Probable catalytic subunit of the gamma-secretase complex, an endoprotease complex that catalyzes the intramembrane cleavage of integral membrane proteins such as Notch receptors (lin-12 or glp-1). Probably works redundantly of lin-12, which provides more presenilin function. Homodimer. 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 (Probable). Weakly expressed. Expressed both maternally and zygotically. Predominantly expressed in the adult. Weakly expressed in the embryo, L2 and L3 stages, and almost absent in L1 stage larvae. Down-regulated by spr-1, spr-3, spr-4 and spr-5. The PAL motif is required for normal active site conformation. Belongs to the peptidase A22A family.