The γ-secretase complex is responsible for intramembrane processing of over 60 
substrates and is involved in Notch signaling as well as in the generation of 
the amyloid β-peptide (Aβ). Aggregated forms of Aβ have a pathogenic role in 
Alzheimer disease and, thus, reducing the Aβ levels by inhibiting γ-secretase is 
a possible treatment strategy for Alzheimer disease. Regrettably, clinical 
trials have shown that inhibition of γ-secretase results in Notch-related side 
effects. Therefore, it is of great importance to find ways to inhibit amyloid 
precursor protein (APP) processing without disturbing vital signaling pathways 
such as Notch. Nicastrin (Nct) is part of the γ-secretase complex and has been 
proposed to be involved in substrate recognition and selection. We have 
investigated how the four evenly spaced and conserved cysteine residues in the 
Nct ectodomain affect APP and Notch processing. We mutated these cysteines to 
serines and analyzed them in cells lacking endogenous Nct. We found that two 
mutants, C213S (C2) and C230S (C3), differentially affected APP and Notch 
processing. Both the formation of Aβ and the intracellular domain of amyloid 
precursor protein (AICD) were reduced, whereas the production of Notch 
intracellular domain (NICD) was maintained on a high level, although C230S (C3) 
showed impaired complex assembly. Our data demonstrate that single residues in a 
γ-secretase component besides presenilin are able to differentially affect APP 
and Notch processing.