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 (PubMed:12716934). 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. Full embryonic lethality. No embryos survive past 10.d dpc. At 9.5 dpc the embryos display a phenotype similar to that of Notch1-deficient mice, with defects in the development of the caudal part of the embryo and in somite segementation, defective vascular morphogenesis in the yolk sac, and patterning defects in the developing heart and neural tube. Assembly of the gamma-secretase complex and APP processing are disrupted. Belongs to the nicastrin family.