Co-expressed presenilin 1 NTF and CTF form functional γ-secretase complexes in cells devoid of full-length protein

The enzyme γ-secretase catalyzes the intramembrane proteolytic cleavage that generates the amyloid β-peptide from the β-amyloid precursor protein. The presenilin (PS) protein is one of the four integral membrane protein components of the mature γ-secretase complex. The PS protein is itself subjected to endoproteolytic processing, generating stable N- and C-terminal fragment (NTF and CTF, respectively) heterodimers. Here we demonstrate that coexpression of PS1 NTF and CTF functionally mimics expression of the full-length PS1 protein and restores γ-secretase activity in PS-deficient mammalian cells. The coexpressed fragments re-associate with each other inside the cell, where they also interact with nicastrin, another γ-secretase complex component. Analysis of γ-secretase-activity following the expression of mutant forms of NTF and CTF, under conditions bypassing endoproteolysis, indicated that the putatively catalytic Asp257 and Asp385 residues have a direct effect on γ-secretase activity. Moreover, we demonstrate that expression of the wild-type CTF rescues endoproteolytic cleavage of C-terminally truncated PS1 molecules that are otherwise uncleaved and inactive. Recovery of cleavage is critically dependent on the integrity of Asp385. Taken together, our findings indicate that ectopically expressed NTF and CTF restore functional γ-secretase complexes and that the presence of full-length PS1 is not a requirement for proper complex assembly.