Proteasome‐mediated degradation of the C‐terminus of the Alzheimer's disease β‐amyloid protein precursor: Effect of C‐terminal truncation on production of β‐amyloid protein

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Abstract
The β‐amyloid protein (Aβ) is derived by proteolytic processing of the amyloid protein precursor (APP). Cleavage of APP by β‐secretase generates a C‐terminal fragment (APP‐CTFβ), which is subsequently cleaved by γ‐secretase to produce Aβ. Our previous studies have shown that the proteasome can cleave the C‐terminal cytoplasmic domain of APP. To identify proteasome cleavage sites in APP, two peptides homologous to the C‐terminus of APP were incubated with recombinant 20S proteasome. Cleavage of the peptides was monitored by reversed phase high‐performance liquid chromatography and mass spectrometry. Proteasome cleaved the APP C‐terminal peptides at several sites, including a region around the sequence YENPTY that interacts with several APP‐binding proteins. To examine the effect of this cleavage on Aβ production, APP‐CTFβ and mutant forms of APP‐CTFβ terminating on either side of the YENPTY sequence were expressed in CHO cells. Truncation of APP‐CTFβ on the N‐terminal side of the YENPTY sequence at residue 677 significantly decreased the amount of Aβ produced, whereas truncation on the C‐terminal side of residue 690 had little effect. The results suggest that proteasomal cleavage of the cytosolic domain of APP at the YENPTY sequence decreases γ‐secretase processing, and consequently inhibits Aβ production. Therefore, the proteasome‐dependent trafficking pathway of APP may be a valid therapeutic target for altering Aβ production in the Alzheimer's disease brain.