The Amyloid Precursor Protein Has a Flexible Transmembrane Domain and Binds Cholesterol

TLDR

C99, the transmembrane C‑terminal domain of amyloid precursor protein, is cleaved by γ‑secretase to release amyloid‑β peptides linked to Alzheimer’s disease, and its flexible, curved α‑helix transmembrane domain facilitates this processive cleavage. The study aims to elucidate C99’s structure and cholesterol‑binding characteristics to inform Alzheimer’s therapeutic design. NMR and EPR revealed that C99’s extracellular N‑helix and N‑loop precede a flexible transmembrane helix, and titration experiments identified a cholesterol‑binding site where membrane‑buried GXXXG motifs play a key role.

Abstract

C99 is the transmembrane carboxyl-terminal domain of the amyloid precursor protein that is cleaved by γ-secretase to release the amyloid-β polypeptides, which are associated with Alzheimer's disease. Nuclear magnetic resonance and electron paramagnetic resonance spectroscopy show that the extracellular amino terminus of C99 includes a surface-embedded "N-helix" followed by a short "N-loop" connecting to the transmembrane domain (TMD). The TMD is a flexibly curved α helix, making it well suited for processive cleavage by γ-secretase. Titration of C99 reveals a binding site for cholesterol, providing mechanistic insight into how cholesterol promotes amyloidogenesis. Membrane-buried GXXXG motifs (G, Gly; X, any amino acid), which have an established role in oligomerization, were also shown to play a key role in cholesterol binding. The structure and cholesterol binding properties of C99 may aid in the design of Alzheimer's therapeutics.