Two domains within the first putative transmembrane domain of presenilin 1 differentially influence presenilinase and γ‐secretase activity

Presenilins (PS) are thought to contain the active site for presenilinase endoproteolysis of PS and γ-secretase cleavage of substrates. The structural requirements for PS incorporation into the γ-secretase enzyme complex, complex stability and maturation, and appropriate presenilinase and γ-secretase activity are poorly understood. We used rescue assays to identify sequences in transmembrane domain one (TM1) of PS1 required to support presenilinase and γ-secretase activities. Swap mutations identified an N-terminal TM1 domain that is important for γ-secretase activity only and a C-terminal TM1 domain that is essential for both presenilinase and γ-secretase activities. Exchange of residues 95–98 of PS1 (sw95–98) completely abolishes both activities while the familial Alzheimer's disease mutation V96F significantly inhibits both activities. Reversion of residue 96 back to valine in the sw95–98 mutant rescues PS function, identifying V96 as the critical residue in this region. The TM1 mutants do not bind to an aspartyl protease transition state analog γ-secretase inhibitor, indicating a conformational change induced by the mutations that abrogates catalytic activity. TM1 mutant PS1 molecules retain the ability to interact with γ-secretase substrates and γ-secretase complex members, although Nicastrin stability is decreased by the presence of these mutants. γ-Secretase complexes that contain V96F mutant PS1 molecules display a partial loss of function for γ-secretase that alters the ratio of amyloid-β peptide species produced, leading to the amyloid-β peptide aggregation that causes familial Alzheimer's disease.