Essential subunit of the gamma-secretase complex, an endoprotease complex that catalyzes the intramembrane cleavage of integral membrane proteins such as Notch. It probably represents a stabilizing cofactor for the presenilin homodimer that promotes the formation of a stable complex. Component of the gamma-secretase complex, a complex composed of a presenilin (Psn) homodimer, nicastrin (Nct), Aph-1 and Pen-2. Belongs to the APH-1 family.