Essential subunit of the gamma-secretase complex, an endoprotease complex that catalyzes the intramembrane cleavage of integral membrane proteins such as Notch receptors and APP (amyloid-beta precursor protein). The gamma-secretase complex plays a role in Notch and Wnt signaling cascades and regulation of downstream processes via its role in processing key regulatory proteins, and by regulating cytosolic CTNNB1 levels. Component of the gamma-secretase complex. The functional gamma-secretase complex is composed of at least four polypeptides: a presenilin homodimer (PSEN1 or PSEN2), nicastrin (NCSTN), APH1 (APH1A or APH1B) and PEN2. Binds to proteolytic processed C-terminal fragments C83 and C99 of the amyloid precursor protein (APP). Interacts with PSEN1 and PSEN2. Identified by mass spectrometry in melanosome fractions from stage I to stage IV. N-glycosylated. Belongs to the nicastrin family.