Essential subunit of the gamma-secretase complex, an endoprotease complex that catalyzes the intramembrane cleavage of integral membrane proteins such as Notch (glp-1 or lin-12). It may represents a stabilizing cofactor required for the assembly of the gamma-secretase complex. Component of the gamma-secretase complex, a complex probably composed of the presenilin homodimer (sel-12, hop-1 or spe-4), nicastrin (aph-2), aph-1 and pen-2. Belongs to the nicastrin family.