Presenilin-1 and presenilin-2 exhibit distinct yet overlapping γ-secretase activities

Abstract

Presenilin-1 (PSI) and presenilin 2 (PS2) are proposed to be transmembrane aspartyl proteases that cleave amyloid precursor protein and Notch. PS1- and PS2-mediated activities were individually characterized using blastocyst-derived (BD) cells and membranes from PS1+/- - PS2-/- and PS1-/-PS2+/+ mice, respectively. The relative amounts of PS1 and PS2 in the various BD cells were determined from the intensities of the anti-PS1 and anti-PS2 immunoblot signals by comparison with standard curves using radiolabeled PS1 and PS2 standards produced by in vitro transcription and translation. Cellular membranes from wild type, PS1-/-PS2+/+, and PS1+/--PS2-/- but not PS1-/-PS2-/- BD cells generated the Aβ40 and Aβ42 products from the C100FLAG substrate. PS1-associated γ-secretase displays considerably higher specific activity than PS2-associated γ-secretase. Moreover, the PS1+/-PS2-/- BD cells and corresponding membranes exhibited much higher γ-secretase activity as compared with other BD cells and membranes. The PS1-mediated γ-secretase activity correlated better with the amount of PS1 that is modifiable by a photoactivated active site-directed γ-secretase inhibitor rather than total PS1; hence, only a small portion (<14%) of the PS1 in wild-type membranes appears to be engaged in an active γ-secretase complex. This finding suggests that PS1 may serve other biological functions in addition to that associated with its γ-secretase activity. Furthermore, the PS1 γ-secretase complex and the PS2 γ-secretase complex activities can be discriminated on the basis of their susceptibility to inhibition by a potent γ-secretase inhibitor. The distinct yet overlapping enzymatic properties of the PS1 γ-secretase complex and the PS2 γ-secretase complex imply that these two putative aspartyl class proteases may contribute to different biological processes.