PEN-2 enhances γ-cleavage after presenilin heterodimer formation

The presenilin (PS) complex, including PS, nicastrin, APH-1 and PEN-2, is essential for γ-secretase activity, which is required for amyloid β-protein (Aβ) generation. However, the precise individual roles of the three cofactors in the PS complex in Aβ generation remain to be clarified. Here, to distinguish the roles of PS cofactors in γ-secretase activity from those in PS endoproteolysis, we investigated their roles in the γ-secretase activity reconstituted by the coexpression of PS N- and C-terminal fragments (NTF and CTF) in PS-null cells. We demonstrate that the coexpression of PS1 NTF and CTF forms the heterodimer and restores Aβ generation in PS-null cells. The generation of Aβ was saturable at a certain expression level of PS1 NTF/CTF, while the overexpression of PEN-2 alone resulted in a further increase in Aβ generation. Although PEN-2 did not enhance PS1 NTF/CTF heterodimer formation, PEN-2 expression reduced the IC ₅₀ of a specific γ-secretase inhibitor, a transition state analogue, for Aβ generation, suggesting that PEN-2 expression enhances the affinity or the accessibility of the substrate to the catalytic site. Thus, our results strongly suggest that PEN-2 is not only an essential component of the γ-secretase complex but also an enhancer of γ-cleavage after PS heterodimer formation.