Increased γ-Site H-Bond Stability Relates to Altered ε-Efficiency and Aβ Levels in the I45T Familial Alzheimer’s Disease Mutant of APP
Cleavage of the amyloid precursor proteins (APP) transmembrane domain (TMD) by {gamma}-secretase is a crucial step in the aetiology of Alzheimers Disease (AD). Mutations in the APP TMD alter cleavage and lead to familial forms of AD (FAD). The majority of FAD mutations shift the preference of initial cleavage from {varepsilon}49 to {varepsilon}48, thus raising the AD-related A{beta}42/A{beta}40 ratio. The I45T mutation is among the few FAD mutations that do not alter {varepsilon}-site preference, while it dramatically reduces the efficiency of {varepsilon}-cleavage. Here, we investigate the impact of the I45T mutation on the backbone dynamics of the substrate TMD. Amide exchange experiments and molecular dynamics simulations in solvent and a lipid bilayer reveal an increased stability of amide hydrogen bonds at the {zeta}-and {gamma}-cleavage sites. Stiffening of the H-bond network is caused by an additional H-bond between the T45 side chain and the TMD backbone, which alters dynamics within the cleavage domain. In particular, the increased H-bond stability inhibits an upward movement of the {varepsilon}-sites in the I45T mutant. Thus, an altered presentation of {varepsilon}-sites to the active site of {gamma}-secretase as a consequence of restricted local flexibility provides a rationale for reduced {varepsilon}-cleavage efficiency of the I45T mutant.