γ-Secretase is a multiprotein, intramembrane-cleaving protease with a growing list of protein substrates, including the Notch receptors and the amyloid precursor protein. The four components of γ-secretase complex - presenilin (PS), nicastrin (NCT), Pen2, and Aph1 - are all thought to be essential for activity. The catalytic domain resides within PS proteins, NCT has been suggested to be critical for substrate recognition, and the contributions of Pen2 and Aph1 remain unclear. The role of NCT has been challenged recently by the observation that a critical residue (E332) in NCT, which had been thought to be essential for γ-secretase activity, is instead involved in complex maturation. Here, we report that NCT is dispensable for γ-secretase activity. NCT-independent γ-secretase activity can be detected in two independent NCT-deficient mouse embryonic fibroblast lines and blocked by the γ-secretase inhibitors N-[N-(3,5-difluorophenacetyl-L-alanyl)]-S- phenylglycine t-butyl ester and L-685,458. This catalytic activity requires prior ectodomain shedding of the substrate and can cleave ligand-activated endogenous Notch receptors, indicating presence of this activity at the plasma membrane. Small interfering RNA knockdown experiments demonstrated that NCT-independent γ-secretase activity requires the presence of PS1, Pen2, and Aph1a but can tolerate knockdown of PS2 or Aph1b. We conclude that a PS1/Pen2/Aph1a trimeric complex is an active enzyme, displaying biochemical properties similar to those of γ-secretase and roughly 50% of its activity when normalized to PS1 N-terminal fragment levels. This PS1/Pen2/Aph1a complex, however, is highly unstable. Thus, NCT acts to stabilize γ-secretase but is not required for substrate recognition.