Substrate docking to γ-secretase allows access of γ-secretase modulators to an allosteric site

gamma-Secretase generates the peptides of Alzheimer's disease, A beta(40) and A beta(42), by cleaving the amyloid precursor protein within its transmembrane domain. gamma-Secretase also cleaves numerous other substrates, raising concerns about gamma-secretase inhibitor off-target effects. Another important class of drugs, gamma-secretase modulators, alter the cleavage site of gamma-secretase on amyloid precursor protein, changing the A beta(42)/A beta(40) ratio, and are thus a promising therapeutic approach for Alzheimer's disease. However, the target for gamma-secretase modulators is uncertain, with some data suggesting that they function on gamma-secretase, whereas others support their binding to the amyloid precursor. In this paper we address this controversy by using a fluorescence resonance energy transfer-based assay to examine whether gamma-secretase modulators alter Presenilin-1/gamma-secretase conformation in intact cells in the absence of its natural substrates such as amyloid precursor protein and Notch. We report that the gamma-secretase allosteric site is located within the gamma-secretase complex, but substrate docking is needed for gamma-secretase modulators to access this site.