that APP-PS1 interactions 
occur even when gamma-secretase inhibitors or "dominant-negative" PS1 mutations 
are used to block gamma-secretase activity. Finally, using nicastrin RNA 
interference, we demonstrate that nicastrin is critical for APP association with 
PS1. We interpret these results to suggest that there is a noncatalytic docking 
site closely associated with PS1-gamma-secretase.