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 the last step of maturation of gamma-secretase, facilitating endoproteolysis of presenilin and conferring gamma-secretase activity. Component of the gamma-secretase complex, a complex composed of a presenilin (Psn) homodimer, nicastrin (Nct), Aph-1 and Pen-2. Belongs to the PEN-2 family.