Dilysine retrieval signal-containing p24 proteins collaborate in inhibiting γ-cleavage of amyloid precursor protein

Abstract

γ-Secretase mediates intramembranous γ-cleavage and ε-cleavage of β-amyloid precursor protein (APP) to liberate β-amyloid peptide (Aβ) and APP intracellular domain respectively from the membrane. Although the regulatory mechanism of γ-secretase cleavage remains unresolved, a member of the p24 cargo protein family, named p24δ1 or TMP21, has been identified as an activity-modulating component. The p24 family proteins are divided into four subfamilies (p24α, β, δ and γ). In contrast to p24δ1, p24β1 has reportedly no effect on γ-cleavage. In this study, we determined whether p24α2, p24γ3 or p24γ4 modulates APP processing. Knockdown of cellular p24α2 induced a significant increase in Aβ generation but not in APP intracellular domain production in cell-based and cell-free assays, whereas p24α2 over-expression suppressed Aβ secretion. By contrast, Aβ secretion was not altered by p24γ3 or p24γ4 knockdown. Endogenous p24α2 co-immunoprecipitated with core components of the γ-secretase complex, and the anti-p24α2 immunoprecipitate exhibited γ-secretase activity. Mutational disruption of the conserved dilysine ER-retrieval motifs of p24α2 and p24δ1 perturbed inhibition of γ-cleavage. Simultaneous knockdown, or co-over-expression, of these proteins had no additive or synergistic effect on Aβ generation. Our findings suggest that dilysine ER-retrieval signal-containing p24 proteins, p24α2 and p24δ1, bind with γ-secretase complexes and collaborate in attenuating γ-cleavage of APP