Essential subunit of the gamma-secretase complex, an endoprotease complex that catalyzes the intramembrane cleavage of integral proteins such as Notch receptors precursor protein. It probably represents a stabilizing cofactor for the presenilin homodimer that promotes the formation of a stable complex (By similarity). Component of the gamma-secretase complex, a complex composed of a presenilin homodimer, nicastrin, aph1 and pen2. Belongs to the APH-1 family.