Lysine 624 of the Amyloid Precursor Protein (APP) Is a Critical Determinant of Amyloid β Peptide Length SUPPORT FOR A SEQUENTIAL MODEL OF γ-SECRETASE INTRAMEMBRANE PROTEOLYSIS AND REGULATION BY THE AMYLOID β PRECURSOR PROTEIN (APP) JUXTAMEMBRANE REGION

gamma-Secretase is a multiprotein intramembrane cleaving aspartyl protease (I-CLiP) that catalyzes the final cleavage of the amyloid beta precursor protein (APP) to release the amyloid beta peptide (A beta). A beta is the primary component of senile plaques in Alzheimer's disease (AD), and its mechanism of production has been studied intensely. gamma-Secretase executes multiple cleavages within the transmembrane domain of APP, with cleavages producing A beta and the APP intracellular domain (AICD), referred to as gamma and epsilon, respectively. The heterogeneous nature of the gamma cleavage that produces various A beta peptides is highly relevant to AD, as increased production of A beta 1-42 is genetically and biochemically linked to the development of AD. We have identified an amino acid in the juxtamembrane region of APP, lysine 624, on the basis of APP695 numbering (position 28 relative to A beta) that plays a critical role in determining the final length of A beta peptides released by gamma-secretase. Mutation of this lysine to alanine (K28A) shifts the primary site of gamma-secretase cleavage from 1-40 to 1-33 without significant changes to epsilon cleavage. These results further support a model where epsilon cleavage occurs first, followed by sequential proteolysis of the remaining transmembrane fragment, but extend these observations by demonstrating that charged residues at the luminal boundary of the APP transmembrane domain limit processivity of gamma-secretase.