Intramembrane Proteolysis of β-Amyloid Precursor Protein by γ-Secretase Is an Unusually Slow Process

Intramembrane proteolysis has emerged as a key mechanism required for membrane proteostasis and cellular signaling. One of the intramembrane-cleaving proteases (I-CLiPs), gamma-secretase, is also intimately implicated in Alzheimer's disease, a major neurodegenerative disease and leading cause of dementia. High-resolution crystal structural analyses have revealed that I-CLiPs harbor their active sites buried deeply in the membrane bilayer. Surprisingly, however, the key kinetic constants of these proteases, turnover number k(cat) and catalytic efficiency k(cat)/K-M, are largely unknown. By investigating the kinetics of intramembrane cleavage of the Alzheimer's disease-associated beta-amyloid precursor protein in vitro and in human embryonic kidney cells, we show that gamma-secretase is a very slow protease with a k(cat) value similar to those determined recently for rhomboid-type I-CLiPs. Our results indicate that low turnover numbers may be a general feature of I-CLiPs.