Essential subunit of the gamma-secretase complex, an endoprotease complex that catalyzes the intramembrane cleavage of integral proteins such as Notch receptors (lin-12 or glp-1). It may represent a stabilizing cofactor for the presenilin homodimer that promotes the formation of a stable complex. Required for the localization of aph-2. 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 APH-1 family.