Probable catalytic subunit of the gamma-secretase complex, an endoprotease complex that catalyzes the intramembrane cleavage of integral membrane proteins such as Notch receptors. Requires the other members of the gamma-secretase complex to have a protease activity (By similarity). Homodimer. Component of the gamma-secretase complex, a complex composed of a presenilin homodimer, nicastrin, aph1 and pen2 (By similarity). The PAL motif is required for normal active site conformation. Belongs to the peptidase A22A family.