Influence of the familial Alzheimer's disease–associated T43I mutation on the transmembrane structure and γ-secretase processing of the C99 peptide

Extracellular deposition of -amyloid (A) peptides in the brain is a hallmark of Alzheimer's disease (AD). Upon -secretase-mediated cleavage of the C-terminal fragment (-CTF) from the A precursor protein, the -secretase complex produces the A peptides associated with AD. The familial T43I mutation within the transmembrane domain of the -CTF (also referred to as C99) increases the ratio between the A42 and A40 peptides largely due to a decrease in A40 formation. A42 is the principal component of amyloid deposits within the brain parenchyma, and an increase in the A42/A40 ratio is correlated with early-onset AD. Using NMR and FTIR spectroscopy, here we addressed how the T43I substitution influences the structure of C55, the minimal sequence containing the entire extracellular and transmembrane (TM) domains of C99 needed for -secretase processing. C-13 NMR chemical shifts indicated that the T43I substitution increases helical structure within the TM domain of C55. These structural changes were associated with a shift of the C55 dimer to the monomer and an increase in the tilt of the TM helix relative to the membrane normal in the T43I mutant compared with that of WT C55. The A21G (Flemish) mutation was previously found to increase secreted A40 levels; here, we combined this mutation in the extracellular domain of C99 with T43I and observed that the T43I/A21G double mutant decreases A40 formation. We discuss how the observed structural changes in the T43I mutant may decrease A40 formation and increase the A42/A40 ratio.